Low Frequency PWM

Introduction: Low Frequency PWM

About: Hello Instrutables community! It is always a pleasure be on Instrutables, be it reading an instrutable or writing an instrutable. I am a mechanical engineer who is currently working for a large engineering com…

Hello everyone,

In this project I will show you how I made an ultra low frequency PWM machine with very minimal components.

This circuit revolves around a schmitt trigger circuit.

Depending upon the requirements, I have classified the 3 types of circuits into 3 different steps.

This can achieve a high duty cycle of upto 150-200 seconds!

Step 1: Video

I have added a video of this project on youtube, hope you like this video and hope it helps.

Step 2: 50% Duty Cycle, Variable Frequency

Components required are-

1 LM358 ic

1 DIP8 ic socket

1 10k potentiometer

1 perfboard

3 20k resistors.

1 470uF electrolytic capacitor.

solder, soldering station, hookup wire etc

This circuit will deliver a square wave with 50% duty cycle continuously. another great advantage of this circuit is that theoretically, the frequency will not change even with change in input voltage. This is a great advantage in as compared to traditional 555 timer ic whose frequency is highly voltage dependent.

Here, when the circuit is powered, the capacitor will start charging through the resistor R. Once it reaches the set threshold, the capacitor starts discharging through the same resistor until it reaches the lower threshold. This goes on for countless cycles.

The frequency of the PWM will be close to the time constant of the RC circuit which is RxC

Use a 10 turn trimmer for better control over the frequency.

Step 3: Constant Frequency and Variable Duty Cycle



DIP8 socket

470uF electrolytic capacitor

1N007 Diode x2

10k 10 turn trimmer


20k reistors x 3

Here, the capacitor start charging through one half of the potentiometer and starts discharging through the other half of the resistor.This means, for the total cycle, the complete part of the potentiometer was used.

Here, the time period of PWM would be approximately be equal to R x C where R is the total value of the potentiometer.

Step 4: Independent On-off Timing Circuit



DIP8 socket

470uF capacito2 diodes

2 10k trimmers


This circuit can be used to regulate power to very low power applications like gardening or some project that has to be powered by a battery. This means, the battery power will only be consumed when the circuit is on and not when the output goes low.

I personally used this circuit to control a esp32 which consumes 80mA continuous last for more than 3 days!

This was done by keeping the ciruit on for 5 seconds and low for 150 seconds.

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    Question 2 years ago

    Thanks I was looking for this!
    How can we add deadtime to "fixed frequency, variable duty cycle" circuit?


    Reply 2 years ago

    Hi... Thanks for your appreciation.
    I did not exactly get your question. Please clarify it a bit and I will certainly help you out!


    Reply 2 years ago

    I mean to make duty cycle stay in range 0-80%. 20% will be a fixed dead time.
    It should not go 100%. Because if duty cycle goes 100% it will short transformer primary.
    I guess adding a resistor in series in front of discharging part of the potentiometer will do.
    Instructable took 18 days to notify me about your reply hence the delay...


    Reply 2 years ago

    Yes you are right. You only need a resistor. I have attached a pic of your modified circuit diagram. Let me know if you have any questions about the same.
    just in case you can email me on harshad.secondary@gmail.com


    Reply 2 years ago