Mechanical Television (2)

 

This section goes into quite a bit of detail on how I built my rotating disc television. Please note that I'm not responsible if you do or copy anything I describe and something goes wrong!

  

 

The light source must be able to vary in brightness fast enough. If too slow at best the picture will look soft. This rules out a normal bulb with a filament - no doubt something that Baird quickly realised whilst experimenting. Baird's only viable choice was the neon lamp so this is what was used.

The light source must also be bright since only a pin hole of light passes through the disc at any time and less than a thousandth of the light from the light source forms the final image. The best neon lamps were the flat plate type which glowed uniformly over the area of the final image - they were also the most expensive!

 

 

Modern mechanically scanned TV systems usually use LEDs as the light source. They are fast enough and run off low voltages which is safer (neon bulbs typically require 60 to 90 volts which can give an unpleasant electric shock if bare wires are touched). Orange LEDs may be used to simulate the colour of the original neon tubes and it is relatively easy to put together enough of them to give sufficient brightness.

 

A relatively new invention is the white LED. These give a picture which is easier to watch because they are not a strong single colour, though they do have a hint of blue. This is because they are really a blue LED with a coating of phosphor which emits the other colours of the spectrum to appear white.

White LEDs are also available with very high light output, so a working mechanically scanned TV can be made to work well with only six. Do not look directly into them - they are so bright they can damage the eyes. Recently 20000mcd white LEDs have become available, so far I have only used the 10000mcd types and they are very bright!

Take great care not to connect the power to them the wrong way around. When you reconnect the power the correct way around they may light up - but will probably not give out as much light as they should.

 

 

 

For my light source I used 18 white LEDs on a piece of prototyping board. The beam angle is quite narrow so to pack in as many LEDs as possible I raised some of them slightly so their plasic rims did not collide. This number covered the area I needed to illuminate on the disc and therefore helped to minimise the distance between the LEDs and the diffuser.

 

 

 

 

 

 

This side view shows more clearly how I staggered the heights to fit the LEDs more closely together. I could have filed off their plastic rims but this was much easier.

 

 

 

 

 

Whilst colour LEDs are many times fast enough for the usual mechanically scanned TV systems the phosphor in a white LED does not react so quickly. The good news is they are still quite fast and adequate for the purpose but anyone thinking of building a system which scans faster than usual may have to consider this further.

The first sign that the white LED speed limit is being reached may be a change in colour of the light (spectral shift) rather than softening of the picture though I have not so far seen this demonstrated.

Because most applications for white LEDs do not assume fast variations in brightness (e.g. torches/flashlights) the data sheets provide little, if any, information about this.

 

A diffuser between the LED light source and the disc is necessary to create an even area of light, otherwise all you see through the rotating disc are the points of light from inside the LEDs and very little picture. Diffuser material is a difficult choice. Too little diffusion and you will still see blobs of light through the disk. Too much diffusion and the amount of light will be reduced giving a very dim picture. The effectiveness of the diffuser is increased by moving the LEDs further back, but with the same tendency to reduce the brightness of the picture.

The photo shows the mounted diffuser. This is normally behind the Nipkow disk which has been temporarily removed to allow the diffuser to be seen. The LEDs are behind the diffuser.

 

It is worth experimenting with the LED to diffuser distance and the diffuser material. Grease proof paper is a good starting point, though various packaging materials can be tried - I found the lid off a packet of Pringles worked well. Some however had unexpected effects - a piece from a plastic milk carton was found to have directional properties with the diffusion of the light being different in perpendicular directions making the spots of light from the LEDs oval.

Another approach is to have two layers of diffuser material, one close to the LEDs and the other close to the disc. This provides more uniform illumination but will reduce the light available compared to a single layer (the usual trade off).

 

Onwards to the Motor.........

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