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The Twisted Nematic LCD
Schadt and Helfrich, 1970. Every other display in this batch makes light; the LCD is a valve in front of a backlight. Rod-shaped molecules line up with a rubbed surface and twist a quarter-turn between two plates, carrying polarised light round with them — the cell is clear. A few volts stand the molecules up, the twist vanishes, the light is blocked. A valve costs power only to switch, which is why the LCD won. You will build a working cell.
Avancé
6 hours
Consignes
1
1
The molecule whose shape is the whole device
The molecule whose shape is the whole device
5CB — 4-cyano-4'-pentylbiphenyl — is the first room-temperature liquid crystal (Gray, 1972), and almost every early LCD uses it or a relative. Its shape is the device: a rigid biphenyl rod so the molecules line up (the nematic phase), a floppy pentyl tail so it stays liquid, and a polar cyano end that is the handle the electric field grabs to switch it. Rigid enough to align, floppy enough to flow, polar enough to steer.
Outils nécessaires :
Notebook and Pencil2
2
The twist, and why it needs two polarisers
The twist, and why it needs two polarisers
A liquid-crystal cell does nothing visible without a polariser each side — it rotates polarisation, which the eye cannot see. The plates are rubbed at 90 degrees, so the molecules form a quarter-turn helix that guides polarised light round to pass the crossed rear polariser: bright. A few volts stand the molecules up, the helix dies, the light is blocked: dark. Read the embedded polariser blueprint first.
Outils nécessaires :
Notebook and Pencil3
3
Build a working cell and rub the alignment layer
Build a working cell and rub the alignment layer
A real build. Two ITO-coated glass plates (find the conductive side with a meter, ~hundreds of ohms). Wipe a thin PVA or polyimide film on each and RUB firmly one way — the grooves align the molecules. Assemble conductive faces in, rubbing directions at 90 degrees, ~10 um spacers, three edges sealed. Draw in 5CB by capillary action (warm it above ~35 C so it flows), seal the last edge. Add a polariser each side, axes crossed.
Test: crossed polarisers, no volts = bright; touch 3-5 V AC to the two ITO faces and the lit area goes dark. Use AC, not DC — DC electrolyses and kills the cell in minutes.
Matériaux pour cette étape :
ITO-Coated Glass Slide2 pièces
5CB Liquid Crystal1 ml
Polarising Filter Sheet2 feuillesOutils nécessaires :
Digital Multimeter (Lab Grade)
Bench Power Supply (30V/5A)
Precision Tweezers Set4
4
Measure the threshold voltage
Measure the threshold voltage
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Outils nécessaires :
Desktop ComputerMatériaux
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Outils requis
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