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.
Sometimes, it's the mundane things in electrical and electronic devices that make the difference. The strain relief on an air conditioner's line cord saved the day when the unit fell out of the window.
BMW Welt and BMW Museum, located next to BMW headquarters and its engine/automobile plant, has reportedly become a popular destination for tourists visiting Bavaria, almost rivaling King Ludwig's 19th century Neuschwanstein Castle.