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My first try was based off schematics for the Atari Punk Console, a modified version of an old Forrest Mims diagram - those project booklets they used to sell at Radio Shack.

I had some old parts already lying around, mostly from Radio Shack. For the rest I went to Mouser. Radio Shack tends to be a ripoff and poorer quality.

The radio shack board was actually handy since it had 2 tracks down the center for + and - that ran nicely between the pins on the 556 timers. The board was big enough that I decided to make a double APC - two complete circuits running off the same power and mixing into the same output.

I ended up just taking the audio output for circuit 1 and running it directly to the output for circuit 2. The tic marks on the edges of the board are where wires were intended to be soldered to power, potentiometers, or audio out.

I ended up having to buy an enclosure from Radio Shack, because I forgot to put it on the Mouser order. It was a little cramped, and having parts mounted to both the lid and the box proved to be a big hassle, especially with the solid copper wire I had on hand. The copper wire tended to break without much provocation. Lessons for next time. I used Switchcraft 1/4 inch jacks - they're good quality and these will get plugged and unplugged a lot. The output (on the right) has the ring soldered to the sleeve since it's only a mono out. The left side is a bit different.

I decided to have Circuit 1 be controllable like a regular APC, with knobs on the box. The upper knob jumps between intervals (sort of), and the lower knob tunes up or down. That's not an exact description but it doesn't take long to get a feel for it. Circuit 2 I decided to make modular, both because I had my pedal idea in mind and because there just wasn't room for two more knobs. I used a stereo 1/4 inch jack. 9 volts is sent out on the sleeve, and control signals that would come from the interval knob comes back on the ring, tuning knob on the tip.

So anything that applies resistance between the sleeve and the ring/tip can be used as a controller for circuit 2. I built a simple box to supply them with knobs.

At that point, I had a working two tone instrument with room to add control modules. See pedal and keyboard.

Parts list:

You can paste these into Mouser's BOM Import tool, then pick and choose what more / less you want. Many of them were suggested in this project page. They may be outdated, just find something equivalent.

660-CF1/2CT52R102J - 1K resistor. Order another one to put in series with the blue LED if you use it.

660-CF1/2C103J - 10K resistor

660-CF1/2C472J - 4.7K resistor

140-HTRL160V10-RC - 10uF capacitor. This is the big green guy .

140-50Q5-103Z-RC - .01uF capacitor

140-50V5-104Z-RC - .1uF capacitor

31VJ501-F - 100k potentiometer. This is the volume knob. It has a logarithmic (audio) scale.

511-NE556 - 556 timer. A 556 timer is two 555 timers in one chip.

31VA505-F - 500k potentiometer. One each of these for the interval and tuning knobs. They have a linear scale.

502-12B - Stereo 1/4 inch jack. For the audio output. I used an extra one of these for control signals.

502-712A - 2.5mm power jack. This fit with the standard 9V supplies for my pedals and synths. Has a switch to allow battery power too.

859-LTL42CB6N - Superbright blue LED. Put a 1k resistor in series.

45KN013-GRX - Knob. It was cheap and looks nicely classic. Suit your own style, but make it fit 1/4 inch shafts and have a set screw.

You will also need an enclosure, circuit board, and toggle switch. I later used 854-PR405X205 for the board in the Mk II.