This circuit sets up the common 555 timer IC to output a PWM pulse that can be varied by the position of a connected potentiometer. PWM (pulse width modulation) is commonly used to control the speed of motors or brightness of LED's. With just a few common electronic components this is one of the simplest PWM circuits you can make.
Step 1: The Circuit
I've included the circuit schematic and PCB layout, the PCB layout may have to be altered based on the transistor you use. The transistor determines how much current you can control, I used a 2n2222a which can supply up to 800ma. The voltages that can be controlled range from 5-15 volts. I used my 555 PWM to control the speed of 3 12volt DC computer fans that use 180ma each.