Using LEDs, LCDs and GLCDs in Microcontroller Projects

5.1 A Typical LED

Figure 5.1 shows a typical LED with the electronic circuit symbol similar to that of a semiconductor diode. The device has two legs: the longer leg is the anode and the shorter leg the cathode. The cathode is also identified by a flat side on the body.

Figure 5.1 A typical LED

The intensity of the light emitted by an LED depends on the amount of forward current passed through the device. The maximum allowable forward current is denoted by I_Fmax. When designing an LED circuit, we have to know the typical voltage drop, V_Typ across the device, and the maximum allowable voltage drop, V_Fmax.

The brightness of the emitted light is measured in millicandela (mcd) and this is usually referenced to the forward current. For example, standard red LEDs are quoted to have brightness of 5 mcd when operated at 10 mA.

The viewing angle of a standard LED is about 60°, although some LEDs have viewing angles as narrow as 30°.

The colour of an LED depends on the type of chemical material used in its construction, and not on the colour of the glass enclosing the LED. As we shall see later, the colour of an LED is specified by giving the wavelength of the emitted light.

Most LEDs have typical forward voltage drop of 2 V, while blue and white ones can have as high as 4 V. The typical operating current is around 10 mA, although some small low-current LEDs can operate at around 1 mA. The ...