Interfacing with Optocoupler (HF11)

Hi Guys,

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

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?

I would try putting the emitter to ground and connecting the collector to the arduino analogue input and the 500K (surely you mean 470K, you don't get 500K resistors normally) from the collector to +5V.

Make sure you have identified the pins correctly.

Sorry - yeah, you were right about the resistor value - was working from memory!

Cheers! I'm now able to read a 0/5v signal depending on the input value, which is good - now onto the next part.

Apparently these opto's are good for 97% linearity on the input/output ratio (if I'm thinking right?) - but, a 3v input is currently reading a 5v output.

Am I right in thinking that I should be seeing roughly 3v on the output as well? I'm guessing I need to look at the current transfer ratio, and match the output resistance to the fraction of the input required to achieve the linearity of the opto?

Sorry - I'm a bit confused by these things - FET's and trasistors, no worries... but not these!


Yes it is not voltage transfer they produce but current transfer. You have to play about with the resistors on both sides to get what you want.