Simple PWM Generator


Introduction: Simple PWM Generator

this instructable will show you a neat little circuit to generate PWM (Pulse Width Modulation), that is quick and easily built from components that are likely to be in your collection already!

Parts List:

  1. 1x 555 Timer IC
  2. 1x 100K Linear Pot
  3. 1x 100nF Capacitor (Ceramic or Propylene Film)
  4. 1x 10nF Capacitor (Ceramic or Propylene Film)
  5. 2x small signal diodes (1N4148/1N5818 etc)
  6. A 12V DC Power Source

The frequency of the output is dependant on the values of R1 (the pot) and C1. If you require a specific frequency, you can work it out using the following formula:

Frequency = 1.44 / (R1 * C1)

In order to drive inductive loads (motors, solenoids etc.) it is highly recommended to add a transistor/MOSFET onto pin 7 (the PWM output) -- don't forget a free-wheel diode if you do!

If you require finer adjustment of the PWM output you can always add a low value pot in series with R1 (suggested value is ~5K linear).



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Hi, i want to use in for a HHO generator, is it possible.

2 replies


This wouldn't be suitable for a HHO generator as it can not provide enough current to support electrolysis nor is it at a fast enough frequency

Thanks fir the quick response, do you perhaps have any ideas for me that would be suitable?

I have a feeling you are missing a THR connection... Maybe a short to TRIG?

3 replies

Nope, no THR connection required

Trg and Thr are internally connected to the comparators that set and reset the output. When Vtrg < 1/3 Vcc the output flip-flop is set. When Vthr > 2/3 Vcc the output is cleared. How is this circuit supposed to clear it's output with no thr connection?

My apologies, I haven't looked at this circuit in a while - you are correct, connect THR to TRG

Do you have another design for that just with a 10K pot?

1 reply

If you replace the 100K pot with a 10K pot, you'll have to times the value of C1 by 10 - replace C1 with a 1uF capacitor, electrolytic doesn't matter.