The Tuner is an old project, inspired by the tuner sub-circuit proposed by LMNC in his amazing Performance VCO.
The software I use is the one written by Jos Bouten and can be found at https://github.com/josbouten/Tune-O-Matic.
He also describes his own implementation of LMNC circuit at https://lookmumnocomputer.discourse.group/t/tune-o-matic-tuner/36.
MyModularJourney also made his own version and can be found at https://github.com/MyModularJourney/Tuna.
The circuit and the software are pretty basic and can struggle sometimes finding the actual note. But globally, it’s a very useful tool, which should be installed in any system.
In my version, I use an Arduino Nano. It is responsible for both calculating the tuning and displaying the result.
I chose to not use an opamp, in order to keep the component count low. The Arduino input is nevertheless protected by a Zener diode D5, behind an AC coupling capacitor C2 and a protection resistor R6.
The 7 segment display can be either Common Cathode (CC) or Common Anode (CA) and the jumper S1 selects which type of display is installed.
The 4 jack sockets are passively connected. This way, you can insert your signal on any connector, and get 3 (passive) outputs. So, even if you don’t use the tuner feature, the module can still be used as a simple 1 to 3 passive multiple.