LED stands for Light Emitting Diode. LEDs are efficient light sources with various applications.

Since a LED is a diode it conducts, like any diode, current in only one direction. That means polarity is important!

Determining polarity.

  • When you let light pass through a led and look at it, the larger, flag-shaped terminal is the cathode (negative) one, whereas the smaller, I shaped terminal is the anode (positive) terminal.
  • If you look at the led from the top or bottom, the terminal at the flattened side of the housing is the cathode (negative) one.
  • When in doubt, simply take a multimeter and set it to diode measurement. Now, connect the probes and see if the led lights up, or the multimeter displays a forward voltage. Now you also know the polarity form the colors of the probes.

LED light output is proportional to current (and NOT to voltage): if current increases light intensity increases.

Regulating the light intensity by a microcontroller can be done by using PWM (pulse width modulation). The led is switched on and off in very high frequency. The longer the part of a cycle it is on compared to off, the brighter it appears to be.

Most important specifications

  • Forward voltage depending on color of the led:
    • Red: 2.1 – 2.3 V
    • Yellow/Orange: 2.2 – 2.8 V
    • Green: 3.2 – 3.4 V
    • Blue/White: 3.2 – 3.4 V
  • Maximum forward current (typically 20 mA)