Read state from LED in another circuit
Yes the Pi needs a return path for the sensing current.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The Pi needs a ground reference to the original circuit.

If the LED box is fully isolated then you can do this:







simulate this circuit

*Figure 2. For isolated units the contact voltage can be monitored as shown."

With the switch open the Pi GPIO will read 'high' and with the contact closed will read 'low'.