I am building a simplified version of this circuit:
As far as I can tell, it is a pretty standard design. The load will be a dumb resistive heater (this is a sous vide project).
For now, the optoisolator labelled 3042 is connected to the controller, and I have code which flips the LED on and off when I manually send a command to the controller. The triac and a load are also connected. The second optoisolator and the thermometer are not yet connected. There is a mistake somewhere, because the load does not turn on.
I measured the circuit, and found that the optoisolator has 230 volts on its output, no matter what signal it receives, i.e. both pins 4 and 6 are always 230 volts. I think that the low-voltage side works, because the LED labelled "cooking" goes on and off when it is supposed to. I wonder if the constant high voltage means that the optoisolator is broken, or is it more likely that my problem comes from some wiring mistake?
So, my question: Is there a situation in which a correctly working optoisolator can be always turned on, no matter what signal arrives at its LED?