Playing small motors from synth and arduino

[van_der_decken]

@hk_jh

does the ground of J3 should be connected to the psu and mosfet ground or to gnd1 (arduino ground, pot ground and optocoupler ground)

J3's ground should go to what you're calling GND1. Since J3 is an input to A0, it has to use the same ground as the Arduino in order to complete the circuit.