83 Liquid crystal displays

Liquid crystal displays (LCDs) are used in many applications, such as laptop computers, pocket calculators, vehicle instrument displays, watches and clocks, and increasingly in diagnostic equipment for automotive systems. The portable oscilloscope that features prominently in this book uses a liquid crystal display and readers may wish to have an insight into the principles of operation of the LCD.

A liquid crystal that is commonly used in LCDs is one that is known as a twisted nematic (TN). This crystal is composed of rod like molecules that are readily polarized by the application of an electric field. When a layer of TN material about 10 ^m thick is placed between two transparent electrodes, a capacitor is formed and the application and removal of an electrical potential to the electrodes alters the polarity of the rod-like molecules and this in turn, affects the light transmitting properties of the LCD cell.

In the simple LCD cell shown in Fig. 8.3 the electrode plates are made from indium doped tin oxide (ITO), which is transparent. A thin layer of plastic material covers the inside surface of the electrodes and during the manufacturing process, microscopic grooves are introduced into the plastic by a rubbing process. These grooves are mutually perpendicular, as shown by the sets of parallel lines on the upper and lower sections. The effect of these grooves is to cause the rod-like molecules to rotate smoothly through 90° across the width of the TN layer. The outside of LCD cell is covered with glass and on the surface of the glass is a polarizing filter. The left-hand view shows the electric current removed from the

LIGHT DARK

Fig. 8.3 A simplified LCD cell

LIGHT DARK

Fig. 8.3 A simplified LCD cell

LCD cell and this allows the TN molecules to guide the light through 90°. If a reflector is placed on one side of the cell, the reflected light makes the cell appear light. Alternatively, back lighting will also shine through the cell. In the right-hand view, the current is switched on. This causes the rod-like molecules to align with the electric field and the passage of light is effectively blocked. This causes the cell to appear dark.

In order to produce a large display, suitable for a computer or oscilloscope screen, it is necessary to have a large number of cells arranged in a matrix form. Figure 8.4 shows the concept.

Do It Yourself Car Diagnosis

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