When the EDM machine has no spark voltage, a touch probe current goes through the machine and activates a buzzer and a light to indicate the electrode is touching the workpiece. When EDM spark is active (90V), the discharge circuit is isolated from the arduino to prevent damage. Uses opto-couplers and a DPDT 6V relay

Circuit also includes a PWM buffer amplifier to a servo control valve +/- 2.5V and an input from a sony glass scale reader amplifier to go to the arduino.