Download The 555 Timer IC

İzmir University of Economics
EEE 332 Digital Electronics Lab
EXPERIMENT 8
The 555 IC Timer
A. Background
The 555 Integrated Circuit (IC) Timer is very popular IC since it is first introduced in 1972. It can
be used to construct several multivibrator circuits easily.
The basic block diagram of 555 IC Timer is given in Fig. 8.1.
VCC

555 TIMER
R1
Comparator 1
Threshold

Control
R

R1
Q
FF
S
Output

Q'
VTL
Trigger
Comparator 2

R1
R2
5K
Discharge

Q1
Ground

(a) Basic Block Diagram
(b)
Pin Diagram
Fig. 8.1. The 555 Timer IC
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A.1. Astable Multivibrator
When two resistors and one capacitor is connected externally as shown in Fig. 8.2, the 555 Timer
IC forms an astable multivibrator.
Fig. 8.2. Astable Multivibrator 555 IC
The output waveform is shown in F. 8.3. At steady state, the capacitor voltage swings between
VCC/3 and 2VCC/3. The charging period starts with the voltage VCC/3 and tends to go to VCC, with a
time constant of = (RA + RB)C. However it is stopped when vC = 2 VCC/3. The discharging period
starts with the voltage 2VCC/3 and tends to go to 0, with a time constant of = RB C.
TC
vO
vC
TD
VCC
VCC/3
VSS
VCC/3
VSS
0
0
t
Fig. 8.3. Waveforms in Astable Multivibrator 555 IC
The charging period is determined as
TC = (ln2)(RA + RB)C = 0.693(RA + RB)C
The discharging period is then
TD = (ln2)RBC = 0.693RBC
The period of oscillation is then
T = TC + TD = 0.693(RA + 2RB)C
and the frequency f is
f = 1/T
The duty cycle is defined as
Duty cycle = (TC/T) x 100 %
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A.2. Monostable Multivibrator
Connecting one resistor and one capacitor externally, the 555 Timer IC forms a monostable
multivibrator as shown in Fig. 8.4.
Fig. 8.4. Monostable Multivibrator 555 IC
Normally the trigger input is connected VCC. Then the capacitor is discharged to 0 and the output
voltage is 0.
T
vO
vC
Tr
VCC
VCC/3
VSS
VCC/3
t
Fig. 8.5. Voltage waveforms in the 555 IC Monostable Multivibrator
When the trigger input is connected 0 momentarily, the capacitor starts charging towards VCC with
a time constant of = RAC (Fig. 8.5). The output is switched to VCC then. The charging is stopped
when vC voltage reaches 2VCC/3. The output pulse duration is determined as
T = (ln3)RAC = 1.1RAC
Recovery time TR is very small.
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B. Preliminary Work
1. Consider the astable multivibrator circuit given in Fig. 8.6 with C = 100 nF and VCC = 5 V.
Fig. 8.6. Astable Multivibrator 555 IC
Determine the values of the resistors RA and RB to set f = 1 kHz and duty cycle = 60 %
RA = ……………….... RB = ……………………
Duty cycle = ………. %
2. Now consider the monostable circuit of Fig. 8.7. Assume RA = 100 k.
Fig. 8.7. Monostable Multivibrator 555 IC
Determine C to get a monopulse of approximately 5 sec.
C = ……………………..
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C. Experimental Work
1.
Construct the astable multivibrator circuit given in Fig. 8.8 with C = 100 nF, VCC = 5 V, and the
values of RA = ……………. and RB = ……………… as calculated in the Preliminary Work.
(a) Observe and plot vC and vO.







Fig. 8.8. Astable Multivibrator 555 IC
Channel 1 scale:
………… ……./div
Channel 2 scale:
………… ……./div
Time division:
………… ……./div
Vp-p = ……………
T = ……………….
f = ………………..
Fig. 8.9. Waveforms in 555 IC Astable Multivibrator
(b) Measure the frequency f and duty cycle.
f = ……………………….. kHz
Duty cycle = ……………. %
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2.
Construct the monostable multivibrator circuit given in Fig. 8.10 with R A = ……………. as
calculated in the Preliminary Work.







Fig. 8.10. 555 IC Monostable Multivibrator
Measure the monopulse width.
T = .................. sec
Component List:
Exp C(1)
555 Timer
RA= …….(calculated)
Exp C(2)
555 Timer
RA= …….(calculated)
RB=…(calculated)
100 nF
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