Robot voice modulator
Even though days of phone pranking are pretty much in the past, this device is still worth making. I am talking about a real-time voice modulator that transforms audio input into the famous "robot voice".
Heart of this device is Holtek HT8950 voice modulator which needs only few external components and a power supply to work. Modulator can be built in a hand held box with a battery or as a desk sitting tool as shown in the picture. As the author states, this integrated circuit offers a lot more than the "robot voice", it can shift audio input two whole octaves in either direction which can be accomplished by pushing appropriate buttons. There is even an mp3 file on the project page with a sample of modulated voice. HT8950 can be purchased on eBay and it is very cheap for hours of fun that you will have with this device.
[ Source: randofo ]