Thanks for sharing your skills in fabricating the pen and power supply. While I'm a firm believer that people need to take responsibility for their own necks when dealing with mains power supplies. I noticed that you might want to caution others about the high voltage capacitor that can maintain a voltage even when the mains supply has been disconnected from the microwave. The capacitor can be discharged by shorting out the two poles with a insulated screw driver. Thanks again for your generosity in sharing.
GE Rotate On/Off Dimmer Model number. 18021 That is what I used. Just connect (solder) the 2 wires from the transformer directly to the female jack terminals.