Wiring the LED to the nRF52840 feather is very straight forward.

The LED will generate a voltage when it detects light. Therefore, the LED anode is connected to A0 which is 1 of the 6 analog to digital pins on the Feather that can read voltages and convert them to a 16 bit value. The Feather’s ground and the LED cathode are connected to a ground rail on the breadboard. As is, this circuit would be very noisy and unreliable. Therefore, a filter is added which is comprised of an 82nF ceramic capacitor and a very weak 15M Ω pull-down resistor. Both are placed between the LED anode and ground. That’s all it takes for the detector circuit. The Feather should now be able to read a voltage change on A0 when a bright blue light hits the attached green LED.