Sunday, 8 September 2013

A £100 Solar Light Challenge

There comes a time in every young man's life when he becomes, in fact, not so young anymore. At this point instinct takes over and before long the fellow has furnished himself with a shed, just as did generations before. Once all obvious enhancements anticipated by the shed manufacturer have been installed, the time comes to pimp the shed further.  The generation of electricity, a precursor to WiFi and therefore happiness, is a strong contender. Time for a challenge..

That it is possible install a solar powered lighting system for less than £100, without using a kit.

(The kit constraint was easy, because most of them cost more than £100 anyway)

Literature Survey
Now, it has been a long while since I've done anything electrical, but a brief amount of research suggested I'd need the following:

  • A solar panel, obviously
  • Battery to store all the lovely power
  • A solar controller
  • Light switch
  • A high power LED
In the interests of efficiency, I decided not to trouble myself further with any theories of how these components might work together.

Method I
Although tempted to buy something enormous that would either improve or catastrophically degrade the structure of the shed roof, I opted for a 40W 12V solar panel on eBay (£41.99). It's a decent size, about the area of an iMac screen. It seemed like it ought to charge a few batteries.

I bought 10x super 2400mah AA batteries for that purpose (£16.95). Then a weatherproof switch (£6.20), and the solar controller for £8.95 (Total £74.09). The solar controller takes the current from the panel and charges the battery, as well as providing output for the lights. As you can see, it is fairly obvious what you're supposed to do with it:

The solar controller. Looks professional doesn't it? Good UX.
With the big ticket items bought, but with only about £25 left from the budget I headed off to Maplin electronics, where I hadn't been since school. How the UK high street is able to support a store which, as far as I can see, is simply an emporium of wonderful toys for people like me, I do not know. I am, nonetheless, very grateful for its continued existence and spent an endorphiny hour browsing the interesting things. I bought some more stuff:
  • A pack to hold the batteries together, £1.50
  • A hobby box for the battery. Unnecessary but it looked professional. £2.99
  • A couple of blocks of strip terminals for making hassle-free connections, £2.18
Remembering from tedious GCSE physics experiments that thicker cable has less resistance, and not wanting to squander the precious juice from the sun, I also bought:
  • 5m electric cable, conveniently red and black £6.95
Now, my little 12V system would struggle to illuminate even the most efficient of domestic energy saving lights, so I read somewhere that the best thing to do is to build your own bulb from an LED light.

This turned out to be a mistake.

Unawares, I went over to the components desk (surely the most awesome part of any Maplin store?) and bought the brightest looking 0.5W LED I could find (£3.09), a heat sink to stick it on (£1.50), and an LED lens (£1.49) to make it more light bulby. 

Total cost so far £93.79

Being a software developer who likes modular development (and not wanting to spend the next few hours standing on a chair), I cut out a piece of MDF to fit into a space above the door, and laid out
the pieces on top. The piece would slot onto the wall later.

An issue was identified after connecting the batteries to the controller. The controller display shows the battery charge level continuously with three LEDs, ironically draining the battery. Although appreciating that measurement naturally changes the state of things, I didn't want Heisenberg to leave me in the dark either. The best thing to do is to add a switch (£2.49) to be able to disconnect the battery from the controller when not needed.

At this point OCD took over and I decided to run all the cables round the back of the panel so the front would give the impression of competence by the installer and might even add valuable equity to the house.

Connecting all the cables together was straightforward. The next exciting stage was constructing the light bulb. This involved sticking the LED onto to the heat sink (the requirement of which should have been a warning), then soldering on a cable on either side, then sticking on the lens with a hot glue gun.

I hadn't soldered anything for years. Now I can pretend I have a robot helper.

The finished bulb. Professional but doomed.

A fan of unit testing, I decided to test out the light before attaching the solar panel (the batteries were already fully charged). The LED lit up brighter than the sun for an instant, then dispatched itself to the great LED socket in the sky. I didn't think very hard about this, and ran out to buy a bigger 1W bulb. (£4.09).

This took my budget to £100.37

Close enough, I thought. And it would have been if the 1W bulb hadn't blown in exactly the same inevitable way as the 0.5W bulb. I should have paid more attention in the tedious physics lessons.

It turns out that 10 x 2400mAh batteries at 1.2V can readily produce a lot more than 1W for an instant. What you need, I learned, is a 2W LED driver, which manages the current to the LED, which changes continuously as the LED operates. To be safe, I bought a 3W LED (£5.99) to match. Now everything seemed to work OK, albeit at £15.35 over budget.

No point crying over blown LEDs, so I moved on to the exciting and manly phase of installing the solar panel on the roof. I drilled a decent hole in the roof, just under the top layer of roofing felt (still watertight, hah), then attached a baton for the panel to rest on, allowing gravity to take care of its adhesion to the roof (a decision to be regretted as soon as the wind picks up, no doubt). And here it is:

And what's this? The controller lights up like a Christmas tree, the charge light also illuminated. What epic success.

The smugness subsided when I tried out the light properly. Ouch. After two minutes the light and heat sink was too hot to touch. Afraid it would burn out (perhaps taking the tinder dry shed with it) I switched it off. Not many of my planned shed projects will take less than two minutes. In any case, I don't want my precious solar juice wasted heating the back of the light.

Method II
The budget in tatters, I returned to eBay and found an exciting alternative:

The car lights arrive. 4 LEDs in each, with connectors and adjustible brackets. Bargain.
With the £100 challenge failed, I decided to try and make it look as professional as possible and the car lights and spotlight turned out to be just the ticket. But first it's time to massacre the spot light..

Turns out there's simply a latch in there you can pop out with a screwdriver, but hacking it out is its own reward.

By chance the car LED lights were a perfect size, and with another connection and a bit of hot glue I had a low energy adjustible spotlight that looked like, well, a proper light.

Compared with the high power LED, the light is rather clinical in nature. But it burns as cold as it looks and seems to last as long as I need it for. The batteries are sufficient to run both of them at the same time with no dimming. 

Success at last.

So there you have it. A working solar system, admittedly for more than £100. But, I console myself, if I hadn't wasted time and money on the high power LEDs light dead end, it would have been possible to do the whole lot for just about under £100. Well, £105.17. Still not quite within the budget, but on a good day on eBay..

Nonetheless, the trial and error is much more fun than deciphering instructions on a kit. Part of the fun, as with software, is to be able to say "I made that." Then, returning six months later, cringing inwardly, and think "What on earth was I thinking when I did that?"

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