Ben Krasnow found a method to control speed of a universal motor (a 120V AC drill motor in this case) by using a CdS photoresistor instead of the original variable resistor used to control the standard triac dimmer circuit that was the drills trigger. The CdS photoresistor is driven by an LED that is controlled by a PWM pin from an arduino microcontroller. This system provides a very easy way to control 120V AC devices with a single PWM pin, and not use any timing code or worry about zero-crossing of the AC power since this is done intrinsically in the circuit. It also offers true electrical isolation between the AC line voltage and microcontroller.
Simplicity at its best.