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

Components-

Lm358

DIP8 socket

470uF electrolytic capacitor

1N007 Diode x2

10k 10 turn trimmer

perfboard.

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

Components-

LM358

DIP8 socket

470uF capacito2 diodes

2 10k trimmers

perfborard

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.