LED Torch

Actually a thyristor circuit that uses two switches to latch the torch circuit on or off.

The thyristor has an extremely low leakage (I think its called that) and therefore the battery can be kept connected in the circuit and will last for ages.

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Tiny 'Strip' version
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The LED used in class is usually a high power (2500 MCD) red LED costing about 12 pence if you buy a hundred. Pupils can buy higher power red/orange LED's which are 14,000 MCD for an extra 50p. White LED's (Kingbright brand, Rapid Electronics. about 4000 MCD's but appear brighter)make a superb torch but cost even more.

Safet Note: I was told by a technical source that white LED's produce light in wavelengths which may cause temporary damage at high powers. I have not been able to confirm or deny this but considering that one candle flame is the same as 1 MCD at 1 metre (I believe) having a very high power source in a space a few millimetres across is probably asking for trouble. It is now possible to buy red LED's that are 30,000 MCD's in small batches. I would highly reccomend sticking to the 4000 MCD power LED's of any colour for the moment.

I use the 2N5060 T092 package Thyristor. Best tip I can give is to buy transistor sockets (5p) and solder those into the board, then if the pupil puts the thyristor in wrong you can easily change it as the T092 package (tiny little black beady thing with three thin legs) is easily overheated with enthusiastic soldering. You can use the big chunky black and silver ones with a hole in but they take up a lot of space for no extra benefit and cost more than a 2N5060 and a socket..

Like the bike light, the battery holder is the 'N' size ones used for car alarm remotes. The batteries have very little power and will quickly run out within 5 or 6 hours if left on but last a lot longer if used occasionally for the odd burst of light. The torch will last even longer with a single 'AAA' battery but I don't think the thyristor works at such low voltages.

The torch case is made out of polypropylene sheet which is now available from a number of design technology suppliers for about £2.00 for an 800 x 600mm sheet which makes over 60 of the cases. Use the thinner gauge which I believe is 0.4mm.

The ends of the case and PCB have to curve in order for the case to stay closed when folded up. You could add a very low power red LED to the circuit with a 1K resistor so the torch glows faintly all the time so its easy to find!

Use high strength double sided tape to join the long edges on the tab as standard tape peels. First lightly sanding the areas taped helps to make them join. Hot glue peels really easily on polypropylene, as does most other glues. If the ends are cut correctly, they should fold in and stay in place. The buttons on the PCB have small 12mm 'cylinders' as they are designed for keyboards and usually have a keycap fitted over them. If you get it right, the polypropylene case will flex enough when squeezed to actuate the buttons.