DIY electricals: LED lamp

The sun did not shine
It was too wet to play.
So we sat in the house
All that cold, cold, wet day…

Finally, after weeks of waiting for the appropriate rainy day, a mildly snowy day came along. Mum was out and so the 3 kids got to play… making a nice energy-saving, money-saving lamp. Here’s the result:

The completed lamp, shown off by Elizabeth and Rebekah

So here’s what we did:

First, the ingredients:

5m strip of warm white LEDs
- (I bought them for £6.45 from eBay, but you can probably get them for less.)
Old cereal box
- We used Tesco Everyday Value Branflakes 750g box. The only thing that mattered was having a surface area ≥ 0·05 m². That’s less than A4 (0·062 m²).
12V power supply
- One of our neighbours handily threw one out a few weeks ago. It turned out to be working perfectly for us.
Power connector
- Depends on your power supply. I got a 2.1mm socket from Maplin for about £1.50.
Scissors
Sellotape
Small knife
Pencil
Optional, but would be useful: a small soldering iron

Now put them together:

Cut out the right size of cardboard from the cereal packet. [Scissors]
Bend it round into the desired lamp shape.
1. You need the [Sellotape] here to fix the shape. The LED strip will help maintain the shape later.
Poke 2 holes in the cardboard wherever you want the power connector to be (full pencil width) and where the leads go from it to the start of the LED strip (v small). [Pencil]
Thread the wires from the LED strip through the small hole, then start removing the adhesive backing and stick the strip onto the cardboard mount.
1. Make sure that by the time you have rotated once you have twisted just enough to lie the next loop of adhesive strip alongside the bit you started.
2. Aim to cover the mount as tightly as possible without overlapping or gaps.
When you’ve run out of LED strip, then you need to wire up the power connector. This is likely the trickiest part:
1. I stripped a few mm more wire at the end of the red and black leads, threaded them through the plastic surround (don’t forget that!) then wound them into the appropriate terminals of the metal part of the power socket, before screwing the plastic casing around the power socket again.
2. Remember to check the polarity of the power supply. Most Western users will be doing this with an AC-DC power adaptor, but in power-scarce places you might be wiring in a backup battery system that runs at 12V. Remember red is +ve (positive) and black is –ve (negative). Our power supply had centre pin +ve as the diagram on the adaptor made clear.
Finally connect to power.

Things you might want to add:

A switch
A lampshade
Some tracing paper as a surround to scatter the light a little

——

Update: later we…

Soldered the junctions from the wires of the LED strip to the power socket
Trimmed off the extra card at the top, then put a clear Tesco Houmous pot lid (reduced to 10p) on the top and bottom to give a little more firmness and to stop the cylinder flexing and stressing the cables. The two pots were attached to each other with light thread and held onto the cylinder by tension.

Research showed that the LEDs probably develop about 80% efficiency, so that a little heat is generated but not a noticeable amount. (High-power LEDs are decreasingly efficient.) The rig is probably drawing about 20W, but when some more connectors come along I’ll plug in an ammeter to try the old P=IV calculation.

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