U3A Electronic Displays


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Title

Intro. 3 types of Electronic displays - LED, Liquid Crystal, Electrophoretic. All relatively recent inventions 1962, for the first two and 1970 for the last. All are involved with the search for displays to reduce the power required to replace filament bulbs, neon tubes and CRTs

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Diodes

Start here as it helps to explain the light emitting diode. An old device discovered in principle in 1874 in galena crystals and manufactured in 1906. The older amongst us can remember cat’s whisker crystal radio sets in our childhood. It permits the passage of electricity in one direction only.

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Diode diagram

Modern diodes are made of silicon semiconductors in two regions. Each region has impurities added to create an area of negative charge carriers (electrons) - n-type semiconductor, and a region of positive charge carriers (holes) - p-type semiconductor. The join is the pn junction. When a PD is applied, electrons will pass across the junction from - to + but not in the reverse direction. A very useful rectification device

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Phone Charger

Diodes abound in most electronic devices. They convert the AC mains to DC for your mobile phone charger.

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Car Alternator

Early diodes were very current limited and burnt out at the junction if the voltage was excessive (2-3v). Modern ones can handle higher voltages and powers. These on my car alternator have finned heat sinks to get rid of excess heat.

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LED Varieties

So where does the LED fit in. A modern device invented in 1962 by a Nick Holonyak Jr working for GEC in the US. His was the first visible light LED (Red). Infrared radiation had been detected in 1955 and 1961.

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LED Diagram

Very similar to the diode. However, as electrons cross the junction, their energy levels drop and a quanta of light energy (a Photon) is emitted.


NB this was predicted as a phenomenon by Einstein in 1905 in his work on photoelectric effects for which he received a Nobel prize in 1922. He postulated that light existed as a particle as well as a wave. Robert Millikan received a Nobel prize in 1926 for proving the existence of Photons which had been named by a Gilbert Lewis - ironically he was trying to prove Einstein was wrong! Photons are very small. Sunlight provides1012 photons/sec on the head of a pin.


The p-n components of the semiconductor are selected to create the desired frequency/colour of the light output. Once very expensive now very cheap SANTA HAT £1. Assembled in separate units or on chips to create displays. Advantage low power consumption as they run on 2-3v, but until recently were easy to burn out and light levels in LUMENs were low.


NB. From 1 Sep 10 EU regulations state that all light output from lamps has to be expressed in Lumens instead of Watts as there is no real connection between Watts and light output. As an example a typical 5ft fluorescent lamp tube has an output of 12,000 Lumens.

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LED Colour Table

Look carefully there is a test at the end. White LED’s are a special problem as white is a combination of all the colours so UV LEDs are used with appropriate phosphors to create white output. TORCH also £1. How are they made and used for displays and lighting?

9A

LED Construction

This is how they are made.

9B

LED Light

And how they light. How are they used in displays?

9C

Sinclair Watch

Remember this, the 1975 LED Digital watch. £17.50 kit, £25.00 assembled. Hopelessly unreliable, the watch lit for only 10 secs after a button press. The battery lasted just 10 days. It bankrupted Sinclair Radionics which was rescued by the govt National Enterprise Board.

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Car LED

Here’s a modern display from my car. LED’s on a circuit board to display the car computer outputs.

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Barcelona Sign

More advanced advertising sign where the image changes/moves: 3x2m.

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Sign Close Up

The sign is made up of many individual cells about the size of a pea. The upper surface of each is a magnifying lens.

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LED Triad

Within each cell - too small for my camera - is a triad of LEDs, RGB. These are modulated to produce any colour combination. Just as the image is produced on a TV. Are they used on TV’s? No not yet as the triads cannot be produced reliably enough for a TV screen. What is advertised as an LED TV is a back lit liquid crystal display which I’m coming to next.

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High Output LEDs

There is great interest for LED lighting as it is becoming bright and higher outputs are becoming available for use at mains voltages. Currently about 200 Lumens/Watt is possible. This is a similar output to fluorescent lamps but more compact, instant starting, and a 25 year life!


This lamp has 38 individual LEDs. They are used for signs everywhere in a range of colours, eg, traffic lights, the front of busses and increasingly at the front and rear of cars.

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More LED Lights

Likely to overtake fluorescents as they are less hazardous as waste - no mercury vapour. However Watts are Watts and LED’ use is limited where power is restricted. They are not efficient when used on battery power and displays with lower power consumption are useful in these applications. Liquid Crystals fulfil this need.

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Car Display

Liquid crystal light bending properties were discovered in 1888 using chloresterol extracted from carrots. The crystals naturally twist light as it passes through the material. Later it was found that if a voltage is applied across the crystal they rotate to untwist the passage of light.


Marconi patented a Liquid Crystal Light Valve in 1936. Displays were invented in 1962 by Richard Williams of RCA who exploited the effect of applying voltages to liquid crystals. This work was in parallel with that of RRE at the same time. The display was the subject of a Swiss patent in 1970 and licenced shortly afterwards for use in digital watches. How does it work?

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Liquid Crystal Construction

The display consists of multiple thin layers:



1. Polarizing filter film with a vertical axis to polarize light as it enters.

2. Glass substrate with electrodes. The shapes of these electrodes will determine the shapes that will appear when the LCD is turned ON.

Early displays used blocks to form portions of numbers, now they are usually individual pixels as these can now be formed and addressed.

3. Twisted nematic liquid crystal.

4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.

5. Polarizing filter film with a horizontal axis to block/pass light. Thus normally, light cannot pass through 1 and 5 and the display would appear black.

6. Reflective surface to send light back to viewer. (In a backlit LCD, this layer is replaced with a light source).

When no voltage is applied the crystals twist the light which can then pass through both polarising layers and the display appears grey. When a voltage is applied across the electrode layers, the display appears black.. Varying the voltage varies the twist and thus a range of shades of gray are available.

Black displays are still common, eg calculators, clocks etc but colour adds a further dimension to their use.


17A

Clock

Still in use. This clock is 3 weeks old.

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Colour Displays

In colour displays coloured dyes are added to the liquid crystal so that variations in light transmission are seen as variations in intensity of colour. Pixels are grouped in RGB triads so that all colours can be displayed. This is the principle used in TV sets. The screens have to be back lit and fluorescent tubes are usually used but increasingly this is being done with LED’s where RGB combinations give better white light than fluorescents, and because they can be rapidly switched, they can be shut off to give deeper blacks.


This image is from my laptop screen at 1024 x 768 pixels = 786,432.

18 A

LCD TV Screen

On the last picture you could just see each pixel. Below each pixel are the RGB liquid crystal triads, each individually addressed to give the colour and brightness combinations required. These devices are amazingly precise in manufacture.


These displays are universal, your phone, the instrumentation of aircraft etc etc. They have the advantage of being much lighter and less power hungry than CRTs, have no burn out effects of phosphor screens, and last a very long time. However, the back lighting needed for bright/colour displays is still a major power consumer and limits battery life. The search is for displays that use little or no power, but are still fully flexible for rapid display changes.

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Kindle Reader

The Kindle is one of several devices that use electrophoretic, paper like, displays. These use no power to maintain the display only consuming power when switching from one display to another. This reduces total power requirements of the device enormously. The Kindle will run for a month on one 3.7v lipo 1570mAh battery charge. These displays were first developed in the 1970s by Nick Sheridon of Xerox in California. As well as solid Kindle type displays they can be made of flexible plastics that can be rolled up.


How does the display work.

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ED 1

At its simplest, the electrophoretic display consists of two surfaces separated by 10 - 100 micrometers. The surfaces are capable of receiving an electric charge positive or negative. The conducting surface on top is so finely dispersed that light can go through it.


Between the surfaces are microspheres of intensely white titanium oxide suspended in a dark hydrocarbon oil. The microspheres carry an electric charge. In my diagram they are positively charged, and will be attracted to any negative charge, in this case the lower conducting surface. The spheres go to the bottom and the display appears black. If the polarity of the charged surfaces is reversed, the spheres are attracted to the top and the surface appears white. The transition is relatively quick, and only needs a charge current applied for that period; thereafter the particles stay put and no power is needed to keep them in place.


That’s black and white. What about more complex images?


20 & 20A

ED 2

If the conducting plates are constructed such that individual, pixel sized, areas can be separately addressed, then picture areas can be formed with black and white combinations - Kindle can achieve 16 shades of grey using this technique.


In practice, a pulse corresponding to a negative image precedes each image change to ensure the particles all leave the viewing surface. This can be seen at each Kindle page change.


What about colour? Coming to Kindle next year.

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ED 3

To create colour, it is only necessary to add triad RGB coloured filters so that coloured images can be formed in exactly the same way as on the LED displays or the liquid crystal TV sets.

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The End

Any Questions?


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