Drum Triggers (Piezo Sensors)
Piezo Schematic Symbol
In electronic drums, including those used by music videogames like Rock Band or Guitar Hero: World Tour, drum hits are registered via a piezo sensor.
Piezo Sensors
Piezoelectric sensors are crystals sandwiched between two metal disks. When the crystal is deformed (e.g. struck), it produces a small amount of electricity. With an amplification circuit (such as the one found here), this electricity can be used as a signal or trigger for other devices, such as a toy. If you need a trigger to last for a while, try a 555 timer, running the output of the amp circuit into the input of the 555 timer. This will ‘hold’ the signal.
Piezos also work in reverse, as a sort of cheap speaker. When you run electricity through the crystal, it deforms. Run rapidly alternating current through it and it buzzes. Replacing a toy’s speaker (or adding a piezo in parallel) could lead to interesting sounds, though many will be higher-pitched as a result.
Finally, instead of using one as a percussive trigger, piezos can also work as microphones. Some are even used in acoustic guitars to run them to a PA. For a very cheap and easy toy-to-mixer bridge (with some potentially interesting distortion), stick a piezo on the body of the toy or on the speaker itself. Run the leads to a 1/4 jack and plug that into an amp or a mixer.
Where to find:
All Spectrum carries a bunch of cheap varieties.