This small circuit board, based on the ATtiny85 microcontoller, fits onto half the pins of the Nanocomp's I/O connector.
The circuit has the microcontroller as the only IC, with decoupling capacitors, an LED for debugging, a reset switch, and connectors for the Nanocomp expansion port and for connecting an AVR programmer via the usual 6-pin ICSP interface. The ATtiny85 has just enough pins that none of them need be shared, and the internal RC oscillator is plenty accurate enough to generate a reliable audio signal for the Nanocomp: after all, we're comparing it with the motor of a cheap cassette recorder.
I built the prototype with my usual construction technique, a combination of stripboard with additional connections made using 30AWG insulated wire intended for wire-wrapping. The fine wire is easy to strip with a thumbnail, and with tweezers it's possible to loop it around the leads of components before soldering. Basically, these are like mod wires on a PCB, but used as the basis of protoyping.
Most of the track breaks coincide with a hole in the board, and can be made with a drill bit or vero tool. Under the ICSP header, breaks are needed between one hole and the next. I use a small craft knife for this, scoring two parallel lines and angling the knife so a thin sliver of copper is removed.
Images of the completed board:
Those with access to one-off PCB prototyping facilities may prefer a different method of construction.
[The board shown here is the second one I built, with a neater and electrically better layout than the first board, the one shown on the page about the Nanocomp clock.]