Good grief, all you have to do is interrupt current in the primary circuit when the door opens. The conducting path for the primary is between the door frame and the door hardware. The secondary (HV) side attaches to the doorknob on the inside. When you expect your roommate, you simply energize the primary with a hefty battery. The door frame is circuit 'common.'
You could do it with a 555 and a transistor. I once made a friend an electric fence driver with an old valve radio transformer and an ignition coil. On one half cycle the HV winding charged up a cap. On the next half cycle the 6.3v winding triggered an SCR which put the cap across the coil. You just had to get within half an inch and a spark would jump across onto you. My friend was called Frank, so we called my device Frank's Zapper...
There are a few details missing here, since, in the era that he plated this trick, automotive ignition coils did not include any interrupting means integral with the coil assembly. So we could assume that it was probably one of the very old Ford Sparking coil devices. If not, then it could be useful to know just how a more current coil was driven to work in this application.
January 2016 Cartoon Caption ContestBob's punishment for missing his deadline was to be tied to his chair tantalizingly close to a disconnected cable, with one hand superglued to his desk and another to his chin, while the pages from his wall calendar were slowly torn away.122 comments