Analog PWM generator

I was watching a video by W2aew about creating a stair-stepping circuit from analog components, and I felt like trying it out. After starting, I realized I didn’t have a 555 timer, Which is used to create the pulses that step up the voltages. However, I did have some simple LM358 op amps, so I decided to use the circuit for another purpose:

This circuit uses only the first part of W2aew’s schematic, but is modified a bit to provide the correct period and pulse width. Hobby servos use a PWM signal with a period of 20 ms, with pulses ranging from 1 to 2 ms in length. By adding a potentiometer to the circuit , I could change the 2 RC circuits that dictate the rise and fall times of voltages on the op amp. The circuit switches states when the voltage across the capacitor changes from high to low and low to high, compared to the voltage at the non-inverting input of the op-amp. this voltage is created by a resistor divider, and is pulled higher or lower depending on the output. I’ve created a quick schematic of my version of the circuit below.

pwm

 

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