ASTABLE OPERATION

If the circuit is connected as shown in Figure 1  (pins 2 and 6 connected) it will trigger itself and free run as a multivibrator. The external capacitor charges through RA   + RB   and discharges through RB. Thus the duty cycle may be precisely set by the ratio of these two resistors.

In this mode of operation, the capacitor charges and discharges between 1/3 VCC   and 2/3 VCC. As in the triggered mode, the charge and discharge times, and therefore the frequency are independent of the supply voltage. Figure 2 shows the waveforms generated in this mode of operation.

VCC  = 5V

TIME = 20µs/DIV.

RA  = 3.9kΩ

RB  = 3kΩ

C = 0.01µF

The charge time (output high) is given by:

t1  = 0.693 (RA  + RB) C

And the discharge time (output low) by:

t2  = 0.693 (RB) C

Thus the total period is:

T = t1  + t2  = 0.693 (RA  +2RB) C

The frequency of oscillation is: 

It may be used for quick determination of these RC values.

The duty cycle is: 

Posted in: 555,Electronics