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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).

Step 2: Gluing the Positive Legs and Applying the Resistors

The next stage is to apply the adhesive to the positive legs, providing an insulating joint that will prevent it from shorting out to adjacent tracks when the legs are bent over. If using a two-part adhesive, there's a good chance it will have started to cure by the time you're halfway through, which makes application really tricky, so it might be wise to use two small batches rather than one large one.

The end result should appear like the first image below.

Once the glue has dried, and the negative legs have been bent and soldered, it's time to trim the positive legs down to about 5mm, trim the legs on all the resistors to about the same length. Then solder the resistors to the legs, so that it appears like the second image, below.

To enable this LED module to fit into the reflector (or modification of it) the legs will all need to be soldered together in a 'cone' shape. I did this by pulling them all together and giving it a bit of a twist. Solder the end, but don't cut it down yet.

It's important not to let the shortened positive legs bend over far enough to touch the negative bridges. If this happens, you'll be able to tell because the LED to which it connects will not light up during the all-important testing phase. A little insulating tape or careful bending will remedy this.

Finally, solder the 8cm of insulated wire to the point marked GND on the diagram (or any other sensible point to which the negative legs are all wired).

Now is the time to test the LED module! Put three fresh D cells into the battery case, and push the pointy end of the resistor cluster onto the positive. Then, baring the end of the wire, apply it to the metal flange that protrudes from the switch section. If all is well, flicking the switch should light the LEDs... success!

If, of course, some, most or all of the LEDs don't light, it's time to do some faultfinding. Check for shorts between the shortened positive wires and the negative bridges that were made if one or two LEDs don't light. Also, check that the negative bridges extend to the right hole - all it takes is one to be wrongly wired and a whole section of LEDs won't light. It's simply electrical circuits, though, so a logical approach should reveal the culprit!

Step 3: Modifying the Torch Head

This bit varies from torch to torch, and has given me the most grief! The inexpensive torches I've bought are generally formed from a rubber 'overcoat' with (actually, fairly brittle) plastic inserts giving the structure.

The head has, as expected, most of the important components within - there's an exploded view below.

My approach was to dispose of the bulb, and use the soldering iron to melt down the reflector to accommodate the circuit board. I also trimmed the square edges off the board with the wire cutters to help it fit a little better.

The ideal would be for the LED module to fit into the reflector assembly so that the tops of the LEDs lie flush with the front of the reflector - this would mean it can be refitted into the lens assembly without any further modification.

In the event, though, I had to opt for hacking at the reflector to retain the thread for the bottom cap, and use blu-tak to rest the module within it. I found the easiest way to 'cut' the reflector to size was to use the hot soldering iron bit to melt it, trimming off the excess with a craft knife. Ensure you have good ventilation if you choose to do it this way, since it can emit some nasty fumes.

Now's the time to solder the loose wire to the metal negative circular plate. If the reflector assembly is still intact, this will require another hole to be made in it, to allow the negative cable to reach the plate without going near the positive terminal.

Next trim the 'positive leg' so that it comfortably rests on the springy contact of the positive terminal in the screw-on cap. To facilitate this, I bent the terminal up a little so that it could get as good an electrical contact as possible.

Step 4: Assembling the Torch

Finally, I created an adaptor to shroud the LEDs and allow them to fit snuggly into the head section using some plastic packing ribbon wrapped into a circle where the inner diameter was the diameter of the LED array and circuit board, and the outer diameter was the inner diameter of the torch body. Wrapping it in insulating tape kept it together and ensured a nice snug fit.

The inner plastic housing has the screw thread to attach it to the torch body, but the glass is on the rubber cap, so if the two stop sticking together, the spring of the batteries can push the cap off. I drilled some small holes and put some 4mm screws in to keep them together.

Now it's time to screw it all together and see if it works!

Total time taken: 2.5 hours each
Total cost: 12 GBP for two (since I ordered 50 LEDs, I thought I'd make a pair of them!)
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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