How do LCDs work?

Thursday, June 05, 2014

How do LCDs work?

Q. I have seen LCDs in things like calculators and watches. How do these work and why are they used instead of something like LED displays?

A. An LCD basically consists of a glass envelope with a solution inside containing the crystals. This is why it is called a “liquid crystal display.” When light passes through the crystals, they rotate its polarization by 90 degrees. A polarizing filter on the front of the display admits light with only one polar orientation. The light passes through the crystals and is reflected off a mirror behind them. When the light encounters the polarizing ñlter again it is blocked because the crystals have changed its polarization.

Transparent electrodes on the segments are used to apply a voltage to the ones that are to be selected. This voltage changes the alignment of the crystals, so that they scatter the light instead of rotating its polarity. These segments appear to be “off” The segments that are not energized appear black, and thus “on” (In some types of displays, this relationship is just the opposite, so that energized segments appear black and thus are “011,” while de-energized ones appear “off.”) Some types depend on the light from the environment, While others have a light behind them so they can be seen in the dark.

The main reason LCDs are used is that they draw very little current. LEDS, on the other hand, may draw as much as 10 mA in pulses. One drawback to LCDs is that they can be destroyed by DC. They thus require a fairly complicated pulse-drive system that operates them on AC. This is usually not a problem, though, since the necessary circuitry is easily incorporated into an IC. General-purpose display driver chips are available for LCD displays. They are made with CMOS technology, and draw very little current from the power supply. This makes them Well suited for battery operation.

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