What do you meen by Reference point? Wich wires i have measured?I have used a multimeter and stuck the pins of it in the red en white socket utill connected to the wires.When when the LED went on the mulitimeter read 2 V and when the LED went off 0.
If hard wiring does not work and i burn 1 or 2 arduinos i wil try the LDR.
Your idea of LDR has crossed my mind. But i thought first to do it hard wired for the stability of the system.
You all seem cautious about connecting arduino directly to the machine.