NE555/SA555/NA555

Document number: DS35112 Rev. 5 - 2

8 of 14

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February 2021

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NE555

Typical Applications Characteristics (cont.)

Astable Operation

As shown in Figure 4, adding a second resistor, RB, to the circuit of Figure 1 and connecting the trigger input to the threshold input causes

the timer to self-trigger and run as a multivibrator. The capacitor C charges through RA and RB and then discharges through RB. Therefore,

the duty cycle is controlled by the values of RA and RB.

This astable connection results in capacitor C charging and discharging between the threshold-voltage level (

≉0.67VCC) and the trigger-

voltage level (

≉0.33VCC). As in the monostable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are

independent of the supply voltage.

C

RL

RA

RB

THRES

TRIG

DISCH

RESET

CONT

VCC

GND

OUT

Output

4

7

6

2

1

3

8

5

Open

(See Note A)

VCC

(5V to 15V)

Decoupling CONT voltage to ground with a capacitor can

improve operation. This should be evaluated for individual

applications.

0.01µF

Fig. 4 Circuit for Astable Operation

Fig. 5 Typical Astable Waveforms

Figure 5 shows typical waveforms generated during astable operation. The output high-level duration tH and low-level duration tL can be

calculated as follows:

tH = 0.693(RA +RB)C

tL = 0.693(RB)C

Other useful equations are:

period = tH + tL = 0.693(RA + 2RB)C

frequency = 1.44/(RA + 2RB)C

output driver duty cycle = tL/(tH + tL) = RB/(RA + 2RB)

output waveform duty cycle = tH/(tH + tL) = 1 – RB/(RA + 2RB)

low to high ratio = tL/tH = RB/(RA + RB)

Fig. 6 Free Running Frequency