Module tester


Several problems were faced during the development of our Eurorack modules:

We solved this the Mutable Instruments way, rolling up our sleeves and designing a simple box able to provide power and all kinds of test signals (clocks, gates, CVs, audio tones) to a module. This tool is made available under cc-by-sa (hardware) and GPL (firmware) licenses.

Building it

Schematics and layout

Schematics. Eagle .brd/.sch files hosted on github. Top and bottom assembly.


Bill of Materials.

Assembly notes

The large capacitors on the bottom of the board should be bent and rest against the board (rather than be mounted vertically as it was done on Ambika). The same thing applies to the NP capacitors on the top side. See this picture:


Fuses: EFUSE = 0xfd ; HFUSE = 0xd4 ; LFUSE = 0xff

Pre-compiled .hex file.

Source code. Build with make -f module_tester/makefile bootstrap.


Set the CV output to “2 Oct arpeggio” with a period of 2s or 5s. It cycles between a 1V output and a 3V output. Adjusts the two trimmer to accurately get these voltages. Note that the MCP4822’s linearity is not fantastic - so don’t be surprised to get 1V and 1.997V or 2.003V when you change the CV output to “1 Oct arpeggio”. The chance is, the analog circuitry on the other end is much less precise than that anyway!

User manual

Power generation

The tester must be powered by a 12V AC external power supply (Wall-wart) with a 2.1mm jack. The red stripe of the ribbon cable (indicating the -12V pins) must be oriented like on this picture:

Because the tester uses half-wave rectification, and input capacitors and heatsinks of moderate size, it is not recommended to draw more than 200mA from the supply rails.


The tester provides 4 channels of signal generation: clock, gate, CV (LFO or stepped notes), and audio. The tester provides 2 channels of signal analysis: audio signal frequency measurement (with display in Hz, Note units, or intervals), and gate/clock timing measurement.

Modifying settings

Each of the 4 signal generation and 2 signal analysis functions is materialized by an output or input jack, a LED, and a button. To modify the settings of a generator/analyzer, press the button under its I/O jack. The LED indicates which generator/analyzer is being configured. The display shows a list of parameters. Turn the encoder to scroll through the list. Click the encoder to modify an option. Notice that the ‘>’ arrow is now displayed in front of the value. Turn the encoder to modify the value. Click the encoder to confirm. For example, to modify the frequency of the audio signal generator to A3, press the AUDIO OUT button. The display shows:

>shape      sine
 freq.        C5

Turn the encoder to select “freq.”:

 shape      sine
>freq.        C5

Click the encoder to modify the value:

 shape      sine
 freq.      > C5

 shape      sine
 freq.      > A3

Click the encoder to confirm.

 shape      sine
>freq.        A3

Settings reference

Clock signal generator

Gate signal generator

Control Voltage generator

Audio generator

Audio input

The encoder selects between 3 units:

Gate input

The first line displays either the period, frequency, and tempo of the incoming signal. The second line displays either the duty cycle, or the duration of the positive edge of the signal. Note that the Gate Input LED blinks with the input signal.