LED and photodiode – are they really that different? Let’s find out.
LED is a two-terminal PN Junction diode having cathode and anode marked c and a in the diagram. If you supply some voltage across its terminals then the current will flow through the LED. Everyone knows that. A Photodiode is a two-terminal PN junction diode as well. But the working principle is completely different.
In LED, p-type has holes as majority carrier and n-type has electrons. Now you connect a 3V battery across the terminals, it will create a potential difference. So electrons will move to the positive side and the holes to the negative side and current will start flowing through the diode. Eventually, they recombine at the junction.
You know that electron stays at a higher energy state than that of a hole. So the electron will be losing some energy before combining with the hole. This very energy that the electron will be losing will come out of the diode as a photon and the amount of photon and the energy that will come out of the diode will determine the color and strength of that light. So this is how LED works.
If you check the video, then you will see a 3V coin cell and a 5-millimeter blue LED. If we connect the anode and cathode properly to the battery, then the LED will start glowing.
Now we have put this same LED in the breadboard and connected the Multimeter probe across the terminal. The regulator is set to measure a voltage of maximum 2V.
Then we placed mobile near the LED in which the flash is quite strong. As soon as we take the light closer to the LED you will see some voltage reading is coming in the Multimeter. The voltage is very less, not even 1V. But still, it is produced across the terminals. If I take the flash away from the LED then the reading is coming to be 0. So we get a maximum reading of 0.6V that is quite sufficient in terms of electronics.
So you can see that LED can act like a photodiode as well. Cool, isn’t it? 😉
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