LM555

7

The equivalent circuit for discharging capacitor C1, when timer output is low is, as follows:

Since the duration of the timer output low state(

(10)

Consequently, if the timer operates in astable, the period is the same with

time. And since frequency is the reciprocal of the period, the following applies.

3. Frequency divider

By adjusting the length of the timing cycle, the basic circuit of Figure 1 can be made to operate as a frequency divider. Figure

8. illustrates a divide-by-three circuit that makes use of the fact that retriggering cannot occur during the timing cycle.

V

C1

t



2

3

---V

CC

V

=

CC

1

2

3

---e

-

t

H

R

A

R

B

+

C1

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

–







–











=

4



t

H

C

1

R

A

R

B

+

In2

0.693 R

A

R

B

+

C

1

=

=

5



C1

R

B

R

D

V

C

1

dv

C1

dt

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

1

R

A

R

B

+

-----------------------V

C1

0

=

+

6



V

C1

t



2

3

---V

CC

e

-

t

R

A

R

D

+

C1

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

=

7



1

3

---V

CC

2

3

---V

CC

e

-

t

L

R

A

R

D

+

C1

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

=

8



t

L

C

1

R

B

R

D

+

In2 0.693 R

B

R

D

+

C

1

=

=

9



frequency,

f

1

T

---

1.44

R

A

2R

B

+

C

1

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

==

11

