NE555/SA555/NA555

Document number: DS35112 Rev. 5 - 2

10 of 14

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February 2021

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NE555

Pulse Width Modulation

The operation of the timer can be modified by modulating the internal threshold and trigger voltages, which is accomplished by applying an

external voltage (or current) to CONT. Figure 10 shows a circuit for pulse-width modulation. A continuous input pulse train triggers the

monostable circuit, and a control signal modulates the threshold voltage. Figure 11 shows the resulting output pulse-width modulation. While

a sine-wave modulation signal is shown, any wave shape could be used.

THRES

TRIG

DISCH

RESET

CONT

GND

OUT

Output

2

5

1

3

8

4

(5V to 15V)

C

Clock Input

7

6

Modulation

Input

(see Note A)

The modulating signal can be directly or capacitively coupled to

CONT. For direct coupling, the effects of modulation source voltage

and impedance on the bias of the timer should be considered.

Fig. 10 Circuit for Pulse width modulation

Fig. 11 Pulse width modulation timing diagrams

Pulse Position Modulation

As shown in Figure 12, any of these timers can be used as a pulse-position modulator. This application modulates the threshold voltage and,

thereby, the time delay, of a free-running oscillator. Figure 13 shows a triangular-wave modulation signal for such a circuit; however, any

wave shape could be used.

THRES

TRIG

DISCH

RESET

CONT

GND

OUT

Output

2

5

1

3

8

4

(5V to 15V)

C

7

6

Modulation

Input

(see Note A)

The modulating signal can be directly or capacitively coupled to

CONT. For direct coupling, the effects of modulation source voltage

and impedance on the bias of the timer should be considered.

Fig. 12 Circuit for pulse position modulation

Fig. 13 Pulse position modulation timing diagrams