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

Since an 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 from the colors of the probes (when they are connected correctly of course).

LED light output is proportional to current (and NOT to voltage): if current increases light intensity increases. The LED driver circuit or a suitable series resistor take care of limiting the current.

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)
• Size: most common are 3 mm and 5 mm diameter.