CA555, CA555C, LM555, LM555C, NE555
RESET
R
1
7
CA555
6
1
2
C
T
4.7K
0.01µF
S
1
START
5
10K
RELAY
COIL
680
4
EO
3
1N4001
CAPACITANCE (µF)
8
10
R
1
= 1kΩ
10kΩ
100kΩ
0.1
1MΩ
10MΩ
0.01
V+
5V
100
T
A
= 25
o
C
V+ = 5V
1
680
0.001
10
-5
10
-4
10
-3
10
-2
TIME DELAY(s)
10
-1
1
10
NOTE: All resistance values are in ohms.
FIGURE 1. RESET TIMER (MONOSTABLE OPERATION)
FIGURE 3. TIME DELAY vs RESISTANCE AND CAPACITANCE
Since the charge rate and threshold level of the comparator
are both directly proportional to V+, the timing interval is rel-
atively 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 termi-
nal (4) and the trigger terminal (2) during the timing cycle
discharges C
T
and causes the timing cycle to restart.
Momentarily closing only the reset switch during the timing
interval discharges C
T
, 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 R
1
and C
T
.
3V
INPUT
VOLTAGE (TERMINAL 2)
SWITCH S
1
“CLOSED”
0
3.3V
CAPACITOR
VOLTAGE (TERMINALS 6, 7)
0
t
D
5V
OUTPUT
VOLTAGE
(TERMINAL 3)
0
SWITCH S
1
“OPEN”
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 R
1
and R
2.
V+
5V
R
1
7
R
2
6
1
2
CA555
5
4
8
EO
3
RELAY
COIL
C
T
0.01µF
FIGURE 4. REPEAT CYCLE TIMER (ASTABLE OPERATION)
T = 0.693 (R
1
+ 2R
2
) C
T
= t
1
+ t
2
where t
1
= 0.693 (R
1
+ R
2
) C
T
and t
2
= 0.693 (R
2
) C
T
the duty cycle is:
R
1
+
R
2
t
1
---------------
=
-----------------------
-
-
t
1
+
t
2
R
1
+
2R
2
FIGURE 2. TYPICAL WAVEFORMS FOR RESET TIMER
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
(R
1
+ 2R
2
) and C
T
.
8-6
HARRIS [ HARRIS CORPORATION ]
