8-6

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

Momentarily closing only the reset switch during the timing

Figure 2 shows the typical waveforms generated during this

mode of operation, and Figure 3 gives the family of time

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

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

1

CA555

EO

8

5

2

6

7

3

4

680

RESET

4.7K

680

10K

0.01

µF

RELAY

COIL

1N4001

V+

5V

START

NOTE: All resistance values are in ohms.

FIGURE 1. RESET TIMER (MONOSTABLE OPERATION)

3V

3.3V

5V

0

0

0

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

0.1

0.01

0.001

10k

Ω

1

10

100

V+ = 5V

100k

Ω

1M

Ω

10M

Ω

FIGURE 3. TIME DELAY vs RESISTANCE AND CAPACITANCE

1

CA555

EO

8

5

2

6

7

3

4

0.01

µF

RELAY

COIL

V+

5V

FIGURE 4. REPEAT CYCLE TIMER (ASTABLE OPERATION)

t

1

t

1

t

2

+

----------------

=

R

1

R

2

+

R

1

2R

2

+

------------------------

CA555, CA555C, LM555, LM555C, NE555