Monday 28 April 2014


I don't really need bike lights at this time of year, but it's a good time to get ready for winter. So I've ordered some stuff.

The most interesting thing I've ordered, is a 3 watt red LED. It's a light-coloured red, and it'll be very bright. But how to drive it?

It takes 2 to 2.5 volts, 700 milliamps. I want to drive it from the bike battery, and that means at least 8 volts, probably more. So, in order not to burn the led out, you have to put in a resistor, to limit the current and drop the voltage. The problem with that, is that most of the power being drawn from the battery, it being used to heat up the resistor, and so is wasted.

I have an idea. I have a buck converter; this will take 5 to 30 volts input, and put out 1.5 to 27 volts output, and it runs at 92% efficiency. Hardly any waste! I'll set it for 2 volts, then slowly turn up the wick until the led is pulling the 0.7 amps it's rated at.

Then I wondered how to make it blink. I've got two plans. Plan A, is, I've ordered a flashing rear light, that also lays down two lines to the side of the bike and behind me, thus marking out an area that I'm hoping a car driver won't go into. So I'll have one very bright rear light, and one that flashes. Plan B is more interesting. I've ordered a relay flasher; the sort that's used to flash turn indicators. I think I might be able to use that to make the big rear light flash.

As well as the more ordinary front lights I've bought, I've also ordered a 10w white led. That's about 1000 lumens, the equivalent of a 60 watt incandescent light bulb. It was a mere £1, and I think I can drive it with another buck converter (which also cost around £1).

But here's the really exciting proposition. for about £5, I can get a 100 watt led, 8000 lumens. The brightest incandescent that you used to be able to buy was 150 watts, and those are  2700 lumens, so we're talking three of those very bright bulbs. One problem then will be the heat produced by the led; it would need a major heat sink. But the bigger problem would be that I'd blind every car driver coming towards me.

1 comment:

  1. Rather than fixing the voltage, you could just use a constant current supply.

    You can buy them for about 4 quid on eBay.

    If there's a fixed voltage across the LED, and the temperature of the LED changes I'd expect the current to also change. I've not done the maths or the experiment though.