User`s guide

57 | P a g e
independent of supply voltage variations. Typically,
the timing varies only 0.05% for a 1V change in V+.
Applying a negative pulse simultaneously to the reset terminal
(4) and the trigger terminal (2) during the timing cycle
discharges CT and causes the timing cycle to restart.
Momentarily closing only the reset switch during the timing
interval discharges CT, but the timing cycle does not restart.
Figure 2 shows the typical waveforms generated during this
mode of operation, and Figure 3 gives the family of time
delay curves with variations in R1 and CT.
Repeat Cycle Timer (Astable Operation)
Figure 4 shows the CA555 connected as a repeat cycle
timer. In this mode of operation, the total period is a function
of both R1 and R2.
T = 0.693 (R1 + 2R2) CT = t1 + t2
where t1 = 0.693 (R1 + R2) CT
and t2 = 0.693 (R2) CT
the duty cycle is:
Typical waveforms generated during this mode of operation
are shown in Figure 5. Figure 6 gives the family of curves of
free running frequency with variations in the value of
(R1 + 2R2) and CT.
1
CA555
EO
8
5
2
6
7
3
4
680
RESET
R1
CT 4.7K 680
10K
0.01F
RELAY
COIL
1N4001
V+
5V
S1
START
NOTE: All resistance values are in ohms.
FIGURE 1. RESET TIMER (MONOSTABLE OPERATION)
tD
3V
3.3V
5V
0
0
0
SWITCH S1 “OPEN”
SWITCH S1 “CLOSED”
INPUT
VOLTAGE (TERMINAL 2)
CAPACITOR
VOLTAGE (TERMINALS 6, 7)
OUTPUT
VOLTAGE
(TERMINAL 3)
FIGURE 2. TYPICAL WAVEFORMS FOR RESET TIMER
TIME DELAY(s)
10-1 1 10
0.1
0.01
0.001
TA = 25oC
R1 = 1k
10k
CAPACITANCE (F)