Hello, I've been wanted to make a PWM module for a DC motor for a micro controller powered quadcopter. Now it might be easier to just punch the code for PWM, however I would like a voltage of more than 5 volts on the motor. That wouldn't be a problem to adjust the components and source. The problem is that currently I am building the circuit in a simulator, and got stuck at a problem. The purpose of a PWM is to adjust it on the fly (literally) for a variable RPM and the circuit is currently built with a mechanical potentiometer. What I would like is to use transistors as they can act like a variable resistor, however I can't seem to think of a solution that doesn't involve op-amps and think there might be a more efficient solution. [Edit: I think maybe a MOSFET, but don't exactly know how they work.] Here is a link to the circuit in question. The 25 ohm resistor is the motor (don't know an equivalent circuit for a motor, so I just put a linear one, and gonna test it later), using a darlington pair and a 555 timer. The idea is to replace the potentiometer that controls the PWM, with a component or components to act like the potentiometer currently connected.