PPM Using 555 Timer

This circuit allows to create a variable pulse-width signal. It is based on the famous 555 timer that generates a square signal at around 48kHz. An operational amplifier compares the voltage across the capacitor in the 555 circuit and compares it to a voltage set by a voltage divider with a potentiometer (R3). This allows to change the duty cycle from 0 to 100% (it is reversed wrt the potentiometer, ie. 100% pot = 0% duty cycle, 0% pot = 100% duty cycle).