This circuit encodes a voltage with pulse-width modulation using a 555 timer chip. The width of the output pulses varies depending on a control voltage.

The width of the pulses is set by a triangle wave oscillator connected to the "ctl" input of the 555. The rest of the circuit is just like the square wave oscillator. By applying a voltage to the "ctl" input (normally 2/3 V_in), we can control the voltage which ends a timing interval. When the "th" input reaches this value, the output goes low. So the 555 will oscillate faster when the ctl input is lower.

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