I managed to steal a bit of time in the past few weeks to build some circuitry for Halloween. I'd hoped to do something with pneumatics, but that didn't happen, so we had to settle for a few smaller things.
I'm also using this as an opportunity to teach Sam how to solder.
We started with converting a typical "Home depot" motion sensor into an inline version. That's pretty simple - take a extension cord, and wire in the motion sensor inline. A little heat-shrink tubing, and it was done. The motion sensor is hooked up to the power for an old boom box that I have, which is playing spooky music from my MP3 player. Come up the walk, and the music starts playing.
Te second project is a bit more elaborate. Right next to the front door, there's a dummy with a pumpin head sitting in a chair. In his (its?) lap, there's a bowl with a full-size Hershey's chocolate bar, and a sign that says "Help Yourself". If you do help yourself, the eyes of the pumpkin light up and a bell rings. This is all based on detecting somebody trying to grab the candy. I explored a few methods of doing this - there are some that use RF techniques to detect the change in capacitance as somebody approaches, but they're notoriously finicky. I decided on an IR approach, and was lucky to find a site that sold the Sharp GP2D15 IR sensor. You've probably come across this sensor in the hands-free faucet in a public restroom. This sensor is very easy to use - it generates a signal when something comes within about 10" of the sensor. The output of the sensor isn't powerful enough to power a relay, so an added 2N2222 transistor does the amplification. That relay runs the bell, and there's a separate solid-state relay that turns the eyes on.
Finally, I bought an Edirol UM-1X Midi interface. This connects to our Roland digital piano, and is used to play back spooky organ music (Bach, mostly) through the piano's built-in Pipe Organ patch.