The circuit uses two timer/oscillators connected as a Pulse Width Modulator. The chip used is an nmos dual timer/oscillator, NE556. This IC has two 555 timers in one 14-pin IC package.
The second 555 (IC1:A) is configured for Pulse Width Modulation. It is set up in monostable mode. It is triggered with the continuous pulse train from the first 555. However, by also applying a DC voltage to pin 3 the comparator reference levels will be changed from their nominal levels of one-third & two-thirds of the supply voltage. This has the effect of modulating the pulse width as the control voltage varies.
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