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)

  • Can you measure the voltage that the "circuit board" uses to switch the LED on and off?
    – VE7JRO
    Commented Nov 6, 2019 at 23:31
  • If you are talking about the LED voltage drop, its 2.7v.
    – Sophie
    Commented Nov 6, 2019 at 23:41
  • What voltage does the "circuit board" operate at? Is it the same as the ESP8266?
    – VE7JRO
    Commented Nov 6, 2019 at 23:44
  • As its powered via USB, i assume its operating at 5v.
    – Sophie
    Commented Nov 6, 2019 at 23:47

1 Answer 1


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:

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  • An optocoupler's input takes e.g. 1.7V as Vf : With a 100 Ohms resistor you add another 10 mA load to the LED's 2.7V . Pin 4 = GND, Pin3 = INPUT_PULLUP should be LOW when LED is on, and HIGH when off. Commented Nov 7, 2019 at 0:25

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