I was trying to think of a quick toy I could build for my kids. I came up with this idea of an ultrasonic range finder, an Arduino, and a speaker. Basically the Arduino plays a tone out of the speaker based on the distance from the range finder. The result works pretty well and can be built in minutes. You can play this from anything from your entire body, to just your feet.
Step 1: Parts
2. Range Sensor (I used a Parallax Ping))) that I picked up a Defcon a couple of years ago, anything like this would work as well http://www.sparkfun.com/products/639)
3. A small speaker (I pulled mine out of an toy moose that sang Blue Christmas)
Ultrasonic range finders basically work by sending out an ultrasonic signal and then they sense how long it takes to come back. Think sonar for a submarine.
Step 2: Schematic
The Schematic is pretty simple. Speaker to Arduino pin 11 and ground. Ultrasonic range finder to power, ground, and Arduino pin 7
Step 3: The Code
The code is pretty simple. I just combined the Ping demo and the Tone demo that come with the Arduino UI. I did it a couple of different ways. You can either take the distance in inches or cm and multiply it by a number (i.e. inches * 10) to get a frequency that's playable. Then as you move around you'll get different frequencies. To make it more playable I got the frequencies of actual notes and told them to play that note when the distance was within a range (I used 20 cm for each note range). Then you just have it play the note. I've attached the sketch.
Step 4: Now Dance!
There are many ways you can "play" this organ. You can step in front of it. Put your hand or other object in front of it. Or, like my kids, just flail around in front of it and it makes music.