# How can I use a voltage to control the pulse width of a 555?

I have a variable PWM signal at 13.2 kHz. I want to control a servo with this signal, but the servo needs a much lower frequency, at most 80 Hz. My first idea was to use two binary counters, a NOT gate, and an AND gate. That should do the trick, but it's definitely not the best solution. It requires a lot of ICs with a lot of unused pins. Also, the servo is looking at a pulse about 1-2ms wide, not a full PWM, so only about 8% of the PWM range would be useful.

Using an RC filter, I can just convert the original signal to an analog voltage. Is there any way I can use this analog voltage to control a 555 to get the signal I need?

My only experience with the 555 is using a pot to adjust the duty cycle.

• This is a job for an MCU. I'd probably select the cheapest, smallest thing with peripherals that met the need. In this case, I think you could get by with a PIC10F20x or PIC10(L)F32x device. An RC filter will be a trade-off between allowable ripple and settling time. At 80 Hz, your settling time should be well less than 10 ms, I'd imagine. Say 5-6 ms? $4\tau$ might be good enough. But if so, remaining high frequency peak-to-peak ripple will be about 2%, too. Can you accept that? – jonk Oct 31 '18 at 5:59
• I'd love to use an MCU but they are forbidden for my application. I need to use discrete components. I'm not working with high tolerances or anything here, I could settle for a 300ms setting time if I need to. – griffin175 Oct 31 '18 at 7:00
• I'd recommend using a better filter than RC, to start, though it's possible you can get by with the RC. You decide that. But a low-pass will convert PWM to a voltage. I think you could get by with a 555 running at a low duty cycle for 80 Hz. You'd need a voltage-controlled current sink/source to adjust the duty cycle within a 1-2 ms adjustment range. It would take a little thinking to work out exactly how to get what you wanted. – jonk Oct 31 '18 at 14:30