I'm a great fan of Intructables.com, a site that is absolutely dripping with information on making lots of things. Trawling through the site a while ago I stumbled across something called he Nokia BikeLight.
Bike lights are a pain, at the best of times. Basically, they're split into two categories: Lights to see by, and lights to be seen by.
The basic idea is that if you're cycling in a well-lit urban area you don't need a light that is very bright, as you're relying on the streetlighting to see. Your bike light therefore only serves as something to allow other road users to see you better. As a result, such lights are cheap and not very bright - typically a single LED that flashes - and you replace the batteries once every few months, or so.
Of course some cyclists (myself included) cycle in areas where there is no street lighting, so you need to rely on your own lights. It is possible to purchase a 1500 lumens light with high and low beam, but oddly enough I don't happen to have the required seven hundred Pound lying around to buy one.
There are far cheaper light available, but they typically use disposable batteries, and those don't last very long either! My headlight I got brand new from Hong Kong - an Ebay purchase - and it had various different modes:
You could have just one LED light up, to save battery life, or you could have six LED's, for more light. Additionally, you could go for having all 19 LED's on constantly, or flashing. Very nice, except it drained the three AAA batteries in a hurry.
And then I had a battery leak, after which the light didn't want to turn on anymore! Grrrrrr!
Back to the Instructables site - I saw an article that slot into the why-didn't-I-think-of-it category. See, a significant part of the cost of a decent headlight is the rechargable battery, typically some type of Lithium-based type.
Which is exactly what most mobile phones use. In the article the writer describes how he butchered an old Nokia handset and joined it to an old LED torch. Enthused, I set to work!
I hard-wired all 19 LED's to two wires, with a water-proof switch thrown into the mix. This lot was then secured to the battery connectors of an old Nokia phone and I used plastic filler (a.k.a. body putty) to fix it all into one sturdy unit.
Now I have a bright headlight that allows me to ride down the darkest of paths and gives about 12 hours of decent light on a full Lithium polymer battery. After 12 hours the light starts fading and after 24 hours it is practically useless.
Now most decent bike lights only offer around 4 to 5 hours on a single charge, so I'm happy with the performance I'm getting, although I'd love to add another two such lights. Three lights will allow me enough lumens to do off-roading at night, with a main beam shining in font, and two secondary beams shining closer to the bike, slightly off to the left and the right.
Now all I need are three identical phones that have good batteries!