Datasheet NE555, SA555, NA555 (Diodes) - 10

NE555/SA555/NA555 PRECISION TIMERS Typical Applications Characteristics (cont.) 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.
T UC NEW PROD 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.  
Fig 12. Circuit for pulse position modulation Fig 13. Pulse position modulation timing diagrams
NE555/SA555/NA555 10 of 14 February 2012 Document number: DS35112 Rev. 4 - 2
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