Eurorack sequencer: Input regulation and output buffer

Hi guys, I'm building a Arduino based step sequencer (8 steps, 3 tracks) for my Eurorack synth. You can find the planned schematic attached.

Some explanations for the context:

  • The circuitry from pin 3 of the "CV In" jack serves to detect the audio jack (which works really great).
  • R3, R4 and the zener diode are meant to prevent voltages higher than 5.1V on D4.
  • The Neo Pixel visualizes the steps, tracks and modes.
  • Track1-3 control switches set the steps (hits) on each track.
  • Track1-3 output jacks are sending binary gates or triggers (I want to send them to envelope generator modules).
  • The prototype runs on an Arduino Uno and I will ultimately implement it on Arduino Nano.

My questions are:

  • I tested the voltage regulator circuitry (R3, R4, ZD1) without having it connected to the Arduino. With an input of +12V it resulted in ~5V, which is fine. When I applied -12V, I still measured around -0.7V. Can this negative voltage already toast the Arduino and if so, what would be an improved circuitry from your perspective?
  • Would you recommed buffering the output (with a transistor, op amp, etc) before sending it to other modules instead just having R10-12 with 1K Ohm each or is it fine as is? When I tried it with one output on my Eurorack envelope generator, it at least worked.

I thank you in advance!

The input should be OK. Probably the safest way is to use A quad op-amp such as the LM324 or two duels LM358 very inexpensive devices. The simplest way is to set each as a unity gain amplifier i.e. "+" is input - connected to output and output output. If you want to minimize drift, offsets etc (I do not think you will need to) Use a 20K in place of the 1K and the "-" to output use a 20K resistor. You can run the opamp on the 12V supply if you like. The output will swing will be dependent on its load. Logic levels will have no problem.

For the record:

I just found this article "… Pin Over-Voltage Protection Using a Single Resistor!" (, Arduino, Programming, & Electronics: 100+V Arduino/AVR Microcontroller Input/Output (I/O) Pin Over-Voltage Protection Using a *Single* Resistor!) which recommends putting at least a 10K resistor to cover -10V to +15.5V before the pin. Will do that as an addition to the Zener circuitry, as he points out "Redundancy is your friend" :slight_smile: