Quick circuit for LED “high beams”

November 8, 2008

his has come up twice in the last week; I want high/lo beams, and I think Wiley needs them too. Different reasons — I don’t want to be rude to peds and other bikers, and Wiley (I think) just needs to conserve power.
The thing to note is that LEDs in the amber-orange-red family are much more efficient at lower temperatures, and more efficient at lower currents. For a given amount of heat sinking, you get more light per watt. In Wiley’s case, he has 4 LEDs total, and he wants to run them from 8 AA NiMH batteries. If he runs 2 1Amp BuckPucks, with two lights in series on each, those batteries will drain pretty fast.

So, first, just go for half the power, because 12 watts from 8 AA batteries won’t last long.

Second, by default, run all 4 LEDs in parallel. 250 milliAmps per LED will still produce pretty good light. When they need to be brighter, run them in series-parallel. A DPDT toggle works.


On-off-on toggles are not the best choice, because the current regulator will run at a high voltage when its output is disconnected from the diodes, which is too high for the LEDs when they are turned on again (this is a bigger problem if there is a capacitor on the regulator output). Either a on-momentary or an on-on will do.

It’s also possible to use a second 2PST toggle to turn a second BuckPuck on, or a 4PDT on-on-on to give you low/medium/high output.


I’m not 100% sure how the two current regulators work in parallel, but it’s worth a try.

2 Responses to “Quick circuit for LED “high beams””

  1. wsanders Says:

    Quick test – two K3 Yellow Leds ran off a 1A Buck Puck and 8 AA 1800mAh NiMhs for more than two hours, at which point input drive current had fallen from about 400 to about 100mA according to my multimeter (which could we wildly inaccurate.) Toward the end, under load, the battery pack voltage was less than 3V – I was probably reversing a cell or two at that point.

    The front assembly is ready to go on tonight’s run home, two Yellow K3’s in series at full blast. Getting the lights clamed to the handlebars is a bit of a challenge – I’m trying to use two mounts left over from dead Vista-Lites but the bulky ugly V-SRAM shifters are in the way, and there is limited handlebar real estate. I think I will be switching to hose clamps or padded automotive “radiator” clamps.

  2. dr2chase Says:

    Bar attachment — try velo-orange, either the water bottle holder or the bell clamp.

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