I have a very cheap mini drill that uses a small DC motor powered by a 12V wall adapter and I made a little box with a very simple PWM regulator.
The core is a CMOS RC oscillator with variable duty cycle, that can be regulated by a 10k linear potentiometer.
This drives a power mos that switches on and off the drill power supply at around 12 to 13 kHz with a duty cycle that is regulable from few percents to quite close to 100%
The circuit is very simple and can be adapted to different needs. It can be supplied up to 24V or more (depending by the mos you use): a simple zener regulator supplies the CMOS oscillator. I tailored it for a 12V supply, but if you run the motor at 18V or 24V or more you can increase R1 value.
Diode D1 recirculates motor current when the mos is off: I used a standard rectifier (1N4007), but a schottky rectifier would be a better choice.
Despite the fact that I'm not able to run the simulation, the circuit works pretty well in real world: I made two units and they perform as expected.
No more to say. I include the picture of the schematic, a photo of the unit (other photos you can find in the attached rar archive), the Proteus 7.10 files (schematic and pcb layout) , the gerber files (CADCAM.zip) and some photos in Photos.rar archive.
Enjoy! Merry Christmas and happy new year to everybody.
Sorry! I can't upload attachements. Please download all of them at: https://app.box.com/s/vlavp110dty344pwesz9
dec 21th, 2014: I succeded in uploading the most of the attachements. To see the photos, please follow the link above.