Activity 1.2.5 Clock Signals: The 555 Timer
Introduction
Almost all development tools used today in digital electronics have an internal clock that can
be integrated into your circuit design. There are times however, when you may want to
generate your own simple clock signal and not depend on the internal clock of your
development board or equipment like a function generator or digital writer.
The 555 Timer oscillator is one of the most common circuits used in introductory electronics.
It is a favorite among beginners because of its low cost and ease of design. These are
precisely the same reasons the 555 Timer is used in the Random Number Generator design.
In this activity you will simulate and create a 555 Timer oscillator. You will observe the effect
that varying the value of its resistor and capacitor values has on the oscillation frequency and
duty cycle.
Equipment
Circuit Design Software (CDS)
555 timer Integrated Circuit (IC)
Resistors and capacitors
#22 Gauge solid wire
Breadboard
Procedure
1. For the 555 Timer oscillator circuit shown below, calculate the frequency and duty
cycle of the output signal based on the component values shown.
XSC1
VCC
5V
G
T
A
R1
330Ω
B
C
D
U1
555_TIMER_RATED
VCC
RST
OUT
DIS
LED1
THR
TRI
R2
330Ω
C1
22µF
CON
GND
R3
180Ω
C2
0.01µF
GND
© 2014 Project Lead The Way, Inc.
Digital Electronics Activity 1.2.5 The 555 Timer – Page 1
2. Use the CDS to enter and simulate the 555 Timer oscillator circuit. Use the
oscilloscope’s markers to make the necessary measurements. Determine the
frequency and duty cycle of the output signal. How do these values compare to the
calculated values? 49.7 Hz
75% duty cycle, the values are very similar.
3. Repeat steps (1) and (2) for each set of component values in the table shown below.
Note that the shaded areas are the values that were measured from the original circuit.
RA
RB
C2
Period(T)
Frequency(f)
tH
tL
Duty
Cycle
100
330
22 F
11.58ms
86Hz
0.00655
0.005
57
330
330
22 F
15.09ms
66
0.010
0.005
66
560
330
22 F
18.60ms
53
0.0135
0.005
73
330
100
22 F
8.08ms
123
0.00655 0.0015
81
330
330
22 F
15.09ms
66
0.010
0.005
66
330
560
22 F
22.11ms
45
0.0135
0.0085
61
330
330
10 F
6.86ms
145
0.00457 0.0023
67
330
330
22 F
15.09ms
66
0.010
0.005
66
330
330
47 F
32.25ms
31
0.021
0.0107
65
4. Review the results of the data collected in step (3) of the procedure.
What effect did varying the RA have on the frequency and duty cycle?
The higher the RA value, the lower the frequency, the higher the duty cycle.
What effect did varying the RB have on the frequency and duty cycle?
The higher the RB value, the lower the frequency and the lower the duty cycle.
What effect did varying the C2 have on the frequency and duty cycle?
The higher the C value, the lower the frequency value and duty cycle.
In the previous activity you created a 4-bit counter that counted from 0-15 in binary.
We used a provided clock source. (Internal clock in the development board or a
software generated clock source).
Using what you have learned about the relationships between RA, RB, C2 and how
they impact the frequency output of the clock signal, create your own 555 Timer
oscillator circuit on your development board.
(Note: The simulation was helpful in determining how RA, RB, and C2 impact the 555
Timer oscillator design. However, the software simulated frequency can be affected
by the processor on your computer. The frequency rate may appear slightly different in
© 2014 Project Lead The Way, Inc.
Digital Electronics Activity 1.2.5 The 555 Timer – Page 2
your actual design compared to your simulated design. Also, your design may be
limited by the resistors and capacitors available.)
Simulation
5. Once you have your 555 Timer circuit functioning, use the clock signal to trigger the 4bit binary counter you created previously.
VCC
5V
X1
R1
330Ω
U1
555_TIMER_RATED
X3
X2
2.5 V
2.5 V
X4
2.5 V
2.5 V
VCC
5V
VCC
RST
OUT
DIS
4
THR
2
CON
4
U5A
~1PR
1Q
5
2
~1Q
6
3
1D
4
U6A
~1PR
1Q
5
2
~1Q
6
3
1D
U7A
~1PR
1Q
5
2
1D
~1Q
6
3
1CLK
1D
1Q
5
~1Q
6
GND
3
1CLK
~1CLR
1
C1
22µF
4
U4A
~1PR
TRI
R2
330Ω
1CLK
~1CLR
74LS74N
1
1CLK
~1CLR
74LS74N
1
~1CLR
74LS74N
1
74LS74N
C2
0.01µF
GND
The combination of the 555 timer and the binary counter creates a counter where X1 goes on
and off and X2 goes on and off every 2 times X1 blinks and so forth to create a counter up to
15.
Breadboard
6. Create the circuit on your breadboard. (Remember you may need to use different
resistor and capacitor values to find a clock signal rate you are comfortable with.
You will need to locate the datasheets for the 555 timer and 74LS74 D Flip-flop.
© 2014 Project Lead The Way, Inc.
Digital Electronics Activity 1.2.5 The 555 Timer – Page 3
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