November 26th, 2008 Posted in Bikes, General | 3 Comments »
For several months every year, commuting by bike involves riding for quite a while in the dark. Here in Canberra that’s all of Winter and maybe a few weeks either side as well. For years I used a series of battery powered lights, starting with incandescents powered by lead-acid gel batteries, then graduating to brighter incandescents powered by nickel metal hydride batteries. These were all quite good, but the need to keep batteries charged all the time and the constant planning to make sure I’d have enough light to get home was rather inconvenient.
The advent of highly efficient, very bright LED emitters has revolutionised bike lighting, but doesn’t seem to have been applied much to commuter lights. And by commuter lights, I really mean lights powered by the rider’s effort. Dynamo lights.
It so happens that LEDs and dynamos turn out to be pretty much made for each other. LEDs require a current limited power source and dynamos are inherently just that. This means that you need no fancy (and potentially lossy) regulation to get between your LEDs and their power source.
So I set about making a set of dynamo powered bike lights just for commuting.
First I researched bike dynamos online. These tend to come in two broad types. One type is the old style rub-on-the-tyre ‘bottle‘ kind, and in general these are inexpensive but they tend not to be very efficient and tend to wear your tyres. The other type is the hub dynamo which lives inside your wheel’s hub. These are usually quite efficient but they have the disadvantage that you can never remove the slight drag that they introduce. I have found in practise that the drag is almost impossible to detect. There are not many easily available hub dynamos to choose from. The Schmidt and the Shimano are the main contenders these days with Schmidt winning on efficiency and Shimano winning on price. I bought a Shimano.
I also bought a 700C rim and 32 spokes so I could build the hub into a wheel, but if wheel building isn’t your cup of tea you could always get a bike shop to do it for you. I usually get a bike shop to do the final true and tension on my wheels after I have made something mostly wheel-like to work on.
With a working source of power, the next thing to think of was the lights themselves. I chose to use two Cree X-LED emitters wired in series as my light. These were very, very cheap from http://dealextreme.com and were the brightest available when I bought them a year or more ago. Every few months these things get brighter, so I’m sure anyone making something like this now will have better lights than I do.
A dynamo is usually used to power filament lamps rather than LEDs, so the fact that it produces AC is of little consequence there, but LEDs need DC so something must be done to turn the AC into DC before the applying it to the LEDs. A key goal in designing all this stuff is to get as much of the energy as possible from the hub into the LEDs and converted to light with minimal losses. So I made a bridge rectifier from Schottky diodes which have a low forward voltage drop and should introduce minimum loss.
One trouble with dynamo powered lights is that when you stop moving, for example at an intersection, your light stops too. Luckily, these days there’s a simple piece of technology that can take care of this problem. Just a few years ago, these supercapacitors were unimaginable but now they are inexpensive and readily available. I used this One Farad capacitor, connected across one of the two LEDs in my light to provide enough power to illuminate the LED for several minutes after I stop. It’s not bright enough to ride by but it doesn’t need to be - it’s only for being seen when you’re stationary. The supercap starts charging as soon as you start moving, and when you stop, it discharges slowly through the LED, keeping it lit.
The Dynamo, two LEDs, four diodes and the tiny supercapacitor make up the complete circuit of the LED commuter dyno light:
One thing that I have deliberately not included in this design is an on/off switch. There are a couple of reasons for not having a switch. The first is that it creates another possible point of failure where reliability is quite important. Next is that if the dynamo is disconnected from a load, its open circuit voltage can rise to very, very high levels. In itself this doesn’t matter much, but if the instant of switch-on occurs while riding at speed with a high voltage on the output of the dynamo it’s possible that either the LEDs or the supercap may be damaged. No such problem if there’s always a load connected across the dyno.
To build the light, I started by hacking up some old aluminium channel material that I had retrieved from a local recycling centre. It forms a nice heatsink for the LEDs, though I think that in this application it’s probably overkill. As can be seen in the image below, the bolts holding the LED emitters in place have insulating washers under their heads to avoid creating a short circuit through the heatsink where the bolts touch the emitters. Wires from the LEDs pass through holes in the aluminium and into a short length of electrical conduit that houses the diodes and the supercapacitor. Wires from the dynamo enter the same place. The whole assembly is attached to the handlebars using a recycled reflector bracket.
To seal off the short length of conduit, I used a pair of plastic chair leg stoppers from the local hardware shop. One has holes drilled to accommodate all of the wiring. The conduit is zip tied to the aluminium channel.
To get the light to go mostly where I want it, a pair of collimating reflectors fit over the LEDs and are held in place by a couple of cut down plastic plumbing fittings. The plastic fittings are glued into place with a contact adhesive. The photo below was taken about fifteen minutes after parking the bike. You can see that the LED on the right is still slightly illuminated by the supercap. It decays away very slowly.
At this point I had planned to show some mind-blowing beam shots and maybe a short video of just how bright the lights are. And they are incredibly bright - all the more so for running purely from unnoticed pedaling effort. It turns out that riding a bike while photographing your headlights is… tricky. So that may have to wait.
The lack of an on/off switch has prompted just a couple of concerned commuters to advise me in daylight that I’ve left my lights on. At night, though I have a good cutoff at the leading edge with the lights aimed away from the eyes of oncoming traffic, I have had several complaints that the lights are “too bright.” From a dynamo? How can that be?
