Light Emitting Diode - LEDs

Light Emitting Diode - LEDs

Optoelectronics is the branch of electronics that is concerned with the conversion of optical signals into electrical signals and vice versa. Optoelectronics also covers the coupling of optical and electronic signals. Knowledge of optoelectronics forms the foundation for its application in the area of communications engineering and in the transmission of optical signals along optical fibres.

The optical signals can lie within the range of visible light. In principle, however, the range of optical radiation goes all the way from the infrared to the ultraviolet region.

The commonly used light-emitting sources are:

  • LEDs in various visible colours
  • IR LEDs
  • OLEDs 
  • Laser diodes

The commonly used light sensitive receivers are:

  • Photoresistors
  • Photodiodes
  • Phototransistors
  • Photothyristors
  • Solar cells


Light Emitting Diode - LEDs

LEDs (light emitting diodes) exist in various colours. The most common of these are red, green and yellow LEDs. In recent times, blue and white LEDs have also become available. In addition to the visible colours, there are also LEDs which emit light that is invisible to the human eye - infrared light.

The current-voltage characteristics of LEDs differ only slightly from those of normal diodes. The threshold voltage of the characteristic depends on the semiconductor material used and hence on the colour of the LED.


 ColourSemiconductor Threshold voltage
 IR Aluminium gallium arsenide 1.3 V
 Red Gallium phosphide 1.6 V - 2.1 V
 Orange Gallium arsenide phosphide 2.0 V
 Yellow Gallium phosphide 2.2 V
 Green Gallium phosphide 2.4 V
 Blue Gallium nitride 4.0 V - 4.5 V
 White Indium-gallium-nitrogen 3.5 V


The radiated power is, to a good approximation, proportional to the diode current. LEDs do, however, have limiting values which must not be exceeded, or else the component will be destroyed. In each case, the manufacturer's data sheet should be consulted for the exact data.

In our circuit, the IR LED is protected from excess current by a series resistor R7 of 402 ohms and its cathode is already connected to ground.
LEDs are, furthermore, not very tolerant of reverse biasing. For this reason, an additional diode D3 is used in the circuit to keep negative voltages away from the LED.

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