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Topic 1.6 – Output Circuits.
Learning Objectives:
At the end of this topic you will be able to;
 Choose an appropriate output device for a given application from;
buzzer, lamp, led, motor, solenoid;
 Calculate the value of current limiting resistor required when using a
LED or other device on a given DC supply.
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GCSE Electronics.
Unit E1 : Discovering Electronics
Output Circuits
In our last topic we considered a variety of ways of using LDRs and
thermistors to make sensing circuits. In this topic we will investigate some of
the output devices that we could use in our circuits, and any necessary safety
precautions needed to use these devices safely.
Output devices are chosen to provide a specific output signal, whether this is
sound, light or movement. The output devices you will be asked about are as
follows.
1.
Buzzer/Siren
This simple device produces an audible tone when switched on. The
buzzer comes in many different shapes and sizes, from small low power
units to high power alarm siren’s.
Low Power
Buzzers
suitable for
projects
High Power siren
used for alarm and
security applications.
2
Every buzzer will specify
the operating voltage and
current required for the
correct operation of the
buzzer, e.g. 12V, 100mA.
It is important that the
output circuit matches
these requirements for
correct operation.
Topic 1.6 – Output Circuits.
2.
Lamps
Filament Lamp
Indicator Lamp
A lamp is a simple device for providing a visual indication that an output
has switched on. Just like buzzers there is an enormous range of lamps
that can be used as an output indicator. Here are a collection of just a
few you will find in an electronics suppliers catalogue.
Once again the choice of which lamp to use will fall to the circuit
designer to choose the most appropriate type for the application. As
with buzzers, every lamp has a maximum voltage and current, e.g. 6V
0.06A, which must not be exceeded otherwise the lamp can be
permanently damaged.
It is your responsibility as the designer of the circuits to ensure that
these maximum values are not exceeded.
Most lamps produce light from a very hot thin wire called a filament.
This method of light production is not very efficient, and lamps burn out
quite quickly, after about 1000 hours (~46 days) of continuous use.
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GCSE Electronics.
Unit E1 : Discovering Electronics
3.
Light Emitting Diodes (LED)
The LED has revolutionised electronic circuits, because it is a device
that produces light in a variety of colours, without the heating effect of
the filament lamp. It is therefore much more efficient than the
filament lamp and it lasts much longer than a filament lamp, typically
about 100,000 hours (>11 years continuous use).
LEDs are available in a huge variety of different package styles and
colours.
As well as being available in single devices as shown above, groups of
LEDs can now be packaged together and they can be seen all around you,
replacing filament lamps in a variety of everyday objects.
Traffic lights:
Old Filament Style
4
New LED style.
Topic 1.6 – Output Circuits.
Warning signs for traffic flow.
The LED is different from the other components we have met so far
because they must be connected correctly, as it only allows current to
flow through it in one direction. A closer examination of the symbol will
help you to correctly insert the LED into a circuit. The anode must be
connected to the positive part of the circuit. Current then flows
through the LED in the direction of the arrow.
Anode
Cathode
Light given off when conventional
current flows in this direction
LEDs can operate on a very small current, typically 10 to 20mA, which
means that they are very energy efficient. The only downside is that
they have a relatively low voltage rating, ~2.0V. We always have to
protect the LED in a circuit to ensure that this maximum voltage is not
exceeded. More will be covered on this later in this chapter.
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GCSE Electronics.
Unit E1 : Discovering Electronics
4.
Motors
Motors are used where movement is required as the output, e.g. to open
a gate, or turn the drum of a washing machine. Motors again are
available in many different shapes and sizes.
Motors generally require quite large operating currents (>500mA) in
order to perform their role correctly. Their use in this course is likely
to be limited as schools do not usually carry large supplies of motors for
experimental procedures, but may form part of a theoretical
examination question so they should not be discounted as a usable
output device.
5.
Solenoid
Solenoid
The solenoid is a specialised output device which relies on the
electromagnetic effect of an electric current. The solenoid is used to
operate bolts in security doors, and for engaging the starter motor of
the car to the engine when the car is first started. It has a pulling
action when activated, and a spring mechanism, or gravity usually returns
the object to its original position.
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Topic 1.6 – Output Circuits.
The following picture shows a solenoid lock with the bolt retracted on
the left and released on the right.
The lock would be installed in a door, and the bolt would engage with the
floor to secure the door. As with motors, solenoids usually need quite a
large current (>1A) to generate a strong enough magnetic field to pull
the bolt into the device, against a spring or the pull of gravity, and
therefore their use in the electronics laboratory is very limited. They
are useful devices in the industrial and automotive world however, so
once again could be used as part of a theoretical examination question.
With the exception of the LED and low power buzzers, all of the output
devices considered have relatively large output current requirements. This
means that they cannot be connected directly to the sensing systems we
designed in Topic 1.5. As we found out in our first look at systems, an output
driver will be required to provide this additional current for these higher
powered output devices. Now that we have looked at how to create sensing
circuits and the range of output devices available we have all the necessary
tools to look at a range of different driver circuits. This will be covered in
Topic 1.7.
We did exclude the LED from this requirement, because unlike most of the
other devices the LED actually requires just a few milliamps, and a voltage of
just 2V to operate properly.
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GCSE Electronics.
Unit E1 : Discovering Electronics
In many practical circuits the power supply will usually be considerably higher
than these voltages and therefore we have to take precautions to protect the
LED from damage by applying higher voltages than those required. This is
achieved very simply by adding a series resistor to the LED as shown in the
example below.
Example : A red LED is to be used with a 9V battery. Calculate the series
resistor required to limit the current to 10mA if the forward
voltage drop across the LED is 2V.
The circuit is therefore as follows.
10mA
9V
R
2V
The voltage across the LED must be limited to a maximum of 2V, as this is a
series circuit, the voltage across the resistor R and LED must equal 9V.
Therefore the voltage across the resistor R must be –
VR  9  2
 7V
As this is a series circuit, the current flowing through the resistor R and LED
must be the same. So, using Ohm’s law for resistor R, we can obtain a value
for resistor R as follows –
R
VR
7

 0.7k  700
I 10mA
We now have a choice to make as the E24 series of resistors does not have a
700Ω resistor. We have to choose between 680Ω or 750Ω.
8
Topic 1.6 – Output Circuits.
In a practical circuit, there would be little noticeable difference between
either resistor, but in exam questions, if the current given in the question is a
maximum of 10mA then you must choose the higher value resistor, i.e. 750Ω,
to ensure that the current does not exceed 10mA. If the current given in the
question is quoted as being approximately 10mA, then either resistor is
acceptable.
One thing we have to be very careful about with l.e.d’s is that they are not
very good at handling reverse bias, indeed if a reverse bias of more than 5V
is applied to the LED then it will usually be damaged permanently. Correct
identification of the anode and cathode is therefore very important when
using LED’s in practical circuits.
A similar approach can be used if a 3.5V, 0.25A lamp is to be used on a 12V
supply for example.
The circuit is therefore as follows.
0.25A
R
12V
3.5V
The voltage across the lamp must be limited to a maximum of 3.5V, as this is
a series circuit, the voltage across the resistor R and lamp must equal 12V.
Therefore the voltage across the resistor R must be –
VR  12  3.5
 8.5V
As this is a series circuit, the current flowing through the resistor R and
lamp must be the same. So, using Ohm’s law for resistor R, we can obtain a
value for resistor R as follows –
R
VR
8.5

 34
I
0.25 A
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GCSE Electronics.
Unit E1 : Discovering Electronics
We now have a choice to make as the E24 series of resistors does not have a
34Ω resistor. We have to choose between 33Ω or 36Ω. Choosing the 33Ω
resistor will allow slightly more current than specified for the lamp which will
reduce its working life. If we choose the 36Ω resistor slightly less current
will flow and the lamp will not be quite as bright but it will last longer.
Now here’s one for you to do!
1.
A green LED is to be used as an indicator. If the maximum voltage
available for the LED is 5V, and the voltage across the LED is to be no
more than 2.2V, calculate the series resistance required from the E24
series to limit the current to a maximum of 15mA. Draw a diagram to
show the circuit required.
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
Note : Even though different colour LEDs will have slightly different forward
voltage drops, for exam question this will always be 2V.
Answer = 187Ω therefore E24 value = 200Ω.
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Topic 1.6 – Output Circuits.
Assignment T1.6A
Investigating light emitting diodes
Activity 1:
1a.
Set up the following arrangement in your circuit simulator, using a red
LED initially.
1b.
Close the switch and record the forward volt drop across the light
emitting diode.
VF(red)
1c.
Switch off the circuit and replace the red LED with a green one. Close
the switch and record the forward volt drop across the new LED
VF(green)
1d.
= .................. (V, mV, lV)
= .................. (V, mV, lV)
Switch off the circuit and replace the green LED with a yellow one.
Close the switch and record the forward volt drop across the new LED
VF(yellow)
= .................. (V, mV, lV)
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GCSE Electronics.
Unit E1 : Discovering Electronics
Design Problem 1:
Design a circuit including a current limiting resistor which will allow a
green LED to be operated from a 12V power supply. The maximum
current allowed for the LED is 15 mA. You should select a resistor from
the E24 series only.
Circuit Diagram: Draw the circuit diagram required in the space below.
Calculations : Show clearly any calculations you need to perform below:
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
Set up the system and measure the current flowing through the LED
and the volt drop across it.
Current flowing
=
...................... (A, mA, µA)
Voltage across LED
=
...................... (V, mV, µV)
Save your circuit as “E1-Circuits6-Act1”
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Topic 1.6 – Output Circuits.
Design Problem 2:
Design a circuit including a current limiting resistor which will allow a
red LED to be operated from a 9V power supply. The absolute maximum
current allowed for the LED is 10 mA. You should select a resistor from
the E24 series only.
Circuit Diagram: Draw the circuit diagram required in the space below.
Calculations : Show clearly any calculations you need to perform below:
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
.......................................................................................................................................
Set up the system and measure the current flowing through the LED
and the volt drop across it.
Current flowing
=
...................... (A, mA, µA)
Voltage across LED
=
...................... (V, mV, µV)
Save your circuit as “E1-Circuits6-Act2”
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GCSE Electronics.
Unit E1 : Discovering Electronics
Homework Questions 1
1.
Draw the electrical circuit symbol for a light emitting diode.
[1]
2.
Why are resistors always used in series with light emitting diodes ?
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
3.
........................................................................................................................................
[2]
Give two advantages of LEDs compared to filament lamps.
i)
.............................................................................................................................
ii)
.............................................................................................................................
[2]
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Topic 1.6 – Output Circuits.
4.
A LED with a forward voltage drop VF = 2V has to be powered from a
number of DC sources as given in the following table. IF(max) is the
maximum current which can be allowed to flow through the LED without
causing damage.
Vsupply
IF(max)
A
6V
10mA
B
9V
15mA
C
12V
10mA
D
15V
20mA
E
20V
30mA
For each case calculate:
i)
ii)
A.
i.
The value of series resistor required to operate the LED
within it’s specification.
State the value of the E24 resistor most suited to be used in
the circuit.
...................................................................................................................
...................................................................................................................
...................................................................................................................
ii.
B.
i.
...................................................................................................................
[3]
...................................................................................................................
...................................................................................................................
...................................................................................................................
ii.
...................................................................................................................
[3]
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GCSE Electronics.
Unit E1 : Discovering Electronics
C.
i.
...................................................................................................................
...................................................................................................................
...................................................................................................................
ii.
D.
i.
...................................................................................................................
[3]
...................................................................................................................
...................................................................................................................
...................................................................................................................
ii.
E.
i.
...................................................................................................................
[3]
...................................................................................................................
...................................................................................................................
...................................................................................................................
ii.
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...................................................................................................................
[3]
Topic 1.6 – Output Circuits.
Solutions to Homework Questions
Homework Questions 1
1.
2.
3.
4.
LEDs require much less current to operate than standard bulbs. They
also operate with only 2V across them. Series resistors are normally
required for two reasons,
i)
to reduce the current flowing through the diode to an
acceptable level.
ii)
to reduce the forward voltage dropped across the diode to
prevent damage occurring since many of the LEDs are
operated on power supplies greater than 2V.
Any two from:i)
Last longer.
ii)
Require less current.
iii)
Produce less heat during operation.
A.
i.
RSeries 
ii.
Nearest preferred value = 430Ω.
i.
RSeries 
ii.
Nearest preferred value = 470Ω.
B.
(6V  2V )
4V

 400
10mA
0.010
(9V  2V )
7V

 466.67
15mA
0.015
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GCSE Electronics.
Unit E1 : Discovering Electronics
C.
D.
E.
(12V  2V ) 10V

 1000
10mA
0.010
i.
RSeries 
ii.
Nearest preferred value = 1kΩ.
i.
RSeries 
ii.
Nearest preferred value = 680Ω.
i.
RSeries 
ii.
Nearest preferred value = 620Ω.
(15V  2V ) 13V

 650
20mA
0.020
(20V  2V ) 18V

 600
30mA
0.030
Now for some Examination Style Questions.
18
Topic 1.6 – Output Circuits.
Examination Style Questions.
1.
The LED in the circuit below needs a current of 20mA and a forward voltage drop of 2V across it
to make it light.
(a)
When the LED is lit:
(i)
What is the voltage drop across resistor R?
……………………………………………………………………………………….
(ii)
What is the current flowing through resistor R?
……………………………………………………………………………………….
[2]
(b)
Calculate a suitable resistance for resistor R.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[2]
(c)
Explain why only certain preferred values are available.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[1]
(d)
Choose a suitable preferred value for resistor R from the E24 series of resistors.
……………………………………………………………………………………………….
[1]
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GCSE Electronics.
Unit E1 : Discovering Electronics
2.
The LED in the circuit below needs a current of 10mA and a forward voltage drop of 2V across it
to make it light.
(a)
Calculate how much power is used in the LED.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[2]
(b)
(i)
What is the current flowing through resistor R?
……………………………………………………………………………………….
(ii)
What is the voltage drop across resistor R?
……………………………………………………………………………………….
[2]
(c)
Calculate a suitable resistance for resistor R.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[2]
(d)
Choose a suitable preferred value for resistor R from the E24 series of resistors so that the
LED has about 10mA going through it.
……………………………………………………………………………………………….
[1]
20
Topic 1.6 – Output Circuits.
3.
The LED in the following circuit is fully on when a current of 20mA goes through it. The voltage
drop across it is 2V.
(a)
The LED is fully on.
(i)
What is the voltage drop across resistor R?
……………………………………………………………………………………….
(ii)
What is the current flowing through resistor R?
……………………………………………………………………………………….
[2]
(b)
Calculate a suitable resistance for resistor R.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[1]
(c)
Choose a suitable preferred value for resistor R from the E24 series of resistors, so that the
LED has about 20mA flowing through it.
……………………………………………………………………………………………….
[1]
21
GCSE Electronics.
Unit E1 : Discovering Electronics
4.
The LED in the following circuit has a current of 15mA and a forward voltage drop of 2V across
it when it is ON.
(a)
(i)
What is the current flowing through resistor R?
……………………………………………………………………………………….
(ii)
What is the voltage drop across resistor R?
……………………………………………………………………………………….
[2]
(b)
(i)
Use your answers to part (a) to calculate the power used in resistor R.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[2]
(ii)
Which one of the following power ratings given below is the smallest that must be
used for resistor R? Circle the correct answer.
125mW
250mW
500mW
1W
[1]
22
Topic 1.6 – Output Circuits.
(c)
Calculate a suitable resistance for resistor R.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[1]
(d)
Choose a suitable preferred value for resistor R from the E24 series of resistors.
……………………………………………………………………………………………….
[1]
23
GCSE Electronics.
Unit E1 : Discovering Electronics
5.
The LED in the following circuit is fully on when a current of 20mA goes through it. The voltage
drop across it is 2V.
(a)
The LED is fully on.
(i)
What is the voltage drop across resistor R?
……………………………………………………………………………………….
(ii)
What is the current flowing through resistor R?
……………………………………………………………………………………….
[2]
(b)
(i)
Calculate the power used in resistor R when the LED is lit.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[2]
(ii)
Which one of the following power ratings given below is the smallest that must be
used for resistor R? Circle the correct answer.
75mW
125mW
250mW
500mW
[1]
24
Topic 1.6 – Output Circuits.
(c)
Calculate a suitable resistance for resistor R.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[1]
(d)
Choose a suitable preferred value for resistor R from the E24 series of resistors, so that the
LED has no more than 20mA flowing through it.
……………………………………………………………………………………………….
[1]
25
GCSE Electronics.
Unit E1 : Discovering Electronics
6.
The LED in the following circuit is needs a current of 15mA going through it and voltage drop of
2V across it to make it light.
(a)
The LED is on.
(i)
What is the voltage drop across resistor R?
……………………………………………………………………………………….
(ii)
What is the current flowing through resistor R?
……………………………………………………………………………………….
[2]
(b)
Calculate a suitable resistance for resistor R.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[1]
(c)
Choose a suitable preferred value for resistor R from the E24 series of resistors, so that the
LED has no more than 15mA flowing through it.
……………………………………………………………………………………………….
[1]
26
Topic 1.6 – Output Circuits.
7.
The LED in the circuit below needs a current of 10mA and a forward voltage drop of 2V across it
to make it light.
(a)
How much power is used in the LED?
Circle the correct answer from the following list:
50mW
20mW
50W
20W
[1]
(b)
The LED is on.
(i)
What is the current flowing through resistor R?
……………………………………………………………………………………….
(ii)
What is the voltage drop across resistor R?
……………………………………………………………………………………….
[2]
(c)
Calculate a suitable resistance for resistor R.
……………………………………………………………………………………………….
……………………………………………………………………………………………….
[2]
(d)
Choose a suitable preferred value for resistor R from the E24 series of resistors so that the
LED has about 10mA going through it.
……………………………………………………………………………………………….
[1]
27
GCSE Electronics.
Unit E1 : Discovering Electronics
Self Evaluation Review
Learning Objectives
My personal review of these objectives:



Choose an appropriate output device
for a given application from;
buzzer, lamp, led, motor, solenoid;
Calculate the value of current
limiting resistor required when using
a LED or other device on a given DC
supply.
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
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