I’m sorry, I wasn’t meaning for you to have to remove the optocouplers. The LED’s polarity should be tested using your meter’s diode tester. With the red probe on the LED’s square pad and the black probe on the round pad, the meter should read around 1.8V (which is the forward voltage drop of the LED). If it reads around 1.5V then reverse the probes and check again. If when reversing the leads (red on round, black on square) it gives a reading of 1.8V, it means the LED is in backwards.
The LED’s turn-on current is rather specific for this application so please don’t use just any 5mm (not 3mm) green LEDs. I don’t remember the specs off the top of my head, so if you do need to replace the LEDs then let me know and I’ll try to find the specs you need to use…hopefully you don’t have to replace the LEDs.
If the LEDs in the optocouplers were in the correct orientation, I was going to have you make a small cut in the heat shrink so you can see what the LEDs are doing as the treadle is moving. You can always repair the cut with caulk or sticky tack or opaque hot glue or something. But this will give you a good idea if the problem is with the digital circuit or analog circuit.
-Brach