Download Components in Sensing Circuits Word Document

Topic 1.4 – Components in Sensing Circuits.
Learning Objectives:
At the end of this topic you will be able to;
1.4.1 Resistors
 understand that resistance can be increased by connecting resistors
in series;
 understand that resistance can be decreased by connecting resistors
in parallel;
 select and use the equation R  R1  R2 to perform calculations
involving the combined resistance of two resistors in series;
RR
 select and use the equation R  1 2 to perform calculations
R1  R2




involving the combined resistance of two resistors in parallel;
describe how fixed and variable resistors can be used in voltage
dividers;
describe how potentiometers can be used as variable resistors and
voltage dividers;
use the colour and printed code to work out the value and tolerance
of a resistor;
select appropriate preferred values from the E24 series;
1.4.2 Light Dependent Resistors (LDRs)
 state that the resistance of an LDR falls as light intensity increases
(non-linear);
1.4.3 NTC Thermistors
 state that the resistance of ntc thermistors decreases as
temperature increases (non-linear);
1.4.4 Switches
 distinguish between the following types of mechanical switches:
push, toggle, reed, micro, tilt, rotary
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GCSE Electronics.
Unit E1 : Discovering Electronics
1.4.1 – Resistors
Using resistors to control and limit current
If you used the Alpha Kit during Topic 1.2, you would have used 6V, 0.06A
bulbs in the Alpha Kit. Such bulbs are designed to work on a 6V supply. When
6V is applied across a bulb, its filament offers sufficient resistance to keep
the current down to 0.06A and the bulb lights up to its specified brightness.
At working temperature, the filament provides a resistance of about 100Ω.
If we were to connect the same bulb to a 12V battery, this resistance would
only be sufficient to keep the current down to about 0.12A. This high current
would probably burn out the filament, and the bulb would be destroyed. Extra
resistance is required in the circuit to limit the current to 0.06A. This extra
resistance could be provided by connecting two such bulbs in series across
the supply (See fig 1a).
The second bulb provides an extra resistance of about 100Ω. The same
effect could be produced by using a fixed resistor of value 100Ω (Fig 1b).
The wide range of resistor values offered by manufacturers enable us to
limit the current through a component to almost any desired value.
2
Topic 1.4 – Components in Sensing Circuits.
If the current flowing through a component has to be very precisely set, a
variable resistor is used as shown below.
Potentiometers
Potentiometers can be used for dividing up a voltage into any value between
zero and the full supply voltage. A potentiometer consists of a circular
conducting track, made of carbon or resistance wire, over which a sliding
contact moves.
The voltage to be divided is connected across the end of track tags and the
output voltage is taken between one of these tags and the wiper tag.
W iper
R es is tanc e
A
W ire
C ons tant
Variable
Input
Voltage
Voltage
Spindle
B
When the wiper is at position A the full supply voltage is available at the
output. At position B the output voltage is zero.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Potentiometers can also be set up to act as
variable resistors in circuits. In this case the
wiper tag is connected to one of the end of
track tags and the unit is used as shown
opposite.
The following pictures show some of the different types of potentiometers
available.
Presets
These are similar to potentiometers but are usually smaller and have to be
adjusted using a screwdriver. They are designed to be inserted into a circuit
then adjusted to the required value. Once accurately set they are usually
sealed so that they do not change from this value. The following pictures
illustrate the difference between presets and the continuously variable type.
4
Topic 1.4 – Components in Sensing Circuits.
Selecting a resistor
If you turn to the resistor section in any electronics supplies catalogue you
will find a wide range of values and types on offer. After calculating the ideal
value of the resistor required in a circuit you must consider the following
points before making your selection.
(a)
Preferred values
It is very unlikely that you will be able to find your ideal value within the
range of values. Manufacturers only produce certain preferred values.
You have to select the nearest value of resistor within the range.
In the E24 series, the 24 preferred values are:
10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91
together with multiples of 10 of these values, up to about 10MΩ. The
increase between values in E24 is about 10%.
If we multiply each of the values above by 10 we get the next 24
available resistor values:
100, 110, 120, 130, 150, 160, 180, 200, 220, 240, 270, 300, 330, 360,
390, 430, 470, 510, 560, 620, 680, 750, 820, 910
Followed by
1k, 1.1k, 1.2k …………… and so on up to 10MΩ.
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GCSE Electronics.
Unit E1 : Discovering Electronics
(b)
Tolerance
This provides an indication of how much above, or below, the stated
value the resistor might be. A 1.5kΩ resistor with a tolerance of ±5%
could be as low as 1425Ω or as high as 1575Ω, since 5% of 1500 is 75Ω.
Compare the tolerance of carbon film and metal film resistors in your
catalogue.
Carbon Film Tolerance
=
....................................................
Metal Film Tolerance
=
....................................................
Which type of resistor offers the closest tolerance? .................................
(c)
Wattage
You will find that the same type, and value, of resistor is offered at
different wattage. The resistor with a power rating just above your
required power rating should be selected. A power rating of 0.25W is
sufficient for most of your practical work.
Compare the size of similar resistors but with different power rating.
(d)
Stability
This gives an indication of how well the resistor behaves when physical
conditions change.
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Topic 1.4 – Components in Sensing Circuits.
Types of Resistor
The following types of resistors are commonly found in electronic systems.
(a)
Wire wound
These are made by winding a piece of
resistance wire e.g. constantan or
nichrome, on to a ceramic former and
coating it with an insulating material
such as varnish.
1.
2.
3.
Advantages
Can be made very accurate.
Tolerance of 0.1% available.
Value does not change much when
resistor heats up.
Capable of dissipating high power.
1.
2.
Disadvantages
Tend to be bulky and
heavy.
Rather expensive.
This type of resistor is used where close tolerance or high power
dissipation is required. Values are available up to about 22kΩ with a
power dissipation capability of up to 50W.
(b)
Carbon film
In this type of resistor a film of graphite is deposited on a ceramic
former. A helical groove is cut into the carbon. The value of the resistor
is determined by the size of the groove, the thickness of the film and
the size of the former.
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Unit E1 : Discovering Electronics
1.
2.
(c)
Advantages
Easy to manufacture, and cheap.
Good tolerance e.g. 5%
1.
Disadvantages
Rather poor temp stability
Metal film
These are manufactured in a similar way to the carbon film resistors
but the conducting film is made from metal (e.g. nichrome) or metal
oxide.
They have all the advantages of the carbon film type and have much
better tolerance and temperature stability than carbon film type.
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Topic 1.4 – Components in Sensing Circuits.
Resistor Colour Code
The value of the resistor and its tolerance can be worked out from four
colour bands on its body.
N.B. Some resistors especially metal film resistors use a five
band colour code. Details can be found in suppliers catalogues.
Only the four band code will be tested in the examinations.
The tolerance band is a single band near one end of the resistor and is
normally gold or silver. A gold band indicates a tolerance of ±5%, while a silver
band indicates ±10%. If there is no fourth band then the tolerance will be
±20%.
The value of the resistor (in ohms) can be worked out by looking at the three
other coloured bands and using the colour code table.
Band 1
(1st Digit)
Colour
Black
Brown
Red
Orange
Yellow
Band 2
(2nd Digit)
Value
0
1
2
3
4
Band 3
(No of 0's)
Band 4
(Tolerance)
Colour
Green
Blue
Violet
Grey
White
Value
5
6
7
8
9
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GCSE Electronics.
Unit E1 : Discovering Electronics
Examples:
1.
Red
Red
Orange
Gold
Value = 2
2
(three 0’s) Ω
= 2 2 000 Ω = 22kΩ
Tolerance = ± 5%
2.
Yellow
Violet
Green
Value = 4 7 00000 Ω = 4700kΩ = 4.7MΩ
Tolerance = ± 20%
3.
Brown
Red
Black
Silver
Value = 1 2 (no 0’s) Ω = 12Ω
Tolerance = ± 10%
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Topic 1.4 – Components in Sensing Circuits.
4.
Complete the following diagrams by showing the colour code required
for the following resistors:
a)
75kΩ, ±5% resistor.
75kΩ = 75000Ω = 7
Violet
b)
18Ω ±10% resistor.
Brown
c)
Grey
Red
Yellow
8
Gold
_
±10%)
Black
(3
Blue
2.4MΩ (±5%) resistor.
000 ±5%
Orange
(1
360Ω (±5%) resistor.
Orange
d)
Green
5
6
Silver
0
Brown
(2
4
±5%)
Gold
00000
Green
±5%)
Gold
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GCSE Electronics.
Unit E1 : Discovering Electronics
Printed value
Equipment manufacturers’ circuit diagrams often use the following code for
indicating resistor values. The letters give multiples and the position of the
decimal point.
Examples:
Marking
R33
3R3
33R
330R
3k3
33k
3M3
Resistor Value
0.33Ω
3.3Ω
33Ω
330Ω
3.3kΩ
33kΩ
3.3MΩ
Self Assessment Test.
1.
Use the colour code to find the value of the resistors shown below.
a.
Red
Violet
Orange
Gold
...................................................................................................................
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Topic 1.4 – Components in Sensing Circuits.
b.
Green
Blue
Black
Silver
...................................................................................................................
c.
What is the highest value that the resistor (b) is likely to have?
........................................................................................................................................
........................................................................................................................................
d.
2.
Using the resistor printed code – what are the values of the
following resistors.
i)
470R
..................
ii)
2k2
..................
iii)
5M6
..................
Complete the following diagrams by showing the colour code required
for the following resistors:
i)
270 Ω, ±5% resistor.
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GCSE Electronics.
Unit E1 : Discovering Electronics
ii)
10kΩ ±10% resistor.
iii)
3.9kΩ ±10% resistor.
iv)
8.2MΩ ±5% resistor.
Self Assessment Test
1.
2.
14
a.
27000Ω = 27kΩ ±5%
b.
56Ω ±10%
c.
10%of 56 
10
 56  5.6
100
MaxValue  56  5.6  61.6
d.
i) 470Ω; ii) 2.2kΩ iii) 5.6MΩ
i)
ii)
iii)
iv)
Red Violet Brown Gold.
Brown Black Orange Silver.
Orange White Red Silver.
Grey Red Green Gold
Topic 1.4 – Components in Sensing Circuits.
Calculating the value of a current limiting resistor
Suppose we want to operate a 2.5V, 0.25A bulb on a 6V supply.
For the bulb to operate at its specified brightness, it must have 2.5V
dropped across it. The difference between this voltage and the supply
voltage can be dropped across a series resistor. The resistor value selected
should allow 0.25A to flow through it when there is a voltage of (6-2.5)V
across it.
0.25A
R
3.5V
Applying Ohm’s Law to the resistor:
6V
2.5V
R
V
I
so
R
3.5
 14
0.25
There are no 14Ω resistors available in the E24 series. This provides us with
a dilemma since we have to choose between a 13Ω, or a 15Ω resistor. Let us
look at the effect of each.
If we choose a 13Ω resistor, this will mean that the circuit resistance will be
less than we needed. A larger current than expected will flow and will
therefore put a greater strain on the bulb, and this will reduce its operating
life time.
If we choose a 15Ω resistor, this will mean that the circuit resistance is
slightly higher than that required, which will reduce the current flowing
below 0.25A. The result will be a bulb operating at slightly less than full
brightness, but within its maximum value.
The most suitable preferred value resistor is this case would be 15Ω.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Resistors in series
The following 2 circuits have been set up on a circuit simulator.
Look at Circuit 1 and you will see that the ammeter reading is 6.00mA. We can
apply Ohm’s Law to the circuit to find the total resistance of the circuit.
R
V
I
so
R
12
 2k
6mA
If we add up the values of R1 and R2 from Circuit 1 we get
R1 + R2 = 1kΩ + 1kΩ = 2kΩ
Which is exactly the same answer as we got using Ohm’s Law?
16
Topic 1.4 – Components in Sensing Circuits.
Look at Circuit 2 and you will see that the ammeter reading is 99.88mA. We
can apply Ohm’s Law to this circuit to find
R
10
 100.12
99.88mA
If we add up the values of R3, R4 and R5 from Circuit 2 we get
R3 + R4 + R5 = 47Ω + 33Ω + 20Ω = 100.
Which is nearly but not exactly the same answer as we got using Ohm’s Law?
The difference in this case of 0.12 is due to very small rounding errors that
occur when the simulator is displaying current flow.
So we can see that the total or effective resistance Rs of resistors in series
is given by the general equation:
Rs = R1 + R2 + R3 + ..........
Therefore:
if R1 = 10Ω and R2 = 40Ω, then Rs = 10 + 40 = 50Ω.
if R1 = 15kΩ, R2 = 25kΩ, and R3 = 75kΩ
then Rs = 15k + 25k+ 75k = 115kΩ.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Resistors in Parallel
Look at Circuit 1 and you will see that the ammeter reading is 23.99mA. We
can apply Ohm’s Law to the circuit to find the total resistance of the circuit.
R
V
I
so
R
12
 500.20
23.99mA
Look at Circuit 2 and you will see that the ammeter reading is 513.12mA. We
can apply Ohm’s Law to this circuit to find
R
10
 19.48
513.12mA
There does not seem to be an obvious relationship between the effective
resistances and the resistor values used, other than the effective resistance
in each case is smaller than the individual parallel resistor values.
It can be shown that the total or effective resistance Rp of resistors in
parallel is given by the general equation:
1
1
1
1



 .........
R p R1 R2 R3
For only 2 resistors in parallel this can be simplified to R p 
18
R1  R2
R1  R2
Topic 1.4 – Components in Sensing Circuits.
For example,
If R1 = R2 = 1kΩ, (as given in circuit 1) then
Rp 
R1  R2
R1  R2
Rp 
1000  1000 1000000

 500
1000  1000
2000
If R1 = 33Ω and R2 = 47Ω (as given in circuit 2) then
Rp 
R1  R2
R1  R2
Rp 
33  47 1551

 19.39
33  47
80
Note
1.
2.
3.
4.
You should always check you answer when using the formula to make
sure that the effective resistance of 2 resistors in parallel is smaller
than the individual resistor values.
When 2 resistors of the same value are connected in parallel the
effective resistance is 1/2 (one half) of their individual values.
If 3 resistors of the same value are connected in parallel then the
effective resistance is 1/3 (one third) of their individual values. E.g. If
three 10k resistors are connected in parallel their effective resistance
= 10k/3 = 3.333kΩ.
In general if ‘n’ resistors of the same value are connected in parallel
then the effective resistance is 1/n (one ‘n’th) of their individual values.
E.g. If ‘n’ 10k resistors are connected in parallel their effective
resistance = 10k/’n’ Ω.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Worked examples
1.
Find
(i)
(ii)
(iii)
(iv)
(v)
the current in the 2Ω resistor,
the current in the 4Ω resistor,
the voltage across the 2Ω resistor,
the voltage across the 4Ω resistor, and
the supply voltage.
Solution:
(i)
2A (Since this is a series circuit so current is same everywhere)
(ii)
2A (Same reason as (i))
(iii) Apply V = I x R to 2Ω resistor.
V2Ω = 2 x 2 = 4V.
(iv)
Apply V = I x R to 4Ω resistor.
V4Ω = 2 x 4 = 8V
(v)
20
Supply voltage V = V2Ω + V4Ω = 4 + 8 = 12V.
Topic 1.4 – Components in Sensing Circuits.
2.
Find
(a)
(b)
(c)
(d)
(e)
the current in the 2Ω resistor,
the current in the 2Ω resistor,
the voltage across the 2Ω resistor,
the voltage across the 4Ω resistor, and
the effective resistance of the parallel circuit.
Solution:
(a)
2A (because the current in the 2Ω resistor will be twice that in
the 4Ω resistor.)
(b)
1A (because the current in the 4Ω resistor will be half that in the
2Ω resistor.) Or (Since 3A enters the network, and 2A goes
through the other resistor only 1A is left).
(c)
Apply V = I x R to 2Ω resistor.
V2Ω = 2 x 2 =4V
(d)
Apply V = I x R to 4Ω resistor.
V4Ω = 1 x 4 = 4V
(e)
Rp 
R1  R2
R1  R2
Rp 
2 4 8
  1.333
24 6
Or by inspection V4Ω = 4V since
resistors are in parallel and therefore
the voltage must be the same as V2Ω.
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GCSE Electronics.
Unit E1 : Discovering Electronics
3.
For the network shown below, calculate:
(a)
(b)
(c)
(d)
(e)
(f)
the combined resistance Rp of R1 and R2 in parallel.
the total resistance RT of the network.
I.
V1 and V2.
I1 and I2.
What is the nearest preferred value to RT in the E24 series.
Solution:
(a)
Rp = 20 /2 = 10 equal resistors in parallel)
(b)
RT = R3 + R1 = 10 +30 = 40
(c)
The voltage V across the whole network is 6V and its total
resistance RT is 40Ω therefore
I
(d)
V
6

 0.15 A
RS 40
V1 = I x R3 = 0.15A x 30Ω = 4.5V
But V = V1 + V2 therefore V2 = V – V1 = 6 – 4.5 = 1.5V
22
(e)
I1 = I2 = ½I (since R1 = R2) therefore I1 = ½ x 0.15A = 0.075A.
(f)
The 2 nearest preferred values to 40 are 39 and 43 so in this
case choose 39
Topic 1.4 – Components in Sensing Circuits.
Summary 
1.
Resistors usually exist in combinations of series and parallel
components.
2.
The effective resistance Rs of series resistors is given by the
following formula.
RS  R1  R2  R3  ..........
3.
The effective resistance Rp of two resistors in parallel is given by
the formula.
Rp 
R1  R2
R1  R2
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GCSE Electronics.
Unit E1 : Discovering Electronics
Homework Questions 1
1. Draw a diagram to show how you would connect two 10Ω resistors to
give a total resistance of (a) 20Ω, (b) 5Ω.
(a)
(b)
2.
In the circuit below what is
[2]
(a)
the current in the 3Ω resistor. ………………………
[1]
(b)
the current in the 6Ω resistor. ………………………
[1]
(c)
voltage across the 3Ω resistor.
…………………………………………………………………………………………………………..……………………………… [2]
(d)
voltage across the 6Ω resistor.
…………………………………………………………………………………………………………..……………………………… [2]
(e)
the supply voltage.
………………………………………………………………………………………………..………………………………………… [1]
24
Topic 1.4 – Components in Sensing Circuits.
3.
In the circuit below, calculate
(a)
the current in the 3Ω resistor.
………………………………………………………………………………………………..……………………………………
(b)
the current in the 6Ω resistor.
………………………………………………………………………………………………..……………………………………
(c)
[1]
the voltage across the 6Ω resistor.
………………………………………………………………………………………………..……………………………………
(e)
[1]
the voltage across the 3Ω resistor.
………………………………………………………………………………………………..……………………………………
(d)
[1]
[1]
the supply voltage.
………………………………………………………………………………………………..……………………………………
[1]
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GCSE Electronics.
Unit E1 : Discovering Electronics
4.
For the network shown below calculate the total resistance between
(a)
X and Y,
……………………………………………………………………………………………………………
………………………………………………………………………………………………..……………………………………
(b)
[1]
Y and Z,
……………………………………………………………………………………………………………
………………………………………………………………………………………………..……………………………………
(c)
[1]
X and Z.
……………………………………………………………………………………………………………
………………………………………………………………………………………………..……………………………………
(d)
Use the list of E24 preferred values to select a single resistor to
replace the network of 4 resistors.
………………………………………………………………………………………………..……………………………………
26
[1]
[1]
Topic 1.4 – Components in Sensing Circuits.
Voltage dividing chains
I
R1
V1
R2
V2
V
You should remember the Voltage Divider Rule from topic 1.3. It states:
V2 
V  R2
( R1  R2 )
If we connect a load across the output it could change the voltage level set
by the resistor chain. If the current taken by the load is greater than 0.1I
there will be a significant change in the output voltage.
We will investigate this in the following assignments.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Assignment 1.4A
Investigating resistors
Activity 1:
In this activity you will be investigating the use of resistors to control the
current flowing in a circuit.
1a.
Set up the following circuit using your circuit simulator.
1b.
Close the switch. Note the brightness of the bulb, and record the
reading from the ammeter.
Brightness : ................................................................................................
Reading on ammeter = ........................... (A, mA or µA)
Save your circuit as “E1-Circuits2-Act1”
1c.
28
Open the switch then modify the circuit so that it has a 100Ω resistor
in series with the bulb, as shown below:
Topic 1.4 – Components in Sensing Circuits.
1d.
Close the switch. Note the brightness of the bulb, and record the
reading from the ammeter.
Brightness : ................................................................................................
Reading on ammeter = ........................... (A, mA or µA)
What effect does the resistor have upon the brightness of the bulb and
the current flowing through the filament ?
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
Save your circuit as “E1-Circuits2-Act2”
1e.
Remove the 100Ω resistor and replace it with a 1 kΩ resistor. Close the
switch. Note the brightness of the bulb, and record the reading from
the ammeter.
Brightness : ................................................................................................
Reading on ammeter = ........................... (A, mA or µA)
Report on any change from 1c.
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
Save your circuit as “E1-Circuits2-Act3”
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GCSE Electronics.
Unit E1 : Discovering Electronics
Complete the following:
When a resistor is connected in series with a bulb, it ______________
the current flowing through the filament.
The larger the value of the resistor, the ______________ the
current flowing through the bulb.
Activity 2:
Let us now investigate the use of a potentiometer, set up as a variable
resistor to control current flow in a circuit.
2a.
Set up the following circuit in your simulator.
2b.
Close the switch then move the slider on the potentiometer by sliding
the grey box from left to right. Describe what you observe.
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
Explain this effect : ................................................................................................
.......................................................................................................................................
.......................................................................................................................................
30
Topic 1.4 – Components in Sensing Circuits.
2c.
2d.
Record the maximum and minimum values of current possible in this
circuit.
Minimum Current =
.......................... (A, mA, µA)
Maximum Current =
.......................... (A, mA, µA)
Connect a voltmeter across the bulb. Move the slider control so that you
can measure the following:
Minimum voltage across the bulb
=
.................... (V, mV, µV)
Maximum voltage across bulb
=
.................... (V, mV, µV)
Complete the following:
The bulb is at its dimmest when the variable resistor is set to its
.......................................... value.
Save your circuit as “E1-Circuits2-Act4”
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GCSE Electronics.
Unit E1 : Discovering Electronics
Activity 3:
We shall now investigate the use of two resistors in voltage dividing chains.
3a.
Set up the following circuit in your simulator. Note that the supply
voltage is at 12V.
3b.
Close the switch and record the voltage provided at the output.
Output voltage =
................. volts
Save your circuit as “E1-Circuits2-Act5”
3c.
Open the switch and connect a 6V, 0.06A bulb across the output,
as shown below.
3d.
Close the switch, you will probably find that the bulb does not light up.
Careful examination of the voltmeter might give you a clue as to why the
bulb does not light up.
32
Topic 1.4 – Components in Sensing Circuits.
Voltage across bulb
=
................... (V, mV, µV)
Try to explain why the bulb does not light up. This is quite difficult. If
you cannot answer ask your tutor for help.
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
Save your circuit as “E1-Circuits2-Act6”
3e.
Switch off the circuit and replace the 10kΩ resistors with 18Ω
resistors as shown below.
33
GCSE Electronics.
Unit E1 : Discovering Electronics
3f.
Switch on the circuit and comment on your observations.
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
Save your circuit as “E1-Circuits2-Act7”
3g.
When the switch is closed - move the mouse over each resistor in turn.
If you hold the mouse over the top of the resistor for a few seconds a
small information box will appear, which shows you the current flowing
through the resistor, and also the power being dissipated.
Which resistor is giving off the most power? .................................................
.......................................................................................................................................
Explain your answer: ...............................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
34
Topic 1.4 – Components in Sensing Circuits.
Information
Resistors can be used to form voltage dividing chains. If current is
drawn by a load connected across the chain, the value of the
resistors in the chain should be much lower than the load resistor.
Activity 4:
In this activity we shall be investigating the use of a potentiometer for
dividing up a voltage.
4a.
Set up the following circuit in your simulator. Note that the supply
voltage is set to 6V.
4b.
Close the switch then adjust the setting of the potentiometer.
4c.
What are the maximum and minimum voltages that can be set across the
bulb ?
Minimum voltage
=
........................... (V, mV, µV)
Maximum voltage
=
........................... (V, mV, µV)
What advantage does this circuit have, over that in Activity 2, for
controlling the brightness of a bulb ?
.......................................................................................................................................
.......................................................................................................................................
Save your circuit as “E1-Circuits2-Act8”
35
GCSE Electronics.
Unit E1 : Discovering Electronics
Design Problems:
1.
Design a system to provide two levels of intensity from a 6V, 0.06A
bulb. The first level gives full intensity, with 6V across the bulb, while
the second provides a lower intensity with 4V across the bulb. You have
available the following items:
Start by drawing a circuit diagram for the system.
Ask your tutor to check your circuit, and then set it up and try it.
Write a report about how your circuit works.
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
.......................................................................................................................................
Save your circuit as “E1-Circuits2-Act9”
36
Topic 1.4 – Components in Sensing Circuits.
2.
Design a circuit which will enable you to operate a 6V, 0.06A bulb
correctly from a 9V supply. All steps in your calculation must be clearly
shown.
You are provided with the following items:
Circuit Diagram:
Calculations:
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
What is your preferred resistor choice 30Ω, 39Ω, 47Ω, 56Ω or 68Ω.
................................
Give a reason for your choice................................................................................
.......................................................................................................................................
Save your circuit as “E1-Circuits2-Act10”
37
GCSE Electronics.
Unit E1 : Discovering Electronics
Homework Questions 2:
Answer all questions in the spaces provided; continue on a separate piece of
paper if required.
1.
You have been given a 6V, 0.06A bulb and told to connect it to a 12V
supply.
i)
What will be the result if you carried out this instruction?
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
[1]
ii)
How could you carry out the instruction successfully?
........................................................................................................................................
........................................................................................................................................
2.
........................................................................................................................................
[1]
Resistors can be used to split the voltage from a battery into smaller
voltages for use in other parts of the circuit. This application is called a
potential divider. Draw two circuit diagrams to show how a 10kΩ and
5kΩ resistor can be used to provide output voltages of (i) 4V and (ii) 8V
from a 12V power supply.
i)
ii)
[2]
38
Topic 1.4 – Components in Sensing Circuits.
3.
A 100Ω resistor has a current of 100mA flowing through it when the
voltage across it is 10V. Calculate the power dissipated in the resistor.
........................................................................................................................................
4.
........................................................................................................................................
[2]
A 220Ω resistor has a current of 10mA flowing through it. Calculate
the power dissipated in the resistor.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
5.
........................................................................................................................................
[3]
Give two advantages and two disadvantages of Wire wound resistors.
Advantages:
i)
...................................................................................................................
ii)
...................................................................................................................
Disadvantages:
i)
...................................................................................................................
ii)
...................................................................................................................
[4]
39
GCSE Electronics.
Unit E1 : Discovering Electronics
6.
Give two advantages and one disadvantage of Carbon Film resistors.
Advantages:
i)
...................................................................................................................
ii)
...................................................................................................................
Disadvantage:
i)
7.
8.
...................................................................................................................
[3]
When trying to select a resistor for a particular application, there are
three critical parameters or properties of the resistor that need to be
considered. What are they?
i)
i)
...................................................................................................................
ii)
...................................................................................................................
iii)
...................................................................................................................
[3]
A resistor is marked with the following coloured bands, Red, Red,
Brown, Gold. What is its value and tolerance?
.............................................................................................................................
ii)
A resistor is marked with the following coloured bands, White,
Brown, Red, Silver. What is its value and tolerance?
.............................................................................................................................
40
Topic 1.4 – Components in Sensing Circuits.
iii)
A resistor is marked with the following coloured bands, Brown,
Black, Orange, Gold. What is its value and tolerance?
.............................................................................................................................
iv)
9.
i)
A resistor is marked with the following coloured bands, Blue, Grey,
Black, Gold. What is its value and tolerance?
.............................................................................................................................
[4]
What would be the colour of the bands on a 4.7kΩ ± 10% resistor?
.............................................................................................................................
ii)
What would be the colour of the bands on a 100kΩ ± 5% resistor?
.............................................................................................................................
iii)
What would be the colour of the bands on a 33kΩ ± 20% resistor?
............................................................................................................................
iv)
10.
i)
What would be the colour of the bands on a 39Ω ± 5% resistor?
.............................................................................................................................
[4]
A resistor is stamped with the number 47R. What is its value in
ohms?
.............................................................................................................................
ii)
A resistor is stamped with the number 2k7. What is its value in
ohms?
.............................................................................................................................
41
GCSE Electronics.
Unit E1 : Discovering Electronics
iii)
11.
12.
A resistor is stamped with the number 1M. What is its value in
ohms?
.............................................................................................................................
[3]
Using the printed value method of marking resistors, what would you
see marked on the following resistors.
i)
10,000 Ω
.......................................
ii)
3,300 Ω
.......................................
iii)
5,600,000 Ω
.......................................
iv)
10 Ω
.......................................
[4]
A 3.5V 0.03A bulb is to be run from a 9V power supply. A resistor must
be used to limit the current flowing in the circuit.
i)
Draw a diagram of the circuit required.
[2]
42
Topic 1.4 – Components in Sensing Circuits.
ii)
Calculate the exact value of the resistor required to limit the
current in the circuit.
.............................................................................................................................
.............................................................................................................................
iii)
.............................................................................................................................
[2]
What value of resistor would you choose from the E24 series of
resistors to use in your circuit. Give a reason for your choice.
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
[2]
13.
A resistor has a value of 1kΩ ±5%. Calculate the minimum and maximum
value of resistance that this resistor may have.
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
.............................................................................................................................
[4]
43
GCSE Electronics.
Unit E1 : Discovering Electronics
14.
Calculate the voltage at the output terminals in each of the following
circuits.
i)
.........................................................................
.........................................................................
.........................................................................
.........................................................................
[4]
ii)
.........................................................................
.........................................................................
.........................................................................
.........................................................................
[4]
iii)
.........................................................................
.........................................................................
.........................................................................
.........................................................................
[4]
44
Topic 1.4 – Components in Sensing Circuits.
1.4.2 – Light Dependent Resistors
A Light Dependent Resistor or LDR consists of a cadmium sulphide track set
out on an insulator base. The resistance of the track depends upon the
intensity of light which falls upon it. You can see the track through the
transparent window on the top of the unit.
The symbol for an LDR is as follows:
The LDR comes in a variety of different packages as shown below:
The resistance characteristic for the LDR is shown at the top of the
following page:
45
GCSE Electronics.
Unit E1 : Discovering Electronics
Resistance (kΩ)
Light Intensity (lux)
The light intensity is measured in a unit called ‘lux’, but we can see from the
characteristic curve that the resistance of the LDR falls as light intensity
increases. The decrease in resistance is non-linear, the resistance falls
rapidly at first and then less quickly as the intensity of light increases.
The type of LDR provided in the Alpha kit is a type ORP12. The Data Sheet
for the device quotes a resistance of several MΩ in total darkness, falling to
about 1kΩ in bright light.
One disadvantage of LDRs is that they respond rather slowly to changes in
light intensity. The ORP12 takes about 120 ms to complete its change in
resistance when light level changes from darkness to bright light. This is a
long time in terms of electronic switching circuits!
46
Topic 1.4 – Components in Sensing Circuits.
1.4.3 – NTC Thermistors
The thermistor is a two leaded component that changes its resistance in
response to a change in temperature. The symbol for a thermistor is shown
below:
The ‘-t°’ alongside the symbol indicates that this is a negative temperature
coefficient (or n.t.c.) thermistor, which simply means that the resistance of
the thermistor decreases as temperature increases.
A positive temperature coefficient (p.t.c.) thermistor does exist where the
resistance increases as temperature increases, but these will not be
examined as part of this course. The symbol is the same, but just has a ‘+t°’
alongside it should you see this in any project books you may look at.
The characteristic curve for a thermistor, therefore looks like this.
Once again we can see that the response is non-linear, i.e. resistance falls
quicker at the start and then the rate of decrease in resistance slows down
as higher temperatures are reached.
47
GCSE Electronics.
Unit E1 : Discovering Electronics
Thermistors come in many different physical packages as shown by the
diagram below:
Irrespective of the package style the behaviour of all of these thermistors is
the same, as temperature rises the resistance of the thermistor falls.
The change in package style does however affect the response time of the
thermistors, the ‘rod’ style thermistor is large and bulky and has the slowest
response time, whilst the tiny ‘glass bead’ style has the fastest response.
Depending on the application different styles of package can be selected but
it is important to remember that from circuit design point of view the
package is not important as long as we know the range of resistance the
thermistor has over the temperature range that it will be used.
48
Topic 1.4 – Components in Sensing Circuits.
A typical data sheet for a thermistor is shown below:
The first two rows in this table show that at 25°C the resistance is 300Ω,
and at 50°C the resistance has fallen to 121Ω. All thermistors are different
so it is important to check their data sheets to determine their
characteristics so that a suitable circuit can be designed to use them
effectively.
We will investigate more about how the LDR and Thermistor are used to make
sensing circuits in our next Topic 1.5. For now it is sufficient for you to be
able to recognise their symbol and describe their characteristic.
49
GCSE Electronics.
Unit E1 : Discovering Electronics
1.4.4 – Switches
A mechanical switch is used to physically break the electrical connection
between two points in the circuit, and then allows us to reconnect these parts
safely, without risk of electrical shock, particularly useful when dealing with
high voltages and currents.
There are many different types of switches as you will find out in the
practical assignment, however we will look at the main basic groups now and
some of the jargon which is associated with different types of switches.
a)
Switch Contacts
The contact arrangement of switches is classified in terms of:Poles
Throws
-
The moving part of the switch which is pivoted.
The part of the switch which is fixed and makes
contact with the moving part of the pole.
The following diagram gives the circuit symbol for the four main
categories of switch contact arrangements. There are others but these
are normally restricted to specific applications.
N/C
N/O
Single Pole Single Throw
(S.P.S.T)
(Single Pole On - Off)
Single Pole Double Throw
(S.P.D.T.)
(Single Pole Changeover)
N/C
N/O
N/C
N/O
Double Pole Single Throw
(D.P.S.T.)
(Double Pole On - Off)
Double Pole Double Throw
(D.P.D.T.)
(Double Pole Changeover)
The dotted line indicates that the two poles are mechanically connected
(or ganged) but are not electrically connected.
50
Topic 1.4 – Components in Sensing Circuits.
With changeover switches the contacts may also be called, or marked
N/O or N/C, these stand for Normally Open, or Normally Closed. i.e. the
N/O contacts are those which are not connected together when the
switch is in the unoperated (off) position. These contacts are made
when the switch is operated. Similarly the N/C contacts are connected
when the switch is unoperated (off) and the contact is broken when the
switch is operated.
b)
Uses of switches
i) SPST switches are often used to switch on and off low voltage
electronic circuits
ii)
A SPDT switch is useful when selecting one of two alternative
circuits or to operate a device from one of two positions as is
commonly used for staircase lighting.
Another example of an use for a
SPDT switch is a door bell which
can be switched off at night and
replaced with a bulb so as not to
disturb a sleeping child.
iii)
A DPST switch is really two separate switches controlled by one
lever. A typical use is in mains appliances to disconnect both the
live and neutral wires as a safety precaution.
From
Mains
L
L
To
Appliance
N
N
51
GCSE Electronics.
Unit E1 : Discovering Electronics
iv)
A DPDT switch is often used to reverse the direction of a motor.
1
+
-
2
1
2
M
When the contacts are switched from
position 1 to 2 the current flows in the
opposite direction through the motor
and hence reverses the direction of the
motor.
Note: The arrangement of relay contacts is also described in terms
of poles and throws, but the symbols are different to those of a
mechanical switch, so that they can be identified on circuit
diagrams, as shown below:
N/C
N/O
Single Pole Single Throw
(S.P.S.T)
(Single Pole On - Off)
Single Pole Double Throw
(S.P.D.T.)
(Single Pole Changeover)
N/C
N/O
N/C
N/O
Double Pole Single Throw
(D.P.S.T.)
(Double Pole On - Off)
52
Double Pole Double Throw
(D.P.D.T.)
(Double Pole Changeover)
Topic 1.4 – Components in Sensing Circuits.
c.
Switch types
i)
Press switches are used for momentary contact and are spring
loaded. A common example of a press switch is on a door bell, or
keyboard.
The press switch is a variation of the S.P.S.T. switch and is
available in two variants: Push to make and Push to break.
Symbols are as follows:
ii)
Toggle switches are two position switches and are normally used
as on/off switches.
53
GCSE Electronics.
Unit E1 : Discovering Electronics
iii)
Rotary switches are multiway switches and are used only in very
specific applications. Each of several channels may be selected by
turning a spindle. It is not unusual to have up to twelve separate
channels for selection from just one pole.
The symbol for a rotary switch is shown below.
Channel 1
Channel 2
Channel 3
Channel 4
Channel 5
Channel 6
iv)
54
Slide switches are inexpensive and are found mainly in low voltage
circuits. They are useful for setting the logic levels of gates high
or low.
Topic 1.4 – Components in Sensing Circuits.
v)
Reed switches are made from two pieces of metal sealed inside a
glass case. When a magnet is brought close to the glass the two
contacts inside join and complete the circuit. These switches are
particularly useful in burglar alarm circuits, when the switch is
mounted in a door frame and a magnet inserted into the actual
door. When the door is closed the circuit is complete, but when
opened and the magnet moves away the contacts open causing a
break in the circuit, hence triggering the alarm.
55
GCSE Electronics.
Unit E1 : Discovering Electronics
56
vi)
Microswitches are very sensitive push switches which require a
very low operating force over a very small distance. They are ideal
for sensing very small movements.
vii)
Tilt switches are formed by sealing two contacts in a metal can
with a small amount of mercury. The switch is positioned so that
when a video for example is in its normal position the mercury lies
across the two contacts, completing the circuit. If the video is
lifted and tilted then the mercury will run off the contacts as it is
a liquid and break the circuit triggering an alarm.
Topic 1.4 – Components in Sensing Circuits.
Homework Questions 3
Answer all questions in the spaces provided; continue on a separate piece of
paper if required.
1.
2.
Draw the electrical circuit symbol for a light dependent resistor.
Describe the operation of the LDR.
[1]
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
3.
........................................................................................................................................
[2]
Draw the electrical circuit symbol for a thermistor.
[1]
4.
Describe the operation of the thermistor.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
[2]
57
GCSE Electronics.
Unit E1 : Discovering Electronics
5.
What are mechanical switches used for in modern electronic circuits?
........................................................................................................................................
6.
........................................................................................................................................
[1]
Give a brief description of the key uses of the following types of
switches.
i)
SPST.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
[1]
ii)
SPDT.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
[2]
iii) DPDT.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
[2]
58
Topic 1.4 – Components in Sensing Circuits.
7.
Draw the circuit symbols for (i) Push to Make switch, and (ii) Push to
Break Switch.
i)
iii)
ii)
What is the key difference between these two switches?
[2]
.............................................................................................................................
.............................................................................................................................
[1]
8.
Describe a situation where you might require the use of a Reed Switch.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
[1]
9.
Describe a situation where you might require the use of a Tilt Switch.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
[1]
59
GCSE Electronics.
Unit E1 : Discovering Electronics
10.
i)
ii)
Draw a circuit diagram to show how a 1kΩ resistor and a SPST
switch can be connected to a 12V supply, so that when the switch
is open, the output voltage is 12V, and when the switch is closed
the output voltage is 0V.
[2]
Modify your circuit diagram to show how the same components and
a 9V power supply may be used to provide an output voltage of 0V
when the switch is open, and an output voltage of 9V, when the
switch is closed.
[1]
60
Topic 1.4 – Components in Sensing Circuits.
Solutions to Homework Exercises.
Homework Questions 1
1.
(a)
(b)
2.
(a)
1A.
(b)
1A.
(c)
V3  I  R  1 3  3V
(d)
3.
[2]
[1]
[1]
[2]
V6  I  R  1 6  6V
[2]
(e)
Vsupply = V3Ω + V6Ω = 3 + 6 = 9V
(a)
the current in the 3Ω resistor will be twice as much as that in the
6Ω resistor and total current is 6A. Therefore current in 3Ω
must be 4A.
[1]
the current in the 6Ω resistor will be half that in the 3Ω resistor
and total current is 6A. Therefore current in 6Ω must be 2A.
[1]
(b)
(c)
(d)
(e)
[1]
V3  I  R  4  3  12V
[2]
V6  I  R  2  6  12V
Vsupply = V3Ω = V6Ω = 12V {parallel circuit}
[2]
[1]
61
GCSE Electronics.
Unit E1 : Discovering Electronics
4.
(a)
(b)
(c)
RX Y  R1  R2  1k  3k  4k
RY  Z 
R1  R2 4000  4000 16000000


 2000  2k
R1  R2 4000  4000
8000
[1]
[1]
RX Z  RX Y  RY Z  4k  2k  6k
[1]
(d)
6.2k
[1]
Homework Questions 2
1.
2.
3.
62
i)
The lamp will blow.
ii)
The lamp could be successfully connected into the circuit if a
resistor of suitable vale were added into the circuit in series with
the lamp so that some of the battery voltage was dropped across
the resistor.
i)
Power  Current  Voltage
 100mA  10V
100

 10  1W
1000
ii)
Topic 1.4 – Components in Sensing Circuits.
4.
Either
Or
V  IR
10

 220  2.2V
1000
and
P V  I
10
22
 2.2 

 0.022W  22mW
1000 1000
P  I 2R
10
10


 220
1000 1000
 0.022W  22mW
5.
Advantages:
Any two from : Very Accurate, Value does not change
when resistor heats up, Can dissipate high powers.
Disadvantages: They are bulky and heavy, rather expensive.
6.
7.
8.
Advantages:
Any two from : Easy to manufacture, Cheap, Good
Tolerance ±5%
Disadvantage:
Poor temperature stability.
i)
Tolerance.
ii)
Wattage.
iii)
Stability. (in any order)
i)
220Ω, ±5%
ii)
9100Ω or 9.1kΩ, ±10%
iii)
10000Ω or 10kΩ, ±5%
iv)
68Ω, ±5%
63
GCSE Electronics.
Unit E1 : Discovering Electronics
9.
10.
11.
12.
64
i)
Yellow, Violet, Red, Silver.
ii)
Brown, Black, Yellow, Gold.
iii)
Orange, Orange, Orange.
iv)
Orange, White, Black, Gold.
i)
47Ω.
ii)
2.7kΩ.
iii)
1.8MΩ.
i)
10k
ii)
3k3
iii)
5M6
iv)
10R
i)
Topic 1.4 – Components in Sensing Circuits.
ii)
If the power supply is 9V and the lamp is rated at 3.5V, 5.5V must
be dropped across the resistor. Since it is a series circuit the
same current which flows through the lamp must flow through the
resistor, so I = 0.03A. Therefore, applying Ohms law to the
resistor gives
R
iii)
13.
V
5.5

 183.3
I 0.03
Looking at the E24 series of resistors will show that there is no
183.3Ω resistor available. We have a choice between 180Ω or
200Ω. If we use the 180Ω resistor however we will be putting
less resistance in the circuit than required, this will cause an
increase in current, which will reduce the life of the lamp.
Therefore it would be more appropriate to use the 200Ω resistor,
as this will ensure that the current is kept slightly below the
maximum rated value of the lamp, hence preserving battery life.
5% of 1000Ω is given by the following:
5% 
5
 1000  50
100
The minimum value is therefore given by 1000Ω - 50Ω = 950Ω.
The maximum value is therefore given by 1000Ω + 50Ω = 1050Ω.
14.
i)
VOUT  VS 
R1
R1  R2
10k
10k  10k
10000
 6
20000
1 6
 6    3V
2 2
 6
65
GCSE Electronics.
Unit E1 : Discovering Electronics
ii)
VOUT  VS 
R1
R1  R2
10k
10k  20k
10000
 9
30000
1 9
 9    3V
3 3
 9
iii)
VOUT  VS 
R1
R1  R2
8.2k
8.2k  1.2k
8200
 30 
10000
82 2460
 30 

 24.6V
100 100
 30 
Homework Questions 3
1.
2.
3.
66
The resistance of an LDR changes depending on how much light falls
onto the window of the package. The more light that falls on the window
the lower the resistance of the LDR. Resistance can vary from
approximately 500Ω in bright light to several MΩ in darkness.
Topic 1.4 – Components in Sensing Circuits.
4.
The resistance of the thermistor is dependent upon it’s temperature.
The resistance of the thermistor falls as the temperature rises. The
range of resistance will vary greatly depending on the type of
thermistor used, but is typically from approximately 100kΩ when cold
to 500Ω when hot (100°C)
5.
Mechanical switches are used to connect and disconnect different parts
of an electrical circuit without risk of electrical shock to the user.
Switches will be found on nearly all items of electrical equipment. Their
main use is to disconnect the power supply from the circuit, either to
allow work to be carried out on the equipment or to preserve battery
life in the case of battery operated equipment.
6.
i)
SPST, switches are sometimes referred to as on/off switches.
They have only two contacts, and are capable of only making or
breaking a single connection in a circuit. Main uses are for
switching battery operated equipment on and off.
ii)
SPDT, switches are sometimes referred to as changeover
switches. They have one input terminal but two output terminals.
Their main use is in changing the output path of current, for
example, this type of switch may be used to change the path of
current from a buzzer to a lamp on a doorbell, so that the buzzer
can be switched off at night and the light switched on by the bell
push on the door instead, so preventing waking up small children
that might be asleep.
iii)
DPDT, switches are really two SPDT switches together, but
operated by the same lever. The main use of this type of switch is
for reversing electrical motors, as shown in the following diagram.
1
+
2
1
-
M
2
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GCSE Electronics.
Unit E1 : Discovering Electronics
7.
i)
iii)
ii)
Both of these switches are momentary action switches. The push
to make is normally open, i.e. the circuit is broken and is only
connected when the push button is pressed. The push to break
switch however is normally closed, i.e. the circuit is connected until
the push button is pressed when the circuit is then broken.
8.
Reed switches are operated by a magnet, and as such are particularly
useful for burglar alarm applications where the magnet is fixed to the
door, and the reed switch to the door frame. When the door is opened
the magnet moves away from the reed switch and breaks the circuit,
and setting off the alarm.
9.
A tilt switch as its name suggests operates when it is moved to an angle.
It’s applications are typically in tractors, cranes and heavy machinery
which operate on slopes or at unusual angles to warn the driver that his
machine is close to the point of toppling over so that precautionary
measures can be taken.
10.
i)
68
Topic 1.4 – Components in Sensing Circuits.
ii)
Now for some examination style questions.
69
GCSE Electronics.
Unit E1 : Discovering Electronics
Examination Style Questions
Note: For all examinations the resistor colour code, and voltage divider formula is provided on Page 2,
the pupil information page of each examination paper. They are reproduced here for use with the
following questions.
1.
The resistor colour code is given in the information sheet on page 2.
(a)
Use the information to work out the colour code for a 680Ω resistor.
Write the colours in the correct spaces on the diagram below.
[3]
70
Topic 1.4 – Components in Sensing Circuits.
(b)
Use the colour code to work out the resistance of the following resistor.
Resistance in ohms = ..................................
[2]
(c)
A voltage divider is set up as shown in the following diagram.
(i)
What is the combined resistance of R1 and R2?
......................................................................................................................................
(ii)
Use the equation given in the information sheet on Page 2 to calculate the output
voltage VOUT.
......................................................................................................................................
......................................................................................................................................
[2]
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GCSE Electronics.
Unit E1 : Discovering Electronics
2.
The following diagram shows 3 resistors arranged as a voltage divider:-
(a)
Use the information sheet on page 2 to help you calculate the combined resistance of the
4kΩ and 6kΩ resistors.
..............................................................................................................................................
..............................................................................................................................................
[1]
(b)
Calculate the output voltage VOUT.
..............................................................................................................................................
..............................................................................................................................................
[2]
3.
Which of the following graphs A, B, C or D best shows the way in which the resistance of an LDR
depends on the light level ?
Answer = .............................
72
[1]
Topic 1.4 – Components in Sensing Circuits.
4.
Here is a diagram of a 560Ω resistor with a 5% tolerance.
Complete the following table.
Resistor
Value
Colour of
Band 1
Colour of
Band 2
Colour of
Band 3
Colour of
Band 4
560Ω
[4]
5.
The resistor colour code is given in the information sheet on page 2.
Here is a diagram of a resistor.
The table shows the colours of the bands on a resistor.
(a)
(i)
Band 1
Band 2
Band 3
Band 4
Orange
White
Orange
Silver
Use the colour code to work out the value of the resistor.
Resistance in ohms = ................................................
[2]
(ii)
What is the percentage tolerance of the resistor?
....................................................................................
[1]
(iii)
What is the tolerance in ohms of the resistor?
....................................................................................
[1]
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GCSE Electronics.
Unit E1 : Discovering Electronics
(b)
Three other resistors are arranged as a voltage divider as follows:
(i)
Calculate the combined resistance of the parallel combination R1 and R2.
..............................................................................................................................................
..............................................................................................................................................
[1]
(ii)
Calculate the output voltage VOUT.
..............................................................................................................................................
..............................................................................................................................................
[2]
6.
Put the three measures of resistance in order of size, starting with the biggest, and ending with the
smallest.
kilohm (kΩ)
ohm (Ω)
megohm (MΩ)
[2]
74
Topic 1.4 – Components in Sensing Circuits.
7.
Here is a diagram of a resistor R.
Complete the following table.
(a)
Resistor
Value
Colour of
Band 1
Colour of
Band 2
Colour of
Band 3
Colour of
Band 4
560Ω
Red
Violet
Red
Silver
Use the colour code to work out the value of the resistor R.
Resistance in ohms = ................................................
[2]
(b)
What is the percentage tolerance of the resistor R?
....................................................................................
[1]
(c)
What is the tolerance in ohms of the resistor R?
....................................................................................
[1]
8.
Here is a diagram of an 82Ω ±5% resistor.
(a)
Complete the following table.
Resistor
Value
Colour of
Band 1
Colour of
Band 2
Colour of
Band 3
Colour of
Band 4
82Ω
[4]
(b)
The 82Ω resistor is now placed in parallel with an 18Ω resistor. Calculate the combined
resistance.
..............................................................................................................................................
..............................................................................................................................................
[2]
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GCSE Electronics.
Unit E1 : Discovering Electronics
9.
Here is a list of four mechanical switches:
push,
reed,
tilt,
toggle
Choose the most appropriate switch from the list for the following jobs.
(a)
To warn tractor drivers that the tractor may be in danger of toppling over.
Answer : ................................................
(b)
To be used with a magnet to sense when a door is open.
Answer : ................................................
[2]
10.
Here is a voltage divider.
(a)
What is the combined resistance of R1 and R2?
(b)
.................................................................................................................................................
[1]
Calculate the value of the voltage VOUT.
..............................................................................................................................................
..............................................................................................................................................
[2]
76
Topic 1.4 – Components in Sensing Circuits.
11.
Three resistors are arranged as a voltage divider as follows:
(a)
Calculate the combined resistance of the 10kΩ and 15kΩ resistors.
..............................................................................................................................................
..............................................................................................................................................
[2]
(b)
Calculate the output voltage VOUT.
..............................................................................................................................................
..............................................................................................................................................
[1]
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GCSE Electronics.
Unit E1 : Discovering Electronics
11.
The resistor colour code is given in the information sheet on page 2.
(a)
Use the information to work out the colour code for a 10kΩ resistor.
Write the colours in the correct spaces on the diagram below.
[3]
(b)
Use the colour code to work out the resistance of the following resistor.
(i)
Resistance in ohms = ..................................
[2]
(ii)
Use the tolerance band to calculate the highest and lowest values the resistor could
have.
Highest Value in ohms = .........................................
Lowest Value in ohms = ..........................................
[2]
78
Topic 1.4 – Components in Sensing Circuits.
(c)
The 10kΩ resistor is used with a 5kΩ resistor to make a voltage divider:
(i)
What is the combined resistance of R1 and R2?
(ii)
.....................................................................................................................................
[1]
Calculate the value of the voltage, VOUT.
.....................................................................................................................................
.....................................................................................................................................
[2]
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GCSE Electronics.
Unit E1 : Discovering Electronics
Self Evaluation Review
My personal review of these objectives:
Learning Objectives



1.4.1 Resistors
understand that resistance can be increased by
connecting resistors in series;
understand that resistance can be decreased by
connecting resistors in parallel;
select and use the equation R  R1  R2 to
perform calculations involving the combined
resistance of two resistors in series;
select and use the equation R  R1R2
R1  R2
to
perform calculations involving the combined
resistance of two resistors in parallel;
describe how fixed and variable resistors can
be used in voltage dividers;
describe how potentiometers can be used as
variable resistors and voltage dividers;
use the colour and printed code to work out the
value and tolerance of a resistor;
select appropriate preferred values from the
E24 series;
1.4.2 Light Dependent Resistors (LDRs)
state that the resistance of an LDR falls as
light intensity increases (non-linear);
1.4.3 NTC Thermistors
state that the resistance of ntc thermistors
decreases as temperature increases (nonlinear);
1.4.4 SWITCHES
distinguish between the following types of
mechanical switches:
push, toggle, reed, micro, tilt, rotary
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
80