i just finished this piece a couple of days ago.
the original circuit was from a rather generic toy keyboard from the nineties. all it consists of is a tone generator with i think 3 octaves including sharps. i think it also had some demo songs but i left them out. i took the signal coming from the keyboard, fed it through a comparator, ran it through a frequency divider, ran that through a filter and finally to a little headphone amp. the signal also triggers a one-shot that triggers an attack release generator that modulates the cut off frequency of the filter. sadly though, i built the one shot out of a 555 chip so it does click when the volume levels arent adjusted properly with an external amp. next time i'll use an op-amp or something. the one shot time can be adjusted with the "gate" pot. this kind of works as a sustain. the main vco can be mixed with two sub voices from the frequency divider. with voice one all the way up, it is the only one you can hear. all the way down and its between voice 2 and 3. there is also a 4 step sequencer that is tunable and can be triggered either by an LFO or the main VCO. both of which can be divided by 1, 1/2, 1/4, 1/8, and 1/16 by another frequency divider. the VCO coming to the sequencer divider is actually 1/16 of the main VCO. the one shot can also be triggered with either the LFO or the VCO.
the case was made from an old touch and tell toy that i had in my collection. i added the keys the two big blue holes with all the functions and a big battery compartment. the lettering are dry press decals with lots of clear coat(notice the glaze?). the keys are color coded. this thing was supposed to be an homage to the resistor but the blue is a little too bright.
the lfo speed can be modulated manually too. there is also a resonance pot, envelope depth pot, a fine tuning pot for higher tuning, a coarse pitch bend, and a volume pot. all in all i am pretty happy with this keyboard despite all the grief it was giving me last week. i still have two more of these VCO chips and i think they will be put to similar use.