Component Parts List:
T1 = primary 115[220]/secondary 8 VAC transformer. Center Tap not needed.
Q1 = 2N1613, NTE128, or substitute. (TO-39 case) On coolrib!
BR1 = 40V, 4A. (Check max current of your mini-drill and add 2A)
R1 = 470 ohm, 5%
R2 = 1K, 5%
P1 = potentiometer, 10K
C1 = 1000uF, 25V

Notes:

• C1 filters the noise and spikes off the AC. If you find the circuit output too noisy add another electrolytic capacitor over the output terminals. Value can be between 10 and 100uF/25V.
• The output voltage is variable with the 10K-potentiometer.
• The transformer input voltage refer to your home power source.
• Mount the transistor on heatsink / cooling rib to prevent overheating.

Circuit design by Tony van Roon