Introduction: Simple PWM Generator

Picture of 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).


Johan1979 (author)2017-02-22

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

IAmMe² (author)Johan19792017-02-22


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

Johan1979 (author)IAmMe²2017-02-22

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

ClintZ (author)2017-01-12

I would like to use this circuit to control a treadmill motor at around 100 vdc.

How would you feed that kind of high voltage into a circuit like this?

IAmMe² (author)ClintZ2017-01-12

You would have to use a dedicated power supply unit to bring the high voltage down to something accepted by this circuit, such as 12VDC. You can then use a high voltage/high power transistor to PWM control the motor.

You would connect the base of the transistor to pin 7 of the 555, the negative of the motor to the collector of the transistor and then the emitter of the transistor to the main negative supply.

ClintZ (author)IAmMe²2017-01-17

I took your design and added the Treadmill motor as you see in my diagram.
Also changed R1 to 10K and C1 to 1uF.
Does this look logical to you. I'm not too boned up on tronics, I have a background in controls engineering which deals mostly in on/off states of 120VAC conditions. Tronics always seems to me like everything is going around in circles unlike controls which has a power rung and a neutral rung laid out in a ladder diagram fashion. Even the programming looks like a ladder.
Anyway....I'd like to build this circuit for a drill press conversion to multi-speed in my shop.
Did I hack it up, or am I close?

IAmMe² made it! (author)ClintZ2017-01-17

Very, very close!

You'll need to run the AC line through a fuse and the transformer first and then through a bridge rectifier. I also recommend adding an electrolytic capacitor on the rectifier output to get rid of the 100Hz/120Hz ripple on the DC line. Something from 100uF to 1000uF @ 35VDC rating should do nicely. I've attached a diagram to make this a little easier to understand.

Aside from that, all looks good!

ClintZ (author)IAmMe²2017-01-17

If I under stood you correctly, this is just to feed the control circuit. I would still feed the motor from the BR that I already have in my circuit. Is that correct?

IAmMe² (author)ClintZ2017-01-17

The bridge rectifier from my last comment would be producing 14/15VDC tops when not under load.

The best thing to do would be add in a secondary bridge rectifier, rated for the high current of the motor, directly across the mains. I would then recommend adding a suppression capacitor across the motor too.

ClintZ (author)IAmMe²2017-01-18

Thanks for the help, I really apreciate it. Forgive my lack of knowledge, but your reply was as I thought would be the case. What rating should the capacitor be and should that be in parrelel with the blocking diode?

IAmMe² (author)ClintZ2017-01-18

No worries, I always find giving it a go is a good way to learn!

The capacitor value shouldn't matter too much, something from 22uF to 100uF should do just fine. It will need to be rated for at least 250VDC though and probably be a safety capacitor as it is directly across your mains supply.

ClintZ (author)IAmMe²2017-01-19

I reworked the circuit a little with all your suggestions (see attached).

I'm going to order some parts and build it, and test it.

I'll let you know how its going after I let out all the smoke, which seems like the usual with me and tronics.

Thanks a BUNCH!!

IAmMe² (author)ClintZ2017-01-19

Sounds good! Sometimes letting the magic smoke out is half the fun...

Obviously, play safe around the mains and good luck with your build!

Kuuran (author)2015-10-23

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

IAmMe² (author)Kuuran2015-10-25

Nope, no THR connection required

Kuuran (author)IAmMe²2015-10-25

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?

IAmMe² (author)Kuuran2015-10-25

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

DerpyH (author)2015-07-08

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

IAmMe² (author)DerpyH2015-07-08

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.

DIY Hacks and How Tos (author)2015-07-05

Nice design.

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