 The LED definitely delivers the photons, no doubt. But many are unaware of the LED's ability to detect light as well. Truth. Most light-emitting diodes can also play the role of photodiode. Specifically, a photodiode which responds to the same color light it emits as an LED. Sort of satisfying symmetry. It's less sensitive compared to an actual purpose-built photodiode, but will still pass enough current to be detected by a microcontroller pin. This water-clear LED normally emits red. Since the white light from my phone here includes the red spectrum, I can use it to trigger the LED's photosensitivity, which is in turn detected by the microcontroller. In fact, it's possible to connect two LEDs in a loop. And by exposing one of them to light, produce just enough current to power the other. It's like poetry.