How to check if a LED is on? I have a circuit board / device that can be in different states. It indicates in which state it is by turning on one of three LED's. Now I want to use an ESP8266 to tell me (over the network) the current state. What kind of circuit would I need for this? (I have thought about using photodiodes, but I would much rather solder something directly to the board)
If the device turning the LEDs on / off runs on 3.3 Volts (has ground to 3.3 Volts outputs), then you might consider directly connecting the 3.3 Volt outputs to the GPIO pins of the ESP8266.  If the device turning the LEDs on / off runs on higher voltages consider either using the appropriate level shifter or the appropriate optical isolator: