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ENTC 3320
555 TIMER
555 Timer IC Timer

The 555 timer has essentially two modes
of operation:
• Astable (free running) multivibrator and
• Monostable (one shot) multivibrator
Gnd 1
Trigger 2
Output 3
Reset 4
8 +Vcc
555
Timer
7 Discharge
6 Threshold
5 Control voltage
Pin Functions



PIN 1 Ground--usually connected to ground. The
voltage should be the most negative of any
voltage appearing at the other pins.
PIN 2 Trigger--level-sensitive point to 1/3 VCC.
When the voltage at this pin is brought below
1/3 VCC the flip-flop is set causing pin 3 to
produce a high state. Allowable applied
voltage is between VCC (pin 8) and ground
(pin 1).
• Pin 3 Output--level here is normally low and goes high during
the timing interval. Since the output stage is active in
both directions, it can source or sink up 200 mA.
• Pin 4 Reset--when voltage at this pin is less than 0.4 V, the timing
cycle is interrupted returning the timer to its nontriggered
state. This is an overriding function so that the timer can
not be triggered unless reset is released (pin 4 > 1.0 V).
When not used, connect to VCC.
•Pin 5 Control voltage--internally derived 2/3 VCC point. A
resistor-to-ground or an external voltage may be connected to pin 5
to change the comparator reference points. When
not
used for this purpose, a capacitor-to-ground greater than
or
equal to 0.01 mF is recommended for all applications.
•Pin 6 Threshold--level sensitive point to 2/3 VCC. When the
voltage at this pin is brought greater than 2/3 VCC., the
flip-flop is reset causing pin 3 to produce a low state.
•Pin 7 Discharge--collector of a transistor switch to ground (pin 1).
It is normally used to discharge the timing capacitor.
• Pin 8 VCC--the power-supply voltage connected here can range
from 4.5 to 16 V with respect to ground (pin 1).
Astable Multivibrator
Reset
Discharge
Threshold
Trigger
2
VCC
3
1
VCC
3
The charge/discharge equation for a RC circuit is:
VC  VFinal  (VFinal  VInitial )e
 t / RC
The charging circuit is through RA, RB, and C.
2
1

 t1 /  RA  RB C
VCC  VCC  VCC  VCC e
3
3


0.693  t1 / ( R A  RB )C
t1  0.693( RA  RB )C
The discharging circuit is through RB and C.
1
2

 t2 /  RB C
VCC  VCC  VCC  VCC e
3
3


1
2
 t 2 / RB C
VCC  VCC e
3
3
1
 e  t 2 / RB C
2
1
ln  ln e  t2 / RB C
2
0.693  t 2 / RB C
t2  0.693RB C
T  t1  t2  0.693( R A  2 RB )C
1
144
.
f  
T ( R A  2 RB )C
The duty cycle will always be greater than 50%.
2
VCC
3
0
The charge equation for a RC one shot is:
2
VCC  VCC  VCC  0e t / RC
3
2
VCC  VCC  VCC e t / RC
3
2
t / RC
 1 e
3
1
  e t / RC
3
1
t / RC
e
3

1
ln    ln e t / RC
 3

1.1  t / RC
t  1.1RC