I have been playing with the Weird Sound Generator by Ray Wilson at MusicFromOuterSpace.com. The WSG, as it is affectionately known, is a very cool device with very few parts. It is based on the CD40106 Hex Schmitt Trigger chip. Basically, the triggers are set up as oscillators, one in audio frequency and the others at low frequency modifying the first. It also includes a simple LM741 op-amp filter. The variety of sounds that it produces is amazing. There is a pretty active community of people building and modding WSGs. MusicFromOuterSpace.com sells a bare circuit board and a kit with all the parts, but I made mine from scratch.
Here's a video of it in action:
Many people make works of art out of their WSGs, building them into strange enclosures, like lunch boxes and teddy bears. Mine is a bit more utilitarian, but I rather like the "finished" look, and the fact that it is compact and easy to play. I wanted the device to be self contained, so I used an enclosure that has a built-in 9V battery compartment. Being self contained also means not requiring an external amplifier, so I added a LM386 amplifier chip and a speaker in addition to a 1/4" jack. I added a switch to turn the filter on or off, which is something not in the original design. You can turn the filter cutoff all the way down, but it is not the same effect as turning it off completely. Another modification I made is a cutoff pushbutton. This button cuts power to the oscillators and filter, but leaves the amplifier on. This slowly drains the 220uF cap across the power to the CD40106 and LM741 resulting in a weird trailing-off of the sound.
The finished device has no labels, but the switches and knobs are color coded so it is easier to tell which switches and knobs are related. The controls from upper left to lower right (using WSG terminology) are: power, cutoff, filter, unusual, wackiness, wacky too, filter cutoff, resonance, wacky frequency, zany frequency, zaniness, weird frequency.
Click on a picture for the full-sized version.