I'm trying to interface an Air Conditioner control circuit, but having some difficulties. The AC unit outputs a 0-5v signal (for fan target speed), which would be great for reading - except that its relative to the units GND connection, which is at -132VDC (yes, thats a negative). Measuring between this and signal reads a positive DC voltage, between this is True Earth is -132Vdc. See here for more info on what I mean http://www.mylg.co.uk/data%20for%20site/Prod%20Sht%2083%20-%20Testing%20BLDC%20Fan%20Motors.pdf
So, I've gotten myself some HF11 Optocouplers, since they are linear - or meant to be - I'm simulating the input with a pot for the moment.
The pot feeds through a 260R resistor to the opto part of the IC (pins 1/2). The output side has 1 side (pin 4) to +5v, pin 6 is pulled to ground via a 500K resistor, voltage reading is taken over the resistor.
I dont seem to be able to read any higher than a 0.05V difference between opto fully on, and opto fully off... any ideas?