I've been looking for an opportunity to carry out an LED torch conversion for a while, but the criteria for finding a 'donor' torch were fairly stringent. It needed to be:

- inexpensive
- capable of powering white LEDs (so containing a power source generating over 3.6V)
- actually, very inexpensive (in case I made a catastrophic error during the build and broke it)

The benefits of an LED torch over an incandescent one are simple - with the 'high brightness' 20 candle-power white LEDs it's now possible to buy on eBay (I bought fifty for 9GBP), a great deal of light can be produced, and significantly less current is drawn, so the batteries last a lot longer.

I was really pleased, therefore, to find a donor torch in a shop called 'Poundland' for just one UK pound (hence the name!) - it takes three D cells, which generates 4.5V - plenty to drive an array of LEDs.

Here's a list of things that are needed for the project:
- a cheap 4.5V 3xD cell torch (eg. from Poundland)
- 3 1.5V (so not rechargeable!) D cells.
- a 13x13 piece of 0.1" pitch copper-clad stripboard (like "Veroboard")
- 25 high brightness white LEDs (like these: http://is.gd/2Gaa0 - eBay link)
- 25 47ohm 0.25watt resistors (or whatever's appropriate - there's a handy parallel array calculator here: http://led.linear1.org/led.wiz into which you can type the specifications of the LEDs you buy to get the correct value)
- 8cm of small insulated wire
- plastic packing ribbon
- resin adhesive (two-part if available)
- insulating tape
- blu-tak or equivalent (it's not a proper project if it doesn't involve blu-tak!)
- a couple of small screws

Tools required:
- a soldering iron and solder
- a drill or bradawl
- wirecutters
- needle-nose pliers for bending
- a craft knife
- a hacksaw to trim the board down.

Step 1: Creating the circuit

I designed the circuit to use the stripboard both as an anchor for the LEDs and as a common 'negative' connection, so each LED had one leg wired to the board, and the other poking through, and connected - via a resistor - to the positive of the battery.

The diagram requires some explanation, and forms part of a number of steps. It should be fairly obvious that each circle represents an LED, and each large square is the positive leg.

1. Using a drill or bradawl, remove the copper around the holes marked with a cross. These will be where each positive (denoted by a slightly longer leg on the side without the flat bit) leg will go through, and therefore it's essential that they don't make any electrical contact with the board.

2. (Note before starting this stage: the centre LED needs special treatment, so please read on before inserting it).

Insert each LED in turn. pushing the positive and negative legs through. The negative legs need to be soldered to the board, but for the LEDs in the diagram that have lines extending from them, these are where the legs need to be bent over and snipped to length so they re-enter the board on the reverse side to join all the 'negatives' together.

The easiest way to do this is to solder each LED from left to right, first securing its negative leg with solder, and then snipping only the negative legs that do not require extending (of which there are eleven).

To ensure the centre LED actually appears in the centre, it may be necessary to 'spread' the legs before inserting it, bending them out and back down so that the centre of the LED straddles the centre hole. This may mean it ends up standing a little 'proud' of the board, and for simplicity you might wish to omit it.

This will result in a veritable forest of legs, but it will be tamed! You may wish to do the bending of legs to bring the common 'negative' across the board now, or wait until the next stage.

The other side of the board will look much more promising (see the second image in this step).
The ebay link is no longer valid, does anyone know a place online to get the leds and resistors?
Nice instructuble, I'd like to make something similar myself. But I think it's better to wire some LEDs in series, instead of having all of them in parallel and needing resistors, which is a waste of energy. But in your case you would need more than 4.5V

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Bio: I'm a bloke who lives in Luton - I love music and radio and comedy, and pretty much any combination of the three. I've ... More »
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