Even More Hippies on your Head!
For quite some time I've known about Luxeon Star LEDS. These are very large LEDs that allow for much higher current than a conventional LED - about a 10 fold increase.
They've been available for some time, but only at around the $50 dollar mark. Then Prime Electronics advertised 1W Luxeons for only $20. Of course, I ended up spending considerably more than this, after I added a couple of collimating lenses and a 5W Luxeon as well.
The 1W Luxeon requires around 3.4V at 350mA to run. That's a nice close match to 3 NiMH batteries in series. The trouble is, 3 NiMH cells will drop to 3.3V a fair time before they're totally flat, and they'll also start out at about 4.2V. In fact, the voltage isn't in itself a problem, but causes a large variation in current, which does cause a problem. LEDs like constant current. Lumileds recommend throwing 4 batteries together with a current limiting resistor. That kind of works, but a few more components allows a much better regulator.
This regulator is based on a constant current NiCd battery charger that was in the back of the old Dick Smith cattledog. The 1k resistor lets around 10mA through to the MOSFET gate and the 1N4148 diode. The voltage drop across the diode at this current is around 650mV. That means the gate is held at 650mV, which holds the source at 650mV. Since there is a 650mV drop from here to ground, the value of Vref controls the current through the system. A 2 ohm resistor means there's about 315mA through the LED - which is pretty close to ideal. Underpowering the LED lets me run a small heatsink safely.
The heatsink is a TO-220 model from Jaycar , designed for PCB mounting. I drove the pins in and Dremeled a groove in each corner to match the spacing on the heatsink. The elasticity of the plastic heatsink pins means that this creates a reasonable join - it hasn't vibrated loose so far!
Since the current is relatively low, I just got the cheapest batteries Jaycar had. 1200mAH means I can run for just under 4 hours at a decent brightness. I've left the light on for around 12 hours by accident - the LED is pretty dull, but would still work OK as a visibility only light.
You're not going to want to fang down a firetrail with this thing.
However, the complete system is light enough it makes a perfect
secondary light, and it's actually not too bad for
riding - I've used it
as a commuter light, and you can ride up to about 20km/h if the surface
is reasonably smooth. The long life also means I didn't bother with a
switch. Unplugging it is simple enough.
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