UNIVERSITY OF MASSACHUSETTS DARTMOUTH
COLLEGE OF ENGINEERING
EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I
INTRODUCTION TO THE 555 INTEGRATED CIRCUIT TIMER
INTRODUCTION
The outline and pin configuration for a 555 Integrated Circuit Timer are shown below in
Figure 1. At this point, it is not necessary to discuss the internal devices in any detail.
Figure 1. A typical 555 IC Timer.
In this exploration, the 555 will be used as an Astable Multivibrator. This configuration
will deliver an output voltage that is a rectangular wave. The pulse and space widths
can be controlled by properly selecting two time constants.
An Astable Multivibrator is specified by the frequency, pulse width, and duty cycle of the
output signal. For the 555 circuit, the equations that need to be satisfied are
f0 =
1.44
(R A + 2RB )(C1 )
PW = 0.693(R A +RB )(C1 )
DutyCycle(%) =
R A +RB
×100
R A + 2RB
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PROCEDURE
Connect a 555 Timer as shown below in MultiSim.
XSC1
VCC
G
12V
T
A
B
XFC1
RA
3kOhm
123
8
U1
VCC
RB
9kOhm
C1
1uF
4
RST
7
DIS
OUT 3
6
THR
2
TRI
5
CON
GND
1
LM555CN
Figure 2. A 555 Timer connected as an Astable Multivibrator and the MultiSim model
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Choose your individual resistor values according to the following:
RA = A kΩ, where A is the first non-zero digit in your UMD ID number
RB = B kΩ, where B is the last non-zero digit in your UMD ID number
Choose the value of the capacitor C1 to be your group number in μF.
Using Excel, prepare a table to record the calculated and measured values of frequency,
pulse width, and duty cycle for up to three sets of resistor values.
Calculate the frequency, pulse width, and duty cycle from the equations on page 1.
Simulate your design in MultiSim. Connect the oscilloscope to display the waveforms of
the 555 output voltage (the voltage at pin #3) and the voltage across the capacitor (the
voltage at pin #6). Connect the frequency counter to measure the frequency of the 555
output voltage. From the instrument measurements, determine
the period of the output voltage,
the “on” time, or pulse width, of the output voltage, and
the frequency of the output voltage.
Compare the calculated and measured values of the frequency, pulse width, and duty
cycle, and comment on your results.
As time permits, repeat the exploration as the values of both resistances are first halved,
then doubled while the capacitor value remains unchanged. Be sure to make note of
how the frequency, pulse width, and duty cycle change as a function of these
resistance values!
RESULTS TO BE HANDED IN – ONE SET PER GROUP OF 2
For the original component values:
Circuit diagram of the 555 Astable Multivibrator
Oscilloscope and frequency counter printouts
For each set of component values:
Calculations of frequency, pulse width, and duty cycle
Excel table containing the calculated and measured results
Comparison of calculated and measured results
For the “overall” exploration:
Conclusions about the changes in the 555 output voltage as a function of the resistances
RA and RB.
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