Hi, i am going to control multiple devices (2) with my Arduino, i have a relay board attached to the on/off switch of the two appliances, tre relay boards are safe and they have optoisolators.
For the input i am going to use a technique that i have already used successfully, i take one wire from the ON led of the appliance and the ground, i put the ground in common with the Arduino ground and read the ON/OFF state of the appliance to read it's state.
However i am a little worried (but maybe worried is too much) to do the same thing with multiple devices, since it implies to put all the grounds together, is there a way to read the presence of signal from the led without having to give the grounds in common?
I know that a way can be using a photoresistor to read if the LED is ON, but to get a good reading i must masquerade the led from other sources (room light) and it makes the led hidden to a normal eye sight.
Also using an optoisolator backwards (excited using the led current) may not be possible, drawing current after the led may not be a good idea, and the led in the optoisolator may not light up.
Now that i think about it, maybe putting the led output into a transistor and driving optoisolator from the transistor could be a good way. Any advice?