Today we will build a circuit that uses an LED in a curious way - to detect brightness.

We all know that LEDs have a certain color but some of these LEDs can also detect light. Obviously they do not have the same efficiency to detect light as a photosensitive resistor, a photo transistor, a photo diode or other component specifically designed for this purpose.

In the circuit that was designed we used a green LED to detect the brightness and a red LED to indicate that the luminosity has been detected.
The rest of the circuit that is composed of two transistors that amplify the output of the green LED to serve as a light indicator.

One of the interesting applications for this circuit is a follower of sunlight since the LED only detects the light that directly hits the tip of the LED. It can not detect light unless it is directly facing the light source.

This circuit has been adapted from a circuit that is in the following link.



Bill of materials (BOM)


  • 1x 470 Ohms Resistor (R1)
  • 1x 47K Ohms Resistor (R2)
  • 1x 220K Ohms Resistor (R3)
  • 1x 5mm Green LED (D2)
  • 1x 5mm Red LED (D1)
  • 1x PNP 2N2907 Transistor (Q1)
  • 1x NPN 2N2222 Transistor (Q2)

IC/Components Pin-out


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