Trying to reset, automatically, an HVAC speed control unit. The unit has an output pin H8 that goes to 24VDC when tripped, and 0VDC when running. There is no reliable ground pin (or other ground) that I can use, so I use H5, another output, which is always at 10VDC. Therefore, the differential between H5 and H8 is 14V when tripped, and -10V when running.

The unit can be reset by shorting H1 and H2 (two more pins on the speed control). H1 is always 24V.

I picked up an 8 pin Si826x opto isolator and I am having issues getting it to output anything - would like to know if I'm missing anything obvious.

Pin H8 of the HVAC speed controller is connected to a 1k resistor, and then to pin 2 on the optoisolator (anode). Pin 3 (cathode) of the optoisolator goes to pin H5.

Pin 8 of the optoisolator connected to +5V on an Arduino, and pin 5 of the optoisolator connected to GND on the Arduino. Am expecting pins 6 and 7 on the optoisolator to be at 5V or 0V depending on whether or not the HVAC speed controller is tripped. But pins 6 and 7 stubbornly refuse to be anything other than 0V, whether or not the HVAC speed controller is tripped.

I then tried connecting pins 2 and 3 of the optoisolator to more pins on the Arduino, and still no output on 6 and 7. Then repeated this with a new opto chip, in case I blew something up. Still nothing.

Am I going mad?

  • 1
    \$\begingroup\$ If you look at the spec it says you shouldn't reverse bias the photodiode voltage more than 0.3V. I know you're current limiting this with a 1kohm resistor AND I can't see in the spec what the abs. max reverse current is AND it's equivalent circuit shows an internal reverse diode but I think you may have bust it. Hope you haven't of course. \$\endgroup\$
    – Andy aka
    Commented Sep 15, 2013 at 20:09

1 Answer 1


Start small, one step at a time (old rule!). Check your overall logic with a single red led in series with the opto coupler. This is a way of seeing the internals working. The led will show you what is happening inside the opto coupling. If led logic works, go diagnose the arduino.


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