UNIVERSITY OF MASSACHUSETTS DARTMOUTH
COLLEGE OF ENGINEERING
EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I
555 INTEGRATED CIRCUIT TIMER
THE 555 TIMER USED AS AN ASTABLE MULTIVIBRATOR
The outline and pin configuration for a 555 Integrated Circuit Timer is 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 your project, 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.
In order to operate as an Astable Multivibrator, the 555 is connected as shown here in
Figure 2. The time constant that determines the pulse width is created by resistors RA,
RB, and capacitor C. The pulse width will be the time it takes for the capacitor C to
charge from 1/3VCC to 2/3VCC as shown in Figure 4. While this charging is taking place,
the output voltage will be equal to VCC.
Figure 2. A 555 IC Timer connected as an Astable Multivibrator.
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The capacitor C charges from 1/3VCC towards 2/3VCC via resistors RA and RB as shown
below in Figure 3. Check the “charging” portion of the capacitor voltage and the
associated output voltage displayed in Figure 4.
Figure 3. The capacitor C1 charges via RA and RB.
Capacitor C1 “charging”
VCC
Figure 4. The waveforms of the capacitor and output voltages during charging.
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When the capacitor voltage reaches 2/3VCC, the internal transistor Q1 will be switched
ON, creating a short-circuit path to ground from pin #7 causing C to discharge towards 0
Volts via resistor RB and the output voltage to fall to 0Volts. When the capacitor voltage
reaches 1/3VCC, the internal transistor Q1 is switched OFF, causing the capacitor to
charge towards 2/3VCC again. This “cycle” repeats over and over again.
Figure 5. The capacitor C1 discharges via resistor RB.
Capacitor C1 “discharging”
0V
Figure 6. The waveforms of the capacitor and output voltages during discharging.
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MATHEMATICAL DESCRIPTIONS OF THE OUTPUT WAVEFORM
Pulse Width
The mathematical expression for the capacitor voltage on the “charging” (pulse width)
portion of the output waveform is given by
t
- 


v C (t) = VCC 1- e 


Solving for the pulse width (PW) yields
PW = 0.693(RA +RB )(C1 ) .
Space Width
On the “discharging” (space width) portion of the output waveform, the capacitor voltage
can be expressed as
-
t
v C (t) = VCCe 
The space width (SW) turns out to be
SW = 0.693RBC1
Frequency
The frequency of the resulting rectangular output voltage (in Hz) is determined by
f0 =
1.44
(R A + 2RB )(C1 )
PW = 0.693(R A +RB )(C1 )
Period
R A +RB
×100
R
+
2R
The period (in seconds) of the output voltage isAthe sum
B of the PW and SW, and can
DutyCycle(%) =
also be determined as the reciprocal of the frequency.
(R
T = PW + SW =
A
+ 2R )(C )
B
1
1.44
Duty Cycle
1.44
(R A +the
2Routput
We can define a quantity that relates
B )(C1 ) voltage’s “On-time” (the PW) to the total
f0 =
period (T). This “Duty Cycle”
is expressed
a percentage
and is given by
PW
= 0.693(Ras+R
)(C )
A
DutyCycle(%) =
B
1
R A +RB
×100
R A + 2RB
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