An LED generates light by using its semiconductor materials. Basically, the presence of electrical current forces the electrons and holes within the P- and N-type materials of the LED to recombine, thus producing light energy. (See Back to Basics-Part 1, April 1995 issue for discussion of P- and N-type materials.) This is graphically shown in Fig. 1.
Certain types of LEDS produce infrared light (light not visible to the human eye) while others produce visible red or green light. A plastic lens magnifies the visible light produced so that it's usable for indicating purposes. Different colors are provided by specific colored lenses.
As shown in Fig. 2, an LED has an anode and cathode. The cathode lead is always located at the LED's flat or notch side. Also shown in Fig. 2 is its symbol. Note the direction of the arrows (away from the diode); this is different from that of a photodiode, whose symbol has the arrows pointing toward the diode.
An LED must be forward biased to produce light. As shown in Fig. 3, a resistor is usually present to protect the LED from excessive current.
Applications: Because an LED is so inexpensive, has an unlimited life span, and uses so little power, it's ideal for all types of applications. The predominant use is that of a status indicator, showing ON, OFF, or malfunction.
LEDs are also used as indication of readout on test equipment, usually in the form of a 7-segment display as shown in Fig. 4. Each segment has several LEDs, with a colored plastic overlay on the segment making the individual LEDS glow as a unit.
When an alphanumeric display is needed (symbols and/or alphabetical letters), a dot-matrix display consisting of a 5 x 7 layout of LEDS as shown in Fig. 5 is used. Here one LED is used for each dot of the matrix.
What to know about LCDs
As shown in Fig. 6 (see on page 118), an LCD consists of two pieces of glass, each coated with a very thin, transparent coating of metal, separated by a liquid crystal material called a nematic liquid. The coatings face the liquid. The front glass coating is broken into segments, with the segments brought out through a separator seal to the edges of the display; these are the electrical connection points for the circuitry that drives the display. The rear glass coating covers the entire active surface of display.
The nematic liquid's peculiar behavior is what makes an LCD work. Under normal conditions (no voltage applied), the liquid's molecules are parallel to the plane of the glass. When a voltage is applied, the liquid's molecules rotate 90[degrees], thus altering the passage of light through it. Black or clear digits are generated by using a polarizing filter, a filter that allows light to pass only at certain angles.
LCDs require special operating voltages (an AC voltage with no DC present); applying the wrong voltages (a DC voltage, or an AC voltage with DC present) will reduce the life of the display.
Source of more solid-state
For more detailed information on solid-state devices, their construction, operation, and testing, we suggest you read Solid-State Fundamentals for Electricians by Gary Rockis, published by American Technical Publishers. The book can be ordered by calling 1-800-323-3471.