Project: Simple PWM circuit

Thread Starter


Joined Jul 17, 2007
There have been a large number of requests for an easy-to-build PWM control circuit. You can always use a 555 or 556 timer circuit, but here's something you might build out of components from your "junk box."

Attached is a schematic that uses a CMOS 4093 Schmitt input NAND gate in conjunction with a potentiometer, 1 resistor, a cap and a couple of 1N914 diodes to create a roughly 2% to 98% pulsewidth duration, at around 1kHz. You could use any CMOS gate that has a Schmitt input and an inverting output. You can use just about any general purpose silicon diodes instead of the 1N914, as long as they have a PIV greater than 10V and current of more than 3mA (NOT Zener diodes). The circuit should work with Vdd from about +5v to +16v (if using power MOSFETS, you'll likely need to use a Vdd of at least 10V to fully turn on the gate; check your datasheet to be certain.)

Select R2 to limit the maximum current in the RC network to around 2mA; this will depend upon your Vdd. Multiply your Vdd times 500; that's the minimum resistance you need in Ohms.

With the components shown, center frequency is near 1kHz. Decreasing C1 and/or R1 will increase the frequency. It's best to reduce C1, as reducing R1 will adversely affect the PW control at the extreme limits.

Increasing R1 much above 500k Ohms will likely result in stability problems. To lower the freqency, increase C1 instead.

If you decide to use something other than a FET/MOSFET for the output driver, be sure to use a 10K current limiting resistor to prevent overloading the output of the gate.

And if you need more PWM circuits - there are three more gates in that 4093 that could be used for three more such circuits. :)

"Quickie" fader:
Connect the output of U1A to both inputs of one of the spare 4093 gates (after removing the grounds, of course) and add another MOSFET to the output of that gate. You now have a complimentary PWM driver by adding only one physical part; the MOSFET. Since the 4093 NAND gate inverts the signal, when one MOSFET is turning ON, the other will be turning OFF, and vice versa. If you were using the MOSFETS to control one or more LEDs, those controlled by one of the MOSFETS would increase in brightness while those controlled by the other MOSFET would grow dim.



Joined May 2, 2008
Since this PWM circuit is for the "junkbox" builder :), a good source for MOSFETs you could use for low-power applications is your old dead motherboards, including many of the ones that caps leaked on, as long as the MOSFETs pass the quick multimeter test (with and without power to the gate, which can be provided from the diode-test mode on many DMMs). If you don't mind using SMT parts, you can probably find a suitable logic chip with inverting gates or buffers on it you can use for the main part of this as well. :) I used MOSFETs from an old Compaq Athlon 700mhz system to repair a small electric scooter's LM339-based motor controller; they actually had a higher voltage and current rating (and about the same gate current/voltage) as the ones that came in the controller originally, before whoever owned the scooter before blew it up and discarded it.


Joined Mar 24, 2008
Here is the 555 version I posted in another thread...

They wanted independent frequency and PWM controls.


Joined Mar 24, 2008
Opps, looks like I stuck my foot in it, twice. That design will reduce the PWM range as you decrease the freq. with R2. Oh weel, today the bear got me.


Joined Mar 28, 2010
Would SgtWookie's simple pwm circuit drive a P-channel Mosfet (for a high side drive circuit)?

Also, would the current output be sufficient to overcome gate capacitance for a fast rise time (to avoid switching losses heating the Mosfet)?

Thread Starter


Joined Jul 17, 2007
Hello Zebbo,
I barely saw your other post before it was deleted; had I not seen the other post, it would have been a long time until you would have received an answer in this thread.

This circuit is primarily for a low-frequency PWM for a small (low gate charge) N-ch MOSFET.

Since your circuit sounds like it will be different, why don't you start a new thread in the Projects Forum, and post more details about it?

Then delete your reply to this thread, and I'll do the same. Otherwise, things might get confused.

Generally, it's not a good thing to reply to an old thread if it's not yours to begin with; simply post a link to the old thread in a new topic.


Joined Oct 22, 2011
i have an alternative, im not sure about anyone else, but i have a few hundred surface mount opamps.. i used them to make pwm circuits a while back, excuse the mosfet circuitry (doesnt work) and the mounting holes etc. but the pdf uploaded with this is the opamp circuit, recreate it in ltspice or similar and you have really cheap, simple pwm! (though not full 0-100% it was good enough for what i required for an led brightness signal source).
the frequency can be adjusted using the rc network in the feedback loop.



Joined Mar 24, 2008
The op amps you are referring to are in the configuration of Schmitt Triggers. Basically you are describing the same kind of circuitry.


Joined Feb 11, 2008
Nice useful circuit SgtWookie. :)

If I might make one suggestion, the 3 unused NAND gates could be connected in parallel and driven from the first NAND oscillator. That would provide increased current output that would help driving power FETs.