It's a Theremin!
Probably my first ever, fun, successful electronics project as a kid.
What's a theremin you ask? Well, here's the Wikipedia Theremin Entry entry. In short it is that sound all the old 50's and 60's SciFi shows used to indicate scary, alien things happening. And I watched lots of old SciFi movies on late night TV. What is now Turner Classic Movies. You know, the "woooWOwoAhhhAHHHAaWOOOooo" sound.
So way back then, the 555 timer IC (integrated circuit) was very cheap and popular. What couldn't you make with it? It was easy to get at Radio Shack and came with sample circuit diagrams for making cool stuff. There were whole books devoted to making stuff with 555s. As a matter of fact, I still have some of them on my home office book shelf.
What do you get if you put together lots of watching SciFi movies, a 555 timer, and a little imagination? ...
the bathroom door theremin!! imagine, someone needs to visit the loo, they walk up to the door, reach out their hand towards the door knob and start to hear a quiet woooWOwoAhhhAHHHAaWOOOooo, as they get closer, it gets louder and more insistent, woooWOwoAhhhAHHHAaWOOOooo. They touch the knob and the sound goes crazy! That was my 12 year old dream come true.
How did it work? As shown in this Instructable, the frequency from the 555 timer depends on the value of an RC circuit (Resistance & Capacitance). In the Instructable, they vary the resistance. In my bathroom theremin, I varied the capacitance. One half of the capacitor is the door knob. The other half is the user reaching out to touch the door knob. As the distance between the door knob and their hand varies, the frequency output by the speaker attached to the 555 circuit varies. Providing a spooky bathroom adventure.