Liquid Crystal Displays (LCD's)


Concepts Shown:

LCD's and their uses.


A Duracell copper top tester, thermosensitive postcards, and an LCD watch (one from a student is fine).


Pass a sample of the copper top tester around the class along with an AA cell, so that the students can see the color change on the face of the tester as the battery is placed into the tester through the back of the package, and the top and bottom of the tester squeezed for about 10 seconds. An explanation of how the tester works should follow. Pass around the thermosensitive postcards and talk about how LCD watches function.


Liquid crystals are fluids consisting of stiff rod-like organic molecules which can form structures with some order. They therefore exhibit some of the properties of crystalline solids, hence the name. Ordering of the rod-like molecules means that their bulk properties are highly anisotropic. It is this anisotropy that is exploited in display devices. The figure below demonstrates a seven-segment cell that could be used in a digital watch or pocket calculator. The liquid crystal substance is sandwiched between transparent electrodes and two sheets of polarizers. With no field the molecular order is influenced by molecule-molecule interactions, and light passes through the cell. However, under a d.c. or an a.c. field the molecular alignment is dominated by the external field. As the liquid crystals realign themselves in the field, no light is transmitted. [eq]. The copper top tester has a strip of thermosensitive liquid crystals on the front. On the back is a tapering resistance strip. When the battery is in place, the variable resistance strip heats each part of the crystal strip to a different extent. As the crystals realign themselves, there is a color change (black to yellow) which indicates whether the battery needs to be replaced. The thermosensitive postcards work in quite the same way. However, the heat or cold from ones hands cause the alignment of the crystals, as opposed to a variable resistor. Remarks: This is a very inexpensive demonstration to perform. It is also very educational if people begin to understand the simple principles that run their watches and other LCD gadgets.


Rahul Pinto

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