Pulse position modulator can be easily built using 555 IC. Here is the schematic diagram of the circuit:

pulse position modulator 555 circuit schematic

Pulse position modulator modulate the on-period while keeping the fixed off-period. Pulse position modulation might substitute pulse-width modulation in some application. The main difference between pulse position modulation (PPM)  and pulse width modulation (PWM)  is that  unlike PWM, PPM doesn’t keep constant frequency. [Circuit's schematic diagram source: National Semiconductor Application Notes]