Decoupling an input, is it possible?

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?


The problem with reading LEDs on other devices is that the voltage may be different on each device. EG, one may have 5v via R to LED to gnd while another may have 12v to LED and R to gnd.

For Arduino input safety you need to isolate each input via an optocoupler. Unless you already know the circuit of the the devices to be monitored, then you can make the circuit to suit.


It should work well just to connect the IRLED of the optocoupler in series with the device's power LED. The current is already limited and the only symptom would be a slightly dimmer power LED. Might not be noticeable because the IRLED only drops about 1.2V.