Download Voltage divider circuits

Engineering Science
Component Electronics
Control Circuits
Volts
VS
R1
10A
LDR
0V
mA
V
COM
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Voltage divider circuits
Input transducers
0V
VV
Input transducers are devices that convert a change in physical conditions (for
example, temperature) into a change in resistance and/or voltage. This can then be
processed in an electrical network based on a voltage divider circuit.
If two or more resistors are connected in series (see diagram below), the voltage over
each resistor will depend on the supply voltage and the ratio of the resistances.
Voltage divider circuits work on the basic electrical principle that if two resistors are
connected in series across a supply, the voltage load across each of the resistors will
be proportional to the value of the resistors.
The layouts of voltage divider circuits are conventionally represented as shown above.
A voltage divider circuit can be represented in a number of different ways. Some of
these are shown below.
0V
2
0V
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Voltage divider circuit
If an input transducer changes its resistance as the physical conditions change, then
the resistance change has to be converted into a voltage change so that the signal can
be processed. This is normally done using a voltage divider circuit.
A typical voltage divider circuit is shown below.
VS
R1
V
R2
0 volts
As you can see, this circuit consists basically of two resistors connected in series. As
you already know, if you change the value of R1, the voltage across it will change, as
will the voltage across R2. In other words, the resistors divide the voltage up between
them.
Practical task
Use a prototype board to build the voltage divider circuit shown below. If the supply
voltage is 6 volts, R1 = 220  and R2 = 330 , what is the voltage across R2?
Volts
VS
R1
10A
mA
V
COM
R2
0V
The voltage across R2 is normally called the output or Vo. You should find that the
voltage is divided up according to the formula
R
V
= 2 
V
O
S
R
+
R
1
2
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Worked example: voltage divider circuit
Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 12 volts
R 1 = 80k
R 2 = 40k
V2
0 volts
Applying the voltage proportion formula:
V2 = VS 
R2
R1 + R2
40
40 + 80
V2 = 4 volts
V2 = 12 
The voltage over the 80 K resistor could be calculated in the same way, but this is
unnecessary for this circuit since we can use Kirchoff’s second law to confirm the
answer. The voltages over each of the components in a series circuit must add up to
the supply voltage, hence the voltage over the 80 K resistor is 12 V  4 V = 8 V.
It is also possible to use Ohm’s law to solve these voltage divider problems.
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Exercise 1
Exercises: Voltage divider circuits
1. Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 12 volts
R 1 = 270R
R 2 = 810R
V2
0 volts
2. Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 12 volts
R 1 = 390R
R 2 = 10K
0 volts
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3. Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 6 volts
R 1 = 10K
R 2 = 47K
V2
0 volts
4. Calculate the voltage signal V2 across the resistor R2 in the voltage divider circuit
below.
VS = 9 volts
R 1 = 10K
R 2 = 2.2K
0 volts
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Simple switches can be used in voltage divider circuits to give a digital signal (that is
definitely ON or OFF) to another part of a circuit.
In the example below a normally closed switch is used.
When the switch is pressed, the voltage divider comes into use and power is supplied
to the LED to give a definitely ON signal. Build the circuit in Crocodile Technology
to test this.
Digital switch types
Different types of switch were described earlier but they can be wired up to suit their
application. A switch with its contacts apart when it is not operating is called normally
open.
Double-pole switch symbols
Double-pole single-throw switch (DPST)
Double-pole double-throw switch (DPDT)
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Analogue input transducers
The two most common analogue input transducers are the thermistor and the lightdependent resistor (LDR).
Thermistor
A thermistor is a device whose resistance varies with temperature. It is a temperaturedependent resistor. There are two main types.
1. Negative temperature coefficient (t or NTC) – where resistance decreases as
temperature increases.
2. Positive temperature coefficient (+t or PTC) – where resistance increases as
temperature increases.
The circuit symbols for and typical characteristics of the two types of resistor are
shown below.
NTC thermistors are the most commonly used.
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Graph of temperature versus resistance
The graph below shows accurately how the resistance varies with temperature for an
NTC thermistor.
Thermistor types
Strain gauges
Strain gauges are really load sensors. They consist of a length of resistance wire and
when stretched their resistance changes. Strain gauges are attached to structural
members (beams, etc.) and as they are loaded, a reading on a voltmeter can be
obtained.
Strain gauge
Light-dependent resistor (LDR)
The LDR (sometimes called a photoresistor) is a component whose resistance depends
on the amount of light falling on it. Its resistance changes with light level. In bright
light its resistance is low (usually around 1 K). In darkness its resistance is high
(usually around 1 M).
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The circuit symbol and typical characteristics of an LDR are shown above.
Graph of illumination versus resistance
The graph below shows accurately how the resistance varies as the amount of
illumination falling on an LDR varies.
Voltage divider circuits
One of the main purposes of the voltage divider circuits is to sense and process inputs
from analogue sensors. In this example a thermistor will be used. The resistor R2 of
the previous circuit has been replaced by an NTC thermistor.
VS = 9 volts
1
-t
VO
0 volts
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Practical task 1
Build the above circuit and measure and record the output voltage (Vo) at room
temperature and at a higher temperature (use your fingers to warm up the thermistor).
Volts
VS
R1
10A
mA
V
COM
NTC Thermistor
0V
Measure the voltage drop and the resistance of the thermistor at both these
temperatures. Check your results by calculation and record them in a simple table.
NTC thermistor
Low temperature
High temperature
Voltage
Resistance
This voltage divider circuit uses a light-dependent resistor or LDR. The LDR replaces
R2 from the basic voltage divider circuit.
VS = 9 volts
10K
VO
ORP12
0 volts
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Volts
VS
R1
10A
mA
V
COM
LDR
0V
Practical task 2
Build the above circuit and measure and record the output voltage (Vo) at normal
room light conditions and when the LDR is covered (use your finger or a coin to do
this).
Measure the voltage drop and resistance of the LDR at both light states. Check your
results by calculation and record these in a simple table.
LDR
Light
Dark
Voltage
Resistance
Variable resistor (potentiometer)
A potentiometer configured as a variable resistor can be used in a circuit as a voltage
or current control device. They are often used in voltage divider circuits to adjust the
sensitivity of the input. In Standard Grade Technological Studies the majority of
applications will use a variable resistor or a potentiometer configured as a variable
resistor.
Potentiometers normally have three tags or terminals. The outer ones are connected to
the ends of the resistive material and the centre one is connected to the wiper.
The spindle of the potentiometer is connected to the wiper, which is able to traverse
the whole of the resistive material. As the spindle rotates, a sliding contact puts more
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or less resistive material in series with the circuit. In this way the resistance in a
voltage divider circuit is varied.
Miniature potentiometers
Most modern circuits now use miniature potentiometers or variable resistors.
Examples of miniature potentiometers (not to scale)
Voltage divider circuits
The LDR voltage divider circuit can be set up to detect when it is light or when it is
dark.
VS = 9 volts
VS = 9 volts
ORP12
10K
ORP12
10K
VO
VO
0 volts
Detects when dark
Detects when light
The above circuits should be simulated on Crocodile Technology to confirm their
operation.
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Sensitivity
With an analogue sensor it is normally desirable to adjust the sensitivity of the circuit.
Rather than using a fixed resistor we can replace it with a variable resistor (or
potentiometer).
VS = 9 volts
ORP12
47K
VO
0 volts
Practical task: voltage divider circuits
The picture below shows a typical situation where a light sensor circuit could be
useful.
To save money and inconvenience the residents want the outside light to come on
when it gets dark. They also want to be able to adjust the sensitivity from summer to
winter nights.
Build the following circuit using a prototype circuit board. The variable resistor is
rated at 10 k.
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+ 9 volts
VR
LDR
0 volts
Adjust the sensitivity so that the output voltage (Vo) goes higher when your hand is
moved across the LDR at a distance of approximately 100 mm. You will have to
attach a multimeter to the circuit to see when this is happening.
Check this out using Crocodile Technology.
VS = 9 volts
10K
ORP12
0 volts
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Exercise 2
Exercises: voltage divider circuits
1. Calculate the voltages that would appear across each of the resistors marked ‘X’ in
the circuits below.
5V
9V
0V
0V
2. In each of the following voltage divider circuits determine the unknown quantity.
12 V
0V
16 V
12 V
0V
0V
15 V
20 V
220R
0V
0V
16
0V
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3. An NTC (negative temperature coefficient) thermistor is used in a voltage divider
circuit as shown below. Using information from the graph shown, determine the
resistance of the thermistor and hence calculate the voltage that would appear
across it when it is at a temperature of:
(a) 80C
(b) 20C.
4. What would happen to the voltage across the thermistor in the circuit shown above
as the temperature increased?
5. What would happen to the voltage across the resistor in the circuit shown above as
the temperature increased?
6. A thermistor (type 5) is used in a voltage divider circuit as shown below. The
characteristics of the thermistor are shown in the graph. If the voltage V2 is to be
4.5 V at 100 C, determine a suitable value for R1. State whether V2 will increase
or decrease as the temperature drops. Explain your answer.
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Transistors
The first major breakthrough in electronics came with the invention of the diode valve
at the beginning of the twentieth century. This was the first real electronic component
and was to lead to the modern diode and transistor.
A diode valve consisted of a heater inside a hollow rod that had been coated with a
substance which released electrons when heated. This was surrounded by a thin metal
cylinder, with all of this being contained in a bulb-like glass container. When the rod
was heated, electrons were released but, as in any diode, the electrons could only go
in one direction.
The diode was followed by the triode, which allowed the current flow to be
controlled. These valves could act as electronic switches or amplifiers. Radio and
television were developed using these amplifier valves. In the 1940s the first
computer was built using valves  it contained over 20,000 valves and filled a large
room.
In 1947 the transistor was invented. The transistor had many advantages over valves,
the main ones being size, efficiency, durability and cost. The next big advance in
electronics was the integrated circuit in 1958: two transistors were fitted on a silicon
chip. The developments since then have been rapid and chips now contain over a
million transistors.
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Transistors (bipolar)
The transistor is a semiconductor device. This means that it is sometimes a good
conductor of electricity and sometimes a poor one. A transistor is made up of three
layers of semiconductor materials that are either ‘n type’ or ‘p type’.
There are two types of bipolar transistor available: pnp or npn. We will deal only with
the npn type for convenience. (The only real difference is that the voltages and
currents should be reversed for a pnp transistor.)
C
This diagram represents an npn transistor. It
has three leads or legs.
N
C is the collector
B
P
B is the base
N
E is the emitter
E
When a positive voltage of about 0.6 volts is applied across the base and emitter, the
resistance between the collector and the emitter of the transistor drops from very high
to very low. In other words the transistor changes from being a very poor to a very
good conductor.
This means that to switch the transistor ‘on’ a small voltage of about 0.6 volts is
applied to the base. When the voltage reaches 0.7 volts the transistor is fully ‘switched
on’. In this condition the transistor is said to be fully ‘saturated’.
General-purpose transistor
The BC 639 is common general-purpose transistor. The diagram below shows the
position of the legs when viewed from underneath the case.
B
C
E
The transistor has to be connected into circuits correctly. The arrowhead on the
emitter indicates the direction of ‘conventional’ current flow  that is, opposite to the
electron flow.
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How does the transistor work?
Consider the circuit shown below.
9V
When the switch S1 is open, no current can flow in any part of the circuit. This may
seem strange since a ‘complete’ circuit appears to be made from the voltage source,
through the bulb, the transistor and back to the voltage source. But, as no voltage is
being applied to the base of the transistor, it is acting as a barrier to electric current.
When switch S1 is closed, a very small voltage is applied to the base of the transistor.
When this happens the transistor allows current to flow through it and the bulb will
light; the transistor is said to ’switch on’.
Bipolar transistors amplify current. A small current flowing through the base of a
transistor causes a much larger current to flow from the collector to the emitter.
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The transistor as a switch
One of the main uses of a transistor is that of a very sensitive switch.
Assignment
Use Crocodile Technology or another circuit simulation package to set up the circuit
below. Begin with a value of 2200 K for the base resistor R and then reduce the
resistance using the values given in the table below. This can be carried out manually
if a suitable package is unavailable.
9V
Lamp
c
b
A
BC 108
1K
e
V
0V
Complete the following table during your investigation.
Base resistor
value (K)
2200
1000
470
220
100
47
33
22
10
1
Base/Emitter
Voltage (mV)
Base current (A)
Lamp
on/off
You should find that the circuit will ‘switch on’ the lamp when the base/emitter
voltage drop of the transistor is 0.7 volts. You will also have noted that as the
base/emitter voltage drop rises above 0.7 V the brightness of the lamp does not
increase. This is because once this level has been reached, the transistor is fully
‘switched on’, or saturated.
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Practical tasks
1. Build the circuit below to demonstrate the operation of a transistor switch.
Lamp
A
+6 volts
IK
Flying
lead
c
e
b
0 volts
When the flying lead (a wire connected at one end only) is connected to hole ‘A’
the transistor should switch and the lamp should light.
Connect a multimeter (set at voltage) across the base and emitter of the transistor.
Volts
A
to lamp
10A
mA
V
COM
The multimeter should measure approximately 0.7 volts. As explained earlier, this
will switch the transistor ‘on’ and the lamp should light.
If this reading is incorrect or the lamp does not light when the flying lead is
connected to hole A, check all the connections and fix any faults until the circuit
works as expected.
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To make the circuit even more sensitive, a voltage divider with an LDR and variable
resistor can be used. This will enable small changes in the LDR resistance to switch
the transistor.
Lamp
BC 639
Transistor
Resistor
Variable Resistor
(Potentiometer)
LDR
2. Build the transistor switching circuit below.
Lamp
+6 volts
10K
b
c
e
1K
0 volts
LDR
Instructions
 Place all components as shown in diagram.
 Insert all connection wires.
 Make the 0 volt connection.
 Make the +6 volt connection.
 Set the variable resistor to mid-value.
 Cover the LDR and observe what happens.
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Transistor circuits calculations
Ohm’s law, Kirchoff’s second law, series circuit and parallel circuit calculations are
just as important and appropriate in transistor circuits as they were in the previous
ones.
Example circuit
+ 6 volts
6V
60mA
Lamp
c
b
BC 108
1K
e
0V
The transistor circuit above is basically a parallel circuit. If the circuit is rearranged
slightly this becomes obvious.
Ic
c
6 volts
T
x
e
0V
Ie
b
Rb
Ib
The transistor (marked T) is at the junction of the parallel circuit. If we assume that no
voltage drops across the collector/emitter in the transistor then
Vxe = 6 volts (in the bulb branch)
As the two branches between x and e are in parallel, Vxe across the resistor branch
must also be 6 volts. Thus
VRb + Vbe = 6 volts
We know that Vbe must be 0.7 volts to switch the transistor on; therefore
VRb = 5.3 volts
It is now possible to calculate all other currents and resistance values.
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Relays
Although relays are often considered to be output devices, they are really output
switches from electric or electronic circuits. These output switches are used as inputs
for other circuits. In practice you can hear relays clicking on and off when a car’s
indicators are used.
How the relay works
When an electric current flows into the relay coil, the coil becomes an electromagnet.
This electromagnet attracts the armature and moves the contacts. This movement
provides the switching, just as the contacts in any other switch do.
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The relay is a very useful device because it is the vital link between microelectronics
and high-energy systems that require substantial amounts of current. The relay is
perhaps the most commonly used switch for driving devices that demand large
currents.
Relay symbol
in a circuit
Relays  connections
Miniature
relay
The connections for a typical miniature relay are shown below.
1 +
16 



4
6
13
11
8
9
Connections 1 and 16 are those from the sensing or input circuit.
Connections 4 and 13 are the supply voltage to run the output.
Connections 6 and 11 are the normally closed output terminals.
Connections 8 and 9 are the normally open output terminals.
Relays  protective diodes
As seen earlier, relays have a coil that is energised and de-energised as the relay
switches on and off. During this process of switching, the coil can generate a large
reverse voltage (called a back e.m.f.). This reverse voltage can cause considerable
damage to components, especially transistors.
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The transistors and other sensitive components can be protected by the inclusion of a
diode that provides a path for the current caused by the reverse voltage to escape.
The circuit diagram is shown below.
Relay
Supply
Voltage
BC 639
Transistor
Diode
0V
To Base
Resistor
A solenoid is another output transducer that has a coil inside. Circuits containing a
solenoid require a protective diode as well.
Coil
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Relay circuit
The circuit below shows a typical transistor circuit with a relay as an output.
+ 6 volts
-t
Relay
c
b
639
BC 108
1K
e
10K
0 volts
Practical tasks
1. Build the relay circuit below. When the temperature of the thermistor is increased
you should hear the relay switching and then switching once more as the
temperature decreases again. Note the diode, which is used to protect the transistor
(see later for more information).
2.
+6 volts
NTC
c
b
1K
0 volts
10K
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e
e
d
o
i
d
Relay
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This task requires you to connect a 12 volt lamp to the normally open output of
the relay so that when the temperature of the thermistor rises the light will switch
on.
Note the 12 volt supply for the bulb. Do not mix up the supplies or the 0 volt lines.
+12 volts
+6 volts
NTC
12V Lamp
c
b
1K
0 volts
e
e
d
o
i
d
Relay
10K
0 volts
Alter the circuit so that the lamp comes on when it gets dark. Draw the circuit diagram
before you alter the prototype circuit above.
Relays can also be used to switch on (and off) solenoid-actuated pneumatics valves.
These normally run on a 12 volt supply. This is a method of controlling pneumatic
circuits and systems with microelectronics.
Solenoid-actuated
3/2 pneumatic
valve
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3. As electric motors normally draw larger currents, relays are ideal devices for such
circuits. By using DTDP switching, relays can control the direction of rotation of
motors.
TO SENSOR
CIRCUIT
0V
+V
The circuit below shows a motor control circuit. The motor will reverse direction
when the input switch is pressed.
+6 volts
c
b
e
e
d
o
d
1K
i
0 volts
Change the circuit so that a change in temperature will automatically change the
direction of the motor. Draw the circuit diagram before making any alterations.
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4. The partial circuit below shows a transistor switch circuit with a relay as an output
and an LDR voltage divider circuit as an input.
Relay
BC 639
Transistor
Diode
Supply
Voltage
Base
Resistor
Variable
Resistor
(Potentiometer)
LDR
0V
Build and test a complete circuit showing the relay connected to a motor.
Instructions
Draw a full circuit diagram.
Investigate from earlier work the value of the potentiometer.
Make a layout diagram for building the circuit on a prototype circuit board.
After checking, build and test your circuit.
Note: Alternatively, this circuit could be built and tested using circuit simulation
software such as Crocodile Technology.
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Integrated Circuits: 555 timer
An integrated circuit (or IC) is simply an electronic package that contains a number of
components on a silicon ‘chip’. The 555-timer IC that you are going to use is a very
versatile chip that has many applications.
As you can see, the 555 chip has eight pins. The pin functions are shown below.
1
8 +V (3 -15)
TRIGGER
2
7
OUTPUT
3
6
RESET
4
CONTROL
5 VOLTAGE
(LEAVE
UNCONNECTED)
NE555
0V
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TIMING
PERIOD
CONTROL
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555 timer  capacitors
Capacitors are electronic components that store electricity for short periods of time
within electronic circuits or networks. They are made from two metal plates or films
separated by an insulator. In many capacitors, film is used so that the layers of metal
film and insulator can be wound into a cylinder. Capacitors are especially useful in
timer circuits with the 555-timer chip.
INSULATOR
METAL PLATE
OR FILM
There are two basic types of capacitor normally used in timer circuits: electrolytic and
polyester.
Electrolytic capacitors are polarity conscious. This means that they must be
connected ‘the right way round’. The negative lead must be connected to zero volts
with the positive terminal towards the higher voltage side of the circuit.
It is very dangerous to reverse connect capacitors.
ELECTROLYTIC
AXIAL
Axial
CAPACITATOR
capacitor
RADIAL
Radial
CAPACITATOR
capacitor
Polyester capacitors are for small-value uses and can be connected without regard to
polarity.
POLYESTER
Capacitance in measured in farads, but because this is a very large measurement most
capacitors are rated in F (microfarads) or in nF (nanofarads).
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555 timer  practical tasks
1. This 555-timer circuit is used to switch an LED on for a specific time when the
chip is ‘triggered’. A typical application for this would be an egg timer.
Build the prototype circuit shown below.
+ 6 volts
1K
100K
555
1K
390R
+
0 volts
100uF
LED
Flying
lead
Instructions
 Briefly touch the bare end of the flying lead to 0 volts. The LED should light for a
fixed period.
 Adjust the variable resistor to obtain the longest fixed time for which the LED will
stay on.
 Change the capacitor to the values in the table below and record the maximum
time period for which the LED lights. Crocodile Technology or similar software
could be used for this task.
Maximum time
Capacitor value (F)
100
470
1000
2200
4700


Draw a graph to illustrate your answers.
Estimate what value of capacitor would give a time of approximately 60 seconds.
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1. The 555-timer circuit already built made an LED go on for a specific time when
the chip was triggered. The circuit diagram below shows the altered circuit with
the LED going off when the chip is triggered. Alter your circuit to show this
effect.
+ 6 volts
100K
1K
390R
1K
7
6
8
4
IC1
555
2
3
1
100F
0 volts
This circuit shows the 555 chip operating as a monostable device. This means that it is
stable in only one state, that is, it ‘jumps back’ to its initial state after a set time.
Note: As an alternative, build this new system using circuit simulation software.
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2. This 555-timer circuit is used to make an LED flash on and off at a set frequency.
Build the prototype circuit shown below.
+ 6volts
1K
555
LDR 1K
390R
+
0 volts
1uF
100uF
LED
Instructions
 On powering up the circuit, the LED should flash on and off at a steady rate
(frequency).
 Cover the LDR to see what effect this has.
 Expose the LDR to bright light and observe the effect.
 Complete a table to show your findings.
Light conditions
Dark
Normal
Bright
Frequency
This circuit shows the 555 chip operating as an astable device. This means that it is
unstable in both states; that is, it flips constantly from one state to the other.
Frequency
Frequency is the regular rate at which a physical event repeats itself. In this circuit it
is the rate at which the LED flashes. In electronic circuits the common events are the
flashing of optical devices (LEDs and lamps) and the sounding of buzzers/speakers.
These outputs are driven by an electrical pulse from the electronic system or circuit.
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