TCM5089 DTMF tone generator with VCO tone control. Great mono- and multi-tonic sinewave source for synth projects! Couple the VCO to a digital potentiometer and arduino for digital control? Anyway, here it is with a digital delay board and amp:
Schematic is pretty simple: look up the datasheets for the involved IC's and combine; it's really that simple. Up next for the TLC: arduino tone activation via the keypad inputs.
if it had stuff like schematics or stuff then it could be a bit better-rated, but I like it!