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Topic 2.1 – Timing Circuits.
Learning Objectives:
At the end of this topic you will be able to;
2.1.1 Resistor-Capacitor Network
 know that a capacitor-resistor network can be used to produce a time
delay, and explain in qualitative terms how a time delay may be
changed;
 appreciate that a time delay circuit has to be buffered to be of
practical use;
2.1.2 Monostable Circuits
appreciate that a monostable produces a single pulse when triggered;
describe a range of applications for a monostable;
recognise a 555 timer configured as a monostable;
draw the trigger circuit for a 555 monostable consisting of a pull
up resistor and a push switch / sensor;
 perform calculations using the formula T=1.1RC.




2.1.3 Astable Circuits
 Appreciate that an astable produces a continuous train of ON-OFF
pulses;
 Describe a range of applications for an astable;
 Recognise a 555 timer configured as an astable;
 Explain in qualitative terms how the frequency may be changed;
 Measure the amplitude and period of the output waveform from an
oscilloscope trace or graph;
 Calculate the frequency of an astable from an oscilloscope trace
or graph;
 Perform calculations given the formula for the frequency of an
astable.
1
GCSE Electronics.
Unit E2: Applications of Electronics
Timing Circuits
In Module E1, we used several different types of timing circuit to produce
effects such as holding an output on for a set period of time, making an
output flash on/off continuously, or delaying an output from coming on for a
short time period even though an input had been activated. All of these
applications would not be possible without the use of a timing circuit.
We will start our second module by investigating how these circuits work in
more detail. By the end of this section you should be able to design these
circuits for yourself to produce any of the functions described above.
2.1.1
Resistor – Capacitor Network
We can all think of situations in which an electronic timer controls for how
long something should happen, or when something should happen. A microwave
oven has a timer to control low long food is cooked for. A Pelican crossing has
a timer which activates a sequence of traffic lights for a predetermined time
when a pedestrian presses a switch.
These timer circuits make use of a component called a capacitor.
Capacitors
Capacitors find many applications in electronic circuits. At this stage we will
only be concerned with their use in timing circuits.
Types and symbols
There are two main types of capacitors, polarised and non-polarised.
Polarised capacitors include electrolytic capacitors.
In the practical assignment you will be using electrolytic capacitors. They are
used because they have a much larger capacity than non electrolytic
capacitors of the same physical size.
2
Topic 2.1 – Timing Circuits.
The symbols used for the two types are shown below.
+
Polarised
Capacitor
Non-Polarised
Capacitor
Considerable care is required when using electrolytic capacitors. Their
positive terminal must be nearer the positive supply rail than their negative
terminal.
WARNING: If electrolytic capacitors are connected the
wrong way around they can heat up and explode.
Identifying the leads on electrolytic capacitors
The ‘+’ and ‘-’ leads are always clearly marked on electrolytic capacitors. The
following illustrate some conventions in common use.
Axial Lead - Positive lead is
On left, next to indent on case.
Negative is shown by white
Stripe down side of case.
Radial Lead - Positive
is the longer lead or negative
marked by the white stripe
just visible on the left.
Care must also be taken not to exceed the working voltage of the capacitor.
This is clearly marked on the capacitor, 40V and 25V respectively.
3
GCSE Electronics.
Unit E2: Applications of Electronics
Non-Polarised Capacitors
The following picture shows a variety of non-polarised capacitors which can
be connected either way around in a circuit, but notice that they still have a
maximum working voltage.
Units of capacitance
The charge storage capability of a capacitor is measured in units called
Farads. The Farad (F) is a very large unit and is not normally used in
electronics. Capacitor values are usually given in micro-Farads (µF). There
are two further units called nano-Farads (nF) and pico-Farads (pF) but
these will not be used in this introductory course.
1 Farad  1,000,000F or 1F 
1
F  10 6 F
1,000,000
We will now consider how capacitors can be charged and discharged.
4
Topic 2.1 – Timing Circuits.
Charging and discharging a capacitor
We can think of a capacitor as being two metal plates separated by an
insulator. The insulator is called the dielectric.
1
+
2
+
DC Supply
-
1
+
2
+
DC Supply
-
Charging Capacitor
Discharging Capacitor
When the switch is set to position 1, the power supply draws electrons off
the top plate and transfers them onto the bottom plate. As a result, the
bottom plate carries a negative charge and the top plate a positive charge.
Transfer of charge continues until the voltage across the capacitor is equal
to the supply voltage. The capacitor is then fully charged.
The amount of charge stored, and hence the energy stored within the
capacitor, will depend upon the size of the capacitor and the supply voltage
used.
When the switch is moved to position 2 the capacitor provides a voltage
across the lamp. Electrons flow through the lamp and the capacitor will
rapidly discharge. The lamp will initially glow brightly then dim as the voltage
across the capacitor falls. After a short while the capacitor becomes fully
discharged and no voltage will be present across its terminals.
The action can be repeated by moving the switch to position 1 then back to
position 2. When used in this way the capacitor behaves like a small
rechargeable battery.
5
GCSE Electronics.
Unit E2: Applications of Electronics
Activity 1: Investigating Capacitor Charge / Discharge
CAUTION:
Be very careful to observe polarity when using electrolytic capacitors.
The positive terminal must be connected to the positive side of the
supply. Failure to do this may result in the capacitor heating up and
exploding.
The voltage applied across it must not exceed the working voltage.
The working voltage of the capacitor is shown on its body.
In this activity you will be investigating the charging, and discharging of a
capacitor. These circuits can be set up using real components if you have
them available, however in the notes we will assume the use of a circuit
simulator, like ‘livewire’, or ‘circuit wizard’. The advantage of using a simulator
is that changes can be made to the circuit very quickly, and there is less
likelihood of a problem with bad connections, so the circuits should work first
time.
1a.
Open the simulation package called “Livewire” from your desktop.
1b.
Set up the following circuit. Lamps are 6V, 100mA.
6
Topic 2.1 – Timing Circuits.
1c.
Run the simulation and change the switch over to position 2. Describe
and explain what you observe.
......................................................................................................................................
......................................................................................................................................
......................................................................................................................................
1d.
Change the switch over to position 1. Describe and explain what you
observe.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
1e.
Change the switch over to position 1. Describe and explain what you
observe.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Save your circuit as “E2-Topic2.1-Act1” in your work area.
7
GCSE Electronics.
Unit E2: Applications of Electronics
Using capacitors as timing elements
a)
Charging capacitor
When a capacitor is charged directly from a voltage supply it very quickly
becomes fully charged. We can slow down the charging process by including a
series resistor in the circuit.
Consider the following circuit.
VR
+
D.C.
Supply
+
VS
VC
The capacitor will always charge up in a predictable way in a fixed length of
time until it approximately reaches the power supply voltage VS. The way the
capacitor charges up is shown in the graph below. The time taken is
dependent on both the value of capacitor and value of resistor used.
8
Topic 2.1 – Timing Circuits.
b)
Discharging capacitor
The discharging process can also be slowed down by discharging the capacitor
through a large resistance.
+
C
R
Practical applications
The voltage across the capacitor cannot be used to drive a load directly
since any current drawn from the timing unit would alter the charging, or
discharging rate.
A switching sub-system is required between the timing unit and the load. It
must draw very little current from the timing unit and supply sufficient
current to drive a load.
Timing
Switching
Unit
Load
The switching sub-system acts as a matching unit between the input device
and the load. You should remember this switching circuit from the final
chapter of the E1 module you have just completed.
9
GCSE Electronics.
Unit E2: Applications of Electronics
In Activities 2 and 3 below you will see how the predictable way in which
capacitor charge and discharge can be used to our advantage.
Investigating resistor-capacitor networks
Activity 2: Investigating the charging of a capacitor through a resistor
2a.
Set up the following circuit with the supply voltage set to 6 volts.
2b.
Connect a digital voltmeter across the output terminals to monitor the
output voltage.
2c.
Open the switch 'SW1' to allow the capacitor to charge up. Comment on
the speed at which the voltage changes when the switch is first opened
compared to the speed later.
....................................................................................................................................
....................................................................................................................................
2d.
Repeat the procedure and measure the time taken for the capacitor to
charge up to 3 volts.
Time taken:
............................
Save your circuit as “E2-Topic2.1-Act2a” in your work area.
10
Topic 2.1 – Timing Circuits.
2e.
Replace the 10kΩ resistor with a 20kΩ resistor. Measure the time
taken for the voltage across the capacitor to change from 0 V to 3 volt.
Time Taken: ....................................
Comment .................................................................................................................
......................................................................................................................................
......................................................................................................................................
Save your circuit as “E2-Topic2.1-Act2e” in your work area.
2f.
Replace the 1000µF capacitor with a 2200µF capacitor. Measure the
time taken for the capacitor to charge up from 0V to +3 volt.
Time Taken: .....................................
Save your circuit as “E2-Topic2.1-Act2f” in your work area.
2g.
Look at your results for the time taken in 2d, 2e and 2f to help you
complete the following statements:
The larger the value of capacitor used in a resistor-capacitor network
the ……………… the time taken for the capacitor to charge.
The larger the value of resistor used in a resistor-capacitor network
the ……………… the time taken for the capacitor to charge.
11
GCSE Electronics.
Unit E2: Applications of Electronics
Activity 3: Investigating the discharging of a capacitor through a resistor
3a.
Set up the following circuit with the supply voltage set to 7 volts.
3b.
Close the switch ‘SW1’ to charge up the capacitor.
3c.
Open the switch ‘SW1’ and comment on the speed at which the voltage
changes when the switch is first opened compared to the speed later.
....................................................................................................................................
....................................................................................................................................
3d.
Repeat the procedure but this time measure the time taken for the
capacitor to discharge to 3V.
Time Taken: ........................................
3e.
Replace the 10kΩ resistor with a 20kΩ resistor. Measure the time
taken for the capacitor to discharge to 3V.
Time Taken: ....................................
Save your circuit as “E2-Topic2.1-Act3a” in your work area.
12
Topic 2.1 – Timing Circuits.
3f.
Replace the 1000µF capacitor with a 2200µF capacitor. Measure the
time taken for the capacitor to discharge to 3V.
Time Taken: .....................................
3g.
Look at your results for the time taken in 3d, 3e and 3f to help you
complete the following statements:
The larger the value of capacitor used in a resistor-capacitor network
the ……………… the time taken for the capacitor to discharge.
The larger the value of resistor used in a resistor-capacitor network
the ……………… the time taken for the capacitor to discharge.
Save your circuit as “E2-Topic2.1-Act3f” in your work area.
Activity 4: Investigating a simple time delay circuit
4a.
Set up the following circuit:
13
GCSE Electronics.
Unit E2: Applications of Electronics
4b.
Momentarily close the switch ‘SW1’ and comment on what you observe.
Comment ……………………………………………………………………………………………………
LED stays on for…………………seconds
4c.
Replace the 2200µF capacitor with a 1000µF one and repeat the above
procedure.
Comment ……………………………………………………………………………………………………
LED stays on for…………………seconds
Compare this with the time you obtained in 4b.
……………………………………………………………………………………………………………………………
4d.
Replace the LED with a 6 volt lamp and repeat the above procedure.
Comment ……………………………………………………………………………………………………
4e.
Replace the lamp with a buzzer and repeat one more time.
Comment ……………………………………………………………………………………………………
14
Topic 2.1 – Timing Circuits.
Practical timer circuits
The simple timer circuit investigated in Activity 4 is satisfactory for
demonstrating the idea of a timer circuit, but it is of little use for practical
circuits since it has three main limitations:
i) the output changes gradually as the capacitor gradually discharges from
9V to 0V, resulting in a poorly defined timing period.
ii) the circuit can only supply a very small current which is barely sufficient
to drive a LED.
iii) the timing circuit has to supply the current to drive the load which
affects the predictability of the timing.
We require a timer that produces a single square wave pulse as shown in the
graph below. Its output will usually start off in a LOW state. When its input
is triggered the output will then go HIGH for a predetermined length of time
before returning to its low state. The output will then remain indefinitely in
its low state until triggered again.
Output Voltage
Predetermined time
time
Trigger applied
15
GCSE Electronics.
Unit E2: Applications of Electronics
The signal produced by the resistor-capacitor network has to be processed to
overcome the limitations stated above.
Timing
Processing Unit
Load
A processing sub-system is required between the timing unit and the load. It
must draw very little current from the timing unit and supply sufficient
current to drive a load. Such a sub-system is often referred to as a buffer.
A buffer is in fact any sub-system connected between two other sub-systems
in order to strengthen a signal.
16
Topic 2.1 – Timing Circuits.
Activity 5:
Investigating a buffered time delay circuit
5a.
Set up the following circuit:
5b.
Two inverters (NOT gates) have been added to the simple time delay
circuit investigated in activity 4 to act as a buffer.
Why are two inverters needed?
....................................................................................................................................
5c.
Momentarily close the switch.
(i)
Record the time the LED stays on after the switch is released
Time LED is on
(ii)
…………………
Compare the way in which the LED switches off with what you
observed previously in Activity 4b.
....................................................................................................................................
5d.
Replace the LED with a 6V, 60mA lamp and comment on what you
observe.
Can you give a reason for this …………………………………………………………………
17
GCSE Electronics.
Unit E2: Applications of Electronics
An improved a buffered time delay circuit
You probably realised that that although the buffered time delay
investigated in activity 5 gave a much sharper switching action it still
suffered from the problem of being unable to provide sufficient current to
light the lamp. We can add a transistor to overcome this.
If you have time, try setting up and testing this circuit.
18
Topic 2.1 – Timing Circuits.
Summary
The timer circuits considered so far have several limitations. Many electronic
systems require a more sophisticated timer to control their operation.
The popular 555 timer IC overcomes the limitations of the simple timers and
is very versatile as it can be used to produce two different types of timed
output:
1.
A single pulse of a fixed period of time. This type of circuit is called a
monostable.
2.
A continuous train of on/off pulses. This type of circuit is called a
square wave oscillator or astable.
19
GCSE Electronics.
Unit E2: Applications of Electronics
Homework Questions 1
Answer all questions in the spaces provided, and continue on a separate piece of paper if
required.
1.
There are two main types of capacitor. These are polarised and non-polarised.
Complete the following sentences so that they are correct.
a)
b)
c)
Capacitors which can be connected any way round are called _____________
capacitors.
[1]
_____________________ capacitors must be connected the correct way
around in a circuit to prevent them from being damaged.
[1]
Draw the correct symbols for the following types of capacitor.
i)
Polarised Capacitor
ii)
Non Polarised Capacitor
[2]
2.
Why must we take great care in connecting up electrolytic capacitors?
..............................................................................................................................................................
..............................................................................................................................................................
..............................................................................................................................................................
..............................................................................................................................................................
[2]
20
Topic 2.1 – Timing Circuits.
3.
In the following circuit:
What will be the value of Vc :
i) at the instant switch SW1 is closed;
……………………………………………
ii) after a long period of time?
……………………………………………
4.
In the following circuit:
What will be the value of Vc :
i) at the instant switch SW1 is closed;
……………………………………………
ii) after a long period of time?
……………………………………………
5.
A 100µF capacitor is connected in series with a 100kΩ resistor across a power
supply. The capacitor takes 16s to charge up to 5V.
a)
The value of the resistor is kept at 100kΩ and the value of capacitor changed
to 200µF. Roughly how long will it take now for the capacitor to charge to 5V?
…………………………
b)
The value of the capacitor is kept at 200µF and the value of resistor is
changed to 200kΩ. Roughly how long will it take now for the capacitor to
charge to 5V?
…………………………
21
GCSE Electronics.
Unit E2: Applications of Electronics
2.1.2
Monostable Circuits
In this application of timer circuits we are trying to produce a single pulse of
fixed duration when the circuit is provided with a trigger pulse, as illustrated
in the following diagram.
TRIGGER
VOLTAGE
Time
OUTPUT
VOLTAGE
Time
T
Monostable timer
A monostable has only one stable output state. Normally it is in this stable
state (output 0V) but can be triggered into the other state (output
approximately equal to supply voltage) where it stays for a predetermined
time. This time is determined by two external components, a resistor and a
capacitor. Both a block diagram and a circuit diagram for a 555 monostable
are shown below.
Input
Trigger
Control
555
Output
Timer
V cc
10k
R
555
4
8
2
3
6
C
7
1
S
0V
TRIGGER
22
TIMER
CONTROL
OUTPUT
Topic 2.1 – Timing Circuits.
When switch S is pressed momentarily, the monostable is triggered by the
falling edge of the trigger pulse produced. The output of the 555 timer, (Pin
3), goes high and remains high for a time given by the formula:
T  1.1 R  C
Where: T is in seconds, if R is in Ohms, and C is in Farads.
In practice the capacitor value will usually be in F and the resistor values in
either k or M which allows us to use one of the following rules
 If R is in k, and C is in F then T will be in ms (milliseconds)
 If R is in M, and C is in F then T will be in seconds
Examples: For each of the following values of R and C in a 555 monostable
circuit, calculate for how long the output remains high after the
circuit has been triggered:
a)
R = 10 k and C = 220F
T  1.1 R  C  1.1 10  220  2420ms  2.420s
b)
R = 1.2M and C = 47F
T  1.1 R  C  1.1 1.2  47  62.04s
23
GCSE Electronics.
Unit E2: Applications of Electronics
Exercise 1
1.
For each of the following values of R and C in a 555 monostable circuit,
calculate how long the output remains high after the circuit has been
triggered:
i.
R = 10kΩ, C = 2200µF
.............................................................................................................................
ii.
R = 8.2MΩ, C = 330µF
.............................................................................................................................
iii.
R = 1kΩ, C = 470µF
.............................................................................................................................
iv.
R = 2.2kΩ, C = 100µF
.............................................................................................................................
v.
R = 3.9kΩ, C = 680µF
.............................................................................................................................
vi.
R = 100kΩ, C = 33µF
.............................................................................................................................
vii.
R = 1MΩ, C = 150µF
.............................................................................................................................
24
Topic 2.1 – Timing Circuits.
You will now investigate this application of the 555 timer IC, either using real
components or using a circuit simulator.
Activity 6:
6a.
Set up the 555 monostable circuit as shown below.
6b.
Run the simulation and operate the switch SW1. Describe what you
observe.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Save your circuit as “E2-Topic2.1-Act6a” in your work area.
25
GCSE Electronics.
Unit E2: Applications of Electronics
6c.
The theoretical duration of the output pulse is given by:
T  1.1 R2  C1
Calculate its value from the value of the components, and then measure
its value using a stop watch.
Theoretical duration =
.........................................................................
Measured value
...........................
=
Suggest a practical application for this circuit.
.......................................................................................................................................
6d.
Investigate whether the pulse duration depends upon the supply voltage
by changing the supply voltage to 9V and then to 12V.
Comment on your findings ...................................................................................
.......................................................................................................................................
.......................................................................................................................................
6e.
Connect the lamp between the output pin and the positive supply rail and
momentarily take the trigger output low by flicking the switch on.
Compare the result with that obtained in 6b above.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
Save your circuit as “E2-Topic2.1-Act6e” in your work area.
26
Topic 2.1 – Timing Circuits.
Varying the time delay
In the previous activity, you will probably have found that the actual and
theoretical times were within one or two seconds of each other, which is
usually acceptable. For some applications, we need very accurate timings, and
for other applications we need adjustable timings.
The 1MΩ fixed resistor could be replaced with a 1MΩ variable resistor in
series with a 1kΩ fixed resistor. The fixed resistor is required to limit the
current flowing into pin 7 when the variable resistor is set to zero.
In fact a wide range of ‘timings’ may be obtained by using a 1kΩ fixed
resistor in series with a 1MΩ variable resistor and one of the capacitor
values, as shown in the table below.
Required timing period
11 ms to 11 seconds
52 ms to 52 seconds
0.11 to 110 seconds
0.52 to 520 seconds
1.1 to 1100 seconds
Capacitor
10µF
47µF
100µF
470µF
1000µF
27
GCSE Electronics.
Unit E2: Applications of Electronics
Design Problem
Design and test an electronic egg timer which should produce an audible alarm
after a variable time delay of between 1 - 5 minutes. Ask your teacher to
check your design before testing it. Draw a diagram of your circuit solution in
the space below.
a)
Set up the circuit and measure the shortest and longest time produced.
Shortest Time ..................................
b)
Longest Time ...................................
Modify your design as shown below so that the ‘timed delay’ starts as
soon as you switch on the power.
+6V
10k
2
555
1uF
0V
28
Topic 2.1 – Timing Circuits.
Sometimes we are required to design a monostable to a specification, rather
than have to analyse a given circuit with values in.
For example: Design a monostable to keep an outside light on for a period of
approximately 90 seconds.
The formula for a monostable delay is T  1.1 R  C so in this case this
becomes:
90  1.1 R C
There are two unknowns in this equation, so as it stands it cannot be solved.
We have to ‘guess’ one of the values for R or C, and then work out what the
corresponding value required is to make the formula correct. It is usually
easier to find a resistor of different values than a capacitor because there
are more of them and variable resistors are very common. Finding a variable
capacitor is not so easy (and they are very expensive!).
So for our problem we will try a capacitor value of 100µF. This gives us the
following:
90  1.1 R  C
90  1.1 R  100
90  110  R
90
R
 0.818 M
110
R  818k  820k
Had we chosen C to be 1000µF, then R would have come out to be 82kΩ.
In design problems like these, there is no single correct answer. However we
always try to ensure that R is greater than 1kΩ, to minimise the current
flowing in the timing circuit. In an examination you will be given either the
value of R or C, and will be asked to calculate the missing one.
29
GCSE Electronics.
Unit E2: Applications of Electronics
2.1.3
Astable Circuits
An astable has no stable output state. The output will continually switch
between 0V (‘low’) and a voltage just below the supply voltage, (‘high’)
producing a ‘square’ wave output.
The astable is sometimes called a pulse generator as we did in Unit E1.
The following diagram shows a typical output waveform.
V
Mark
Space
Time
T1
T2
The time when the output on is referred to as the ‘Mark’ and the off time is
usually referred to as the ‘Space’. The frequency of the output pulse can be
calculated using the formula:
f 
1
(T 1  T 2)
Astable circuit based on a 555 timer
The circuit diagram for a 555
astable is shown opposite.
V cc
R1
555
The mark and space timings, and
therefore the frequency of the
output is determined by three
external components, a
capacitor, and two resistors.
4
R2
3
6
2
1
C
0V
TIMER
30
8
7
CONTROL
OUTPUT
Topic 2.1 – Timing Circuits.
The theoretical value for the Mark, Space and Frequency are given by the
following equations.
Mark T 1  0.7  ( R1  R2 )  C
Space T 2  0.7  R2  C
Period  T 1  T 2
Frequency F 
1
1

Period T 1  T 2
We can use similar rules as for the monostable:
 If R1 & R2 are in k, and C is in F then T1 and T2 will be in ms
(milliseconds)
 If R1 & R2 are in M, and C is in F then T1 and T2 will be in seconds
 If the Period is in seconds, the frequency will be in Hertz (Hz)
 If the Period is in ms (milliseconds), the frequency will be in kilohertz
(kHz)
Note:
The mark and space timings of the output waveform can be altered by varying
R1 or R2. As the timing equation shows, the mark will always be greater than
the space. An approximately square waveform may be obtained (i.e. mark =
space) if R2 is much larger than R1.
We will now look at a worked example for you to see how these calculations
are carried out.
31
GCSE Electronics.
Unit E2: Applications of Electronics
Example:
An astable using a 555 timer has the following timing elements:
R1 = 22kΩ
Calculate: i)
ii)
iii)
iv)
R2 = 82kΩ
C = 470µF
the mark time,
the space time,
the period,
the frequency of the astable
Solution:
Mark Time T 1  0.7  ( R1  R2 )  C
 0.7  (22  82)  470ms
 34216ms
 34.216 s  34 s
Space Time T 2  0.7  R2  C
 0.7  82  470ms
 26978ms
 26.978 s  27 s
Period  T 1  T 2
 34  27
 61s
and
32
1
T1 T 2
1

34  27
1

61
 0.016 Hz
frequency F 
Topic 2.1 – Timing Circuits.
Activity 7: Investigating an Astable circuit
7a.
Set up the following circuit with the resistors R1 and R2 both equal to
10kΩ and C = 100µF.
7b.
Switch on the power and describe what you observe.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
The ‘ON’ time of the lamp (which should be just over 1 second),
corresponds to the ‘MARK’ of the output waveform and the ‘OFF’ time
(which should be just less than a second), corresponds to the ‘SPACE’ as
shown in the diagram below.
Mark
T1
Space
T2
33
GCSE Electronics.
Unit E2: Applications of Electronics
For the circuit in Activity 7a, R1 =10kΩ, R2 = 10kΩ and C = 100µF, therefore
T 1  0.7  ( R1  R2 )  C
 0.7  (10  10)  100ms
 1400ms
 1 .4 s
T 2  0.7  R2  C



and
frequency F 
1
T1 T 2



7c.
The values of T1 and T2 above are rather small to be measured
accurately with a stop watch and too large to be measured on a C.R.O.
In each of the following cases calculate the values of T1, T2 and F, and
compare them with the measured values.
34
Topic 2.1 – Timing Circuits.
i)
R1 = R2 = 10kΩ, C = 1000µF
Calculated values
Measured values
T1
T2
F
ii)
R1 = 1kΩ, R2 = 100kΩ, C = 100µF
Calculated values
Measured values
T1
T2
F
35
GCSE Electronics.
Unit E2: Applications of Electronics
In this example the mark and the space are approximately the same because
R2 is much greater than R1, (R2 >> R1). In examples like this the approximate
frequency is given by the formula.
F
0. 7
R 2C
where R2 is in MΩ, and C is in µF.
You will only use this formula in an exam if you are asked to.
For the last example given in 7c (ii) this approximate formula would give the
following result:
F
0 .7
R2 C
0 .7
0.1 100
 0.07 Hz

Compare this approximate result with that obtained from your calculated
value and the measured value in 7c (ii).
Theoretical Value = ..................
Measured Value = .....................
Approximate Formula Value = .........................
Comment: ............................................................................................................................
.................................................................................................................................................
.................................................................................................................................................
.................................................................................................................................................
36
Topic 2.1 – Timing Circuits.
7d.
Sometimes pulse generating circuits need to operate at high speed,
which means the pulse duration cannot be measured accurately on a
stopwatch. In this case we need to use another instrument called an
oscilloscope (CRO). Modify your circuit to the following:
Save your circuit as “E2-Topic2.1-Act7c” in your work area.
37
GCSE Electronics.
Unit E2: Applications of Electronics
In the examination you may be asked to calculate the period and amplitude of
the output of an astable (or pulse generator) from the graph of an
oscilloscope. Here are a couple of examples to show you how this can be
carried out.
Oscilloscope Settings
Time Base = 2ms / cm
Voltage Gain = 5V / cm
The squares on an oscilloscope screen are 1cm x 1cm.
The amplitude is the vertical distance from the lowest to highest point, which
in this case is 2cm. The voltage gain is set to 5V / cm making the amplitude of
this astable output 2 x 5 =10V.
The frequency is calculated from the calculating the sum of the ‘on’ time and
‘off’ time, i.e. the time for one complete cycle.
In this example the ‘on’ time is 2cm = 2 x 2 = 4ms, and the ‘off’ time is 1cm = 1
x 2 = 2ms, giving the complete cycle time to be 6ms.
The frequency can then be calculated from F 
Now here are some for you to try.
38
1
1

 166.6 Hz
T 6ms
Topic 2.1 – Timing Circuits.
Exercise 2
1.
Oscilloscope Settings
Time Base = 5ms / cm
Voltage Gain = 2V / cm
i.
Amplitude = ……………………………………………………………………
ii.
Mark time = ……………………………………………………………………
iii.
Space time = …………………………………………………………………
iv.
Frequency = ……………………………………………………………………
39
GCSE Electronics.
Unit E2: Applications of Electronics
2.
Oscilloscope Settings
Time Base = 1ms / cm
Voltage Gain = 5V / cm
40
i.
Amplitude = ……………………………………………………………………
ii.
Mark time = ……………………………………………………………………
iii.
Space time = …………………………………………………………………
iv.
Frequency = ……………………………………………………………………
Topic 2.1 – Timing Circuits.
3.
Oscilloscope Settings
Time Base = 10ms / cm
Voltage Gain = 1V / cm
i.
Amplitude = ……………………………………………………………………
ii.
Mark time = ……………………………………………………………………
iii.
Space time = …………………………………………………………………
iv.
Frequency = ……………………………………………………………………
41
GCSE Electronics.
Unit E2: Applications of Electronics
4.
On the oscilloscope screen below, draw the waveform you would expect
to see if an astable was operating with the following specification.
Amplitude = 6V
Mark time = 10ms
Space time = 5ms
Oscilloscope Settings
Time Base = 5ms / cm
Voltage Gain = 2V / cm
5.
On the oscilloscope screen below, draw the waveform you would expect
to see if a pulse generator was working with the following specification.
Amplitude = 9V
Mark time = 4ms
Space time = 8ms
Oscilloscope Settings
Time Base = 2ms / cm
Voltage Gain = 5V / cm
42
Topic 2.1 – Timing Circuits.
Homework Questions 2
Answer all questions in the spaces provided. Continue on a separate piece of paper if
required.
1.
Explain what you understand by the term Monostable Timer.
..............................................................................................................................................................
..............................................................................................................................................................
..............................................................................................................................................................
[1]
2.
The following circuit diagram shows a 555 timer connected as a monostable.
Vcc
1k
10k
4
2
8
555
6
470uF
i)
7
3
1
Output
0V
Calculate the theoretical time duration of the output pulse for the circuit.
(T=1.1RC)
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
[1]
43
GCSE Electronics.
Unit E2: Applications of Electronics
ii)
The circuit needs to be modified to provide a pulse delay of approximately 2
minutes. If you have access to the following capacitors and resistors, design a
circuit to meet this requirement. {470µF, 1000µF, 2200µF, 22k, 120k, 270k,
390k}
.................................................................................................................................................
.................................................................................................................................................
.................................................................................................................................................
3.
.................................................................................................................................................
[2]
Explain what you understand by the term Astable circuit.
.............................................................................................................................................................
.............................................................................................................................................................
..............................................................................................................................................................
[1]
4.
The following circuit shows a 555 timer connected as an Astable circuit.
Vcc
10k
4
7
100k
555
6
33uF
44
8
2
3
1
Output
0V
Topic 2.1 – Timing Circuits.
The following graph shows the output observed at Pin 3, when viewed on an
oscilloscope.
a)
What type of wave is this? ..........................................................................................
b)
i)
ii)
c)
On the trace above label the ‘Mark’ part of the wave.
On the trace above label the ‘Space’ part of the wave.
The theoretical value of the ‘Mark’ time is given by
[1]
[1]
[1]
Mark Time  0.7  R1  R 2   C
Calculate the ‘Mark’ Time for the above circuit.
..................................................................................................................................................
d)
..................................................................................................................................................
[2]
The theoretical value of the ‘Space’ time is given by
Mark Time  0.7  R 2   C
Calculate the ‘Space’ Time for the above circuit.
..................................................................................................................................................
e)
..................................................................................................................................................
[2]
Finally calculate the frequency of the output wave.
...............................................................................................................................................
...............................................................................................................................................
[2]
45
GCSE Electronics.
Unit E2: Applications of Electronics
5.
Oscilloscope Settings
Time Base = 20ms / cm
Voltage Gain = 1V / cm
46
i.
Amplitude = ……………………………………………………………………
ii.
Mark time = ……………………………………………………………………
iii.
Space time = …………………………………………………………………
iv.
Frequency = ……………………………………………………………………
[5]
Topic 2.1 – Timing Circuits.
Solutions to Exercises:
Exercise 1
1.
For each of the following values of R and C in a 555 monostable circuit,
calculate for how long the output remains high after the circuit has
been triggered:
i. T  1.1 R  C  1.1 10  2200  24200ms  242s
ii. T  1.1 R  C  1.1 8.2  330  2976s
iii. T  1.1 R  C  1.1 1 470  517ms
iv. T  1.1 R  C  1.1 2.2  100  242ms
v. T  1.1 R  C  1.1 3.9  680  2917ms  2.917s
vi. T  1.1 R  C  1.1 100  33  3630ms  3.630s
vii. T  1.1 R  C  1.1 1 150  165s
Exercise 2
1.
i.
Amplitude = 2 x 2V = 4 V
ii.
Mark time = 3 x 5ms = 15 ms
iii.
Space time = 1 x 5ms = 5 ms
iv.
Frequency =
1
1
1


 50 Hz
T 15 ms  5 ms 20ms
47
GCSE Electronics.
Unit E2: Applications of Electronics
2.
3.
4.
48
i.
Amplitude = 3 x 5V = 15 V
ii.
Mark time = 4 x 1ms = 4 ms
iii.
Space time = 1 x 1ms = 1 ms
iv.
Frequency =
i.
Amplitude = 4 x 1V = 4 V
ii.
Mark time = 3 x 10ms = 30 ms
iii.
Space time = 2 x 10ms = 20 ms
iv.
Frequency =
1
1
1


 200 Hz
T 4 ms  1ms 5 ms
1
1
1


 20 Hz
T 30ms  20ms 50ms
Topic 2.1 – Timing Circuits.
5.
49
GCSE Electronics.
Unit E2: Applications of Electronics
Solutions to Homework Exercises
Homework Questions 1
1.
a)
Non Polarised.
b)
Polarised
c)
i)
Polarised Capacitor
ii)
Non Polarised Capacitor
2.
Great care must be taken connecting up Polarised capacitors since if connected the
wrong way round there is a danger that they may explode.
3.
i)
0V
ii)
9V
4.
i)
9V
ii)
0V
5.
a)
32s
b)
64s
50
Topic 2.1 – Timing Circuits.
Homework Questions 2
1.
A monostable timer has only one stable state. It may visit another state briefly but it will always
return to its original state by itself.
2.
i)
T  1.1 10  470ms
 5170ms
 5.17s
ii)
This problem gives you an example of the typical designers nightmare. You have to
provide a delay of approximately 2 minutes, but only a few components are available. Due
to the fact that the delay must only be approximate, this will help greatly, in the design.
You basically have to choose a resistor and a capacitor to give a delay of approximately
120 seconds (2 mins). Looking at your time delay in (i), this should indicate that larger
values of R and C are needed. The solution to the problem therefore becomes a little bit of
trial and error. By picking a value of C, working out R and seeing if you have this value of
resistor available, as shown below:
Assume C = 2200µF since we know it must be bigger than the 470µF used before.
120s  1.1  R  2200
120
R
M
1.1  2200
 0.049586M
 49.586k
No resistors are available around this value, so we must try a different value of capacitor,
let us try C=1000µF
120s  1.1  R  1000
120
R
M
1.1  1000
 0.109090M
 109.090k
The closest resistor we have available to this would be 120k. If this value was used then
the delay would be approximately 132 seconds (Re-calculate if you do not agree) which
would be acceptable as an answer. Another possible solution is with R = 270k and C =
470µF. What time delay would this give?
3.
An Astable timer has no stable state and the output continuously changes from one state to the
other without any external input.
51
GCSE Electronics.
Unit E2: Applications of Electronics
4.
a)
Square Wave.
b(i) Mark
b(ii) Space
c)
Mark Time  0.7  R 1  R 2   C
 0.7  10  100  33ms
 2541ms
 2.541s
d)
Space Time  0.7  R 2   C
 0.7  100  33ms
 2310ms
 2.31s
e)
Frequency 
5.
1
1
1


 0.206 Hz
T 2.541  2.31 4.851
i.
Amplitude = 2.4 x 1V = 2.4 V
ii.
Mark time = 2 x 20ms = 40 ms
iii.
Space time = 2 x 20ms = 40 ms
iv.
Frequency =
1
1
1


 12.5 Hz
T 40ms  40ms 80ms
Now for some Examination Style Questions
52
Topic 2.1 – Timing Circuits.
Examination Style Questions
1.
The circuit diagram below shows a simple timer connected to a LED indicator.
(a)
Describe what happens when the switch S is momentarily pressed and released.
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
[1]
(b)
Give one disadvantage of this simple timer.
..................................................................................................................................................
..................................................................................................................................................
[1]
(c)
The period of a 555 timer monostable is given by the formula
T = 1.1 RC
where R is in ohms and C is in farads.
Calculate the period of the monostable when R = 100kΩ and C = 100µF.
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
[2]
53
GCSE Electronics.
Unit E2: Applications of Electronics
2.
A design for a car alarm system is shown below.
The system provides a square wave output repeatedly for a period of 66 seconds after the input
sensor is activated.
(a)
The period of the monostable is given by the formula T= 1.1RC, where R is in ohms, and
C is in farads.
Determine a suitable value for a resistor to be used with a 1000µF capacitor to give a
period of 66 seconds.
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
[3]
54
Topic 2.1 – Timing Circuits.
(b)
The output of the astable was displayed on an oscilloscope to check its frequency.
Calculate the period and frequency of the astable if the oscilloscope timebase was set to
1ms per division.
Period:
..................................................................................................................................................
..................................................................................................................................................
[1]
Frequency:
..................................................................................................................................................
..................................................................................................................................................
[2]
55
GCSE Electronics.
Unit E2: Applications of Electronics
3.
A pupil has designed a thermometer for a blind person.
When a thermistor is placed in water of different temperatures a different note can be heard on the
loudspeaker.
The circuit diagram is shown below.
(a)
In what way is the 555 timer being used ? Choose your answer from the following list.
ASTABLE
MONOSTABLE
LATCH
BISTABLE
..................................................................................................................................................
[1]
(b)
Which components, other than the thermistor, affect the frequency of the note heard on the
loudspeaker ?
..................................................................................................................................................
..................................................................................................................................................
[2]
56
Topic 2.1 – Timing Circuits.
4.
(a)
Study the following diagram.
(i)
What type of capacitor is shown in the diagram ?
..................................................................................................................................................
(ii)
What happens to the switch is moved to position ?
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
(iii)
Describe what you see when the switch is moved to position ?
..................................................................................................................................................
..................................................................................................................................................
(b)
..................................................................................................................................................
[4]
The capacitor is replaced with a larger value capacitor.
What effect would this have?
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
[1]
57
GCSE Electronics.
Unit E2: Applications of Electronics
5.
Monostable and astable circuits are common forms of timing subsystems.
(a)
Which one of the following A, B, C, or D is the output signal produced by an astable
circuit?
Answer: ....................................
[1]
58
Topic 2.1 – Timing Circuits.
(b)
Here is the circuit diagram for a monostable, using a 555 timer.
Use the formula:
T=1.1RC
To calculate the delay period, T, in seconds.
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
[2]
59
GCSE Electronics.
Unit E2: Applications of Electronics
6.
(a)
Which of the following is another name for an astable circuit ?
A
Inverter
B
Time Delay
C
Pulse Generator
D
Latch
Answer: ................................
[1]
(b)
The output of an astable circuit is connected to an oscilloscope.
The following trace is produced.
The oscilloscope controls are set as follows:
Sensitivity (voltage gain) = 100mV / cm
Time base (speed) = 5 ms / cm
Use the trace to find:
(i)
the amplitude of the signal;
..................................................................................................................................................
[1]
(ii)
the period of the output signal.
(c)
..................................................................................................................................................
[1]
Give one use of an astable circuit.
..................................................................................................................................................
[1]
60
Topic 2.1 – Timing Circuits.
7.
The diagram below shows a 555 timer astable circuit.
(a)
Sketch the shape of the output signal produced by an astable.
[1]
(b)
The approximate value of frequency f of the astable is given by the formula:
f 
0.7
R2 C
Where R2 is in ohms and C is in farads.
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
[3]
61
GCSE Electronics.
Unit E2: Applications of Electronics
8.
(a)
Here is the signal produced by an astable circuit.
(i)
What is the amplitude of the signal ?
..........................
(ii)
What is the period of the signal ?
..........................
(iii)
Calculate the frequency of the signal.
..................................................................................................................................................
(b)
62
..................................................................................................................................................
[3]
The circuit diagram shows a monostable circuit using a 555 timer.
Topic 2.1 – Timing Circuits.
(i)
Using the axes provided to sketch the output signal produced by a 10 second
monostable circuit, which is triggered at the time shown.
[2]
(ii)
The monostable time delay can be found from the formula:
T=1.1 RC (where T is in seconds, R is in MΩ, and C is in µF)
Here are four resistor/capacitor sets.
Set
A
B
C
D
Resistor
47kΩ
82kΩ
47kΩ
82kΩ
Capacitor
100µF
100µF
220µF
220µF
Which one will produce a time delay nearest to 10 seconds?
Show how you obtained your answer.
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
The answer is set ..........................
[2]
63
GCSE Electronics.
Unit E2: Applications of Electronics
9.
Part of the block diagram for an electronic game is shown below.
(a)
The first graph shows the output signal produced by system X when the switch is pressed
and released. After 3s, the signal remains at 0V until the switch is pressed again.
Name subsystem X.
Choose your answer from the following list
LATCH
MONOSTABLE
COUNTER
ASTABLE
Answer: ......................................................
[1]
(b)
The second graph shows the output signal produced by subsystem Y.
Name subsystem Y.
Choose your answer from the following list
LATCH
MONOSTABLE
COUNTER
ASTABLE
Answer: ......................................................
[1]
64
Topic 2.1 – Timing Circuits.
(c)
Which one of the following graphs, A, B, C, or D shows the output signal Q of the AND
gate ?
Answer: ....................................................................
[1]
65
GCSE Electronics.
Unit E2: Applications of Electronics
10.
An alarm system contains a number of timing subsystems.
(a)
A switch unit is used to trigger timing system Y.
The graph shows the output signal produced when the switch is pressed and released.
Name subsystem Y.
Choose your answer from the following list
LATCH
MONOSTABLE
COUNTER
ASTABLE
Answer: ......................................................
[1]
(b)
The graph shows the output signal produced by another timing subsystem, Z.
Name subsystem Z.
Choose your answer from the following list
LATCH
MONOSTABLE
COUNTER
ASTABLE
Answer: ......................................................
[1]
66
Topic 2.1 – Timing Circuits.
(c)
Both Y and Z make use of the following RC Circuit:
To begin with, the output is 0V. The switch is then closed. What is the output voltage
(i)
at the instant that the switch is closed;
..............
(ii)
when the switch has been closed for a very long time.
..............
(iii)
[2]
The capacitor has a value of 100µF.
Which of the following resistors would give the RC circuit the biggest time delay?
1kΩ
10kΩ
100kΩ
1MΩ
Answer: .........................................
[1]
(d)
Here are four signals, labelled P, Q, R and S.
Which one, P, Q, R or S has amplitude of 5V, and a period of 2s ?
Answer: .....................................
[1]
67
GCSE Electronics.
Unit E2: Applications of Electronics
11.
A pulse generator is made from a 555 timer circuit, shown below.
The approximate value of the frequency f, in Hz, of the pulse generator is given by the formula:
f 
0.7
RB C
Where RB is in ohms, and C is in farads.
Use this formula to calculate the frequency of the pulse generator.
..................................................................................................................................................
..................................................................................................................................................
..................................................................................................................................................
[2]
68
Topic 2.1 – Timing Circuits.
Self Evaluation Review
Learning Objectives
My personal review of these objectives:



2.1.1 Resistor-Capacitor Network.
know that a capacitor-resistor network can be used
to produce a time delay, and explain in qualitative
terms how a time delay may be changed;
appreciate that a time delay circuit has to be
buffered to be of practical use;
2.1.2 Monostable Circuits.
appreciate that a monostable produces a single
pulse when triggered;
describe a range of applications for a monostable;
recognise a 555 timer configured as a monostable;
draw the trigger circuit for a 555 monostable
consisting of a pull up resistor and a push switch
/ sensor;
perform calculations using the formula T=1.1RC.
2.1.3 Astable Circuits.
Appreciate that an astable produces a continuous
train of ON-OFF pulses;
Describe a range of applications for an astable;
Recognise a 555 timer configured as an astable;
Explain in qualitative terms how the frequency may
be changed;
Measure the amplitude and period of the output
waveform from an oscilloscope trace or graph;
Calculate the frequency of an astable from an
oscilloscope trace or graph;
Perform calculations given the formula for the
frequency of an astable.
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
69