SQUASCILLATOR: Square-wave AVR VCO (4HP)
A square-wave oscillator with voltage-controlled duty cycle - this one is built around a microcontroller, rather than an analogue, core so far fewer components are required to get things up and running and it's far less hassle to calibrate; there's a fine-tune trimmer on the board but most of the heavy-lifting is done by the firmware.
Note the two CV inputs - one has an attenuator (CV1) whilst the other hasn't. Inputs from the two are mixed, and the CV2 input is left floating, but if you'd prefer it pulled to ground there's solder jumper for that.
Like my other AVR-based VCOs, this one will track 1V/oct pretty well over 5 octaves, although if you're expecting 'to-the-cent' accuracy then you may well be disappointed unless you're prepared to hack on the firmware (specifically, fine-tune the values in the pitch lookup table - have fun!)
Apropos of which, the firmware is available for modification and has a permissive license so feel free to hack on it.
Square waves aren't as boring as they seem - with a bit of judicious filtering (resonant or otherwise) you can get some pretty crazy sounds from your bog-standard square, and team a couple of these up with something like the CLOCK-LOGIC module and you've got a recipe for wild and crazy sounds.
The module is supplied complete with power cable and mounting hardware - it sits behind a 4HP panel fabricated from dark purple FR4 fibreglass (aka PCB material) with gold ENIG lettering.