The Sidekick: Build it!

Schematics and PCB

The Sidekick is a simple circuit:

The AR envelope generators are simply passive RC filters filtering a 0-5V pulse created by closing the trigger switch (when left open, the line is pulled down to ground with a 1k resistor). A pair of diodes allow a different (variable) resistor value to be used for the charge and discharge of the cap. The output is then buffered into an op-amp.
The LFOs are made with a pair of op-amp, one acting as a comparator with hysteresis, the other as an integrator. “Ramp up, hit threshold, change polarity, ramp down, hit threshold, change polarity” and we get a triangle ; the square wave is taken straight from the comparator output.
The CV mixing is done with an inverting summer, and an inverter which restores the original signal polarity. DC signals summers can be tricky to get right in single supply designs (it’s much easier with AC signals!), but here we take advantage of the fact that we have exactly 3 inputs to sum, and pretend that the signals are referenced at Vcc/4: the output is (do the maths) exactly (4 . Vcc / 4 – Va – Vb – Vc) = Vcc – Va – Vb – Vc, which is reversed to Va + Vb + Vc with the next stage, an inverter referenced at Vcc / 2.

You can find the Eagle files for this board in the sidekick/hardware_design/pcb directory of the source code hosted on github.

The schematics in PDF format are here.

In bold, the editors’ picks

Bwa bwa bwa!

Rebranded the envelope generators as AR instead of AD.
Since the 10uF caps are going to be bent on the board, left more space around them.
Silkscreen tweaks.
Changed the direction of the LFO pots. This feels weird, but at least we get them to oscillate in the audio frequencies on more than 5% of their range!