How to Read Relay State using Digital Pin?

Hi everyone, I am having trouble reading a value here in my setup:

There are 2 arduino uno rev3 units:


  • has a 12v Relay Card connected to it to control a warning light (on/off) if certain conditions are met (such as if a Condition is satisfied, ARD2 will send signal to Pin 7 then pass it to the relay card to activate the warning light).


  • is located in another location, it needs to read the state of the Relay1 Only(in the Relay Card) , not the Pin 7 . ARD1 will use Digital Pin 8 (as input) to read the state of the Relay1 (if it is triggered or not, 0 or 1)

Problem is, I don’t know where in the Relay Card will I connect the ARD1’s Digital Pin 8 & a reference ground to do the reading, please help me with this . Can I use the Dry Contact Port of the Relay Card?

below is my setup diagram .

As I read it, you want to confirm that the light is receiving the voltage from the battery.

I presume you will have a pair of wires between the two arduinos.

Simply use an opto connected to the lamp such that it is lit when the lamp is lit. The output then acts as a switch to the monitoring arduino.


what if i removed the light from the relay ? replace it with only the Pin 8 and ground from ARD1 then connect it to the RELAY1 NO and COM, when the NO and COM are shorted, will they activate pin 8 from ARD 1?

I assumed you wanted to get feedback that the light is being powered. The only way to be sure, you need to activate the signal from the actual output.

If you don't actually need to know that the relay has operated, but was simply told to operate, you can turn both relays on at the same time and use one for the light and one for feedback.

Otherwise, give a full description of what you want to do.