LED as photodiode

In a LED, light is produced by a recombination of electrons that emit energy in form of a photon, when recombining occurs in the valence band. This process is reversible: electrons are dettached from the valence band by photons entering due to ambient light, allowing a very low current ow between the depletion layers. This effect is many times weaker than that of the inverted normal operation of the LED. We can observe this physical process, we need a darlington circuit with two transistors and a very high gain. The 100k resistor in series with the blue LED is needed to protect the LED agains the reverse voltage of the voltage source, which can lead to a breakthrough in the depletion layer. If now the semiconductor of the LED is luminated by a bright light source, a minimal current ow into the base of the first transistor is generated, this triggers the darlington circuit and the red LED lights up. If the blue LED is dimmed, the darlington amplier closes and the yellow LED gets darker.