Nick’s LED headlight experiment

For some time, I’ve been tinkering with LED lighting on my Mk3 Commando.  Having read some articles that made LED conversions  more complicated than necessary,  I though I’d have a go at a simpler solution.  My first article in Roadholder (No. 344 page 37) was on LED indicators, using off the shelf components.

Since then, I’ve added a hazard warning option to my original setup by modifying the diode array that usually prevents all lamps flashing because of current leakage through the indicator pilot lamp.  I’ll post the details in another article.

So why bother with a LED headlight assembly rather than changing the existing H4 bulb for a LED type?
MOT regulation changes previously banned LED conversions of halogen headlight systems hence my investigation of alternative approaches.  However, in the interim, the latest amendments to the MOT regulations for March 2021 have relaxed the constraints for class 1 and 2 motorcycles and in addition for vehicles in classes 3, 4, 5 & 7 that were first registered before 1986.

The MOT should not be tied to any particular technology.  Functional tests should suffice.  My view on this is that the existing beam pattern test should be sufficient.  If that is passed, who cares what bulb technology is used?  However, I would go further and add two additional tests: beam intensity (some LEDs can be overly bright) and beam colour “temperature” (e.g. daylight colour).  That would classify the light output without stipulating any particular technology thus leaving room for technological improvements without having to change the MOT’s small print!

The MOT regulations for older (non-homologated) vehicles don’t appear to mention the use of dedicated LED headlight systems only halogen bulb substitutions. On that basis one could have a purpose designed LED headlamp assembly and use that instead of the older filament lamp headlight assemblies. This falls in line with most modern bikes that have native LED headlight systems rather than LED bulb conversions and as a consequence, tend to have better beam patterns

The assembly that I’m using is one intended for Harleys but it fits a standard Lucas 7 inch rim and shell.  However, the gotcha is that these lower cost headlamp systems (typically £40 to £50) are designed for negative earth vehicles.  The ones that are suitable for positive earth vehicles are nearer £200!  At this point, after the wailing and gnashing of teeth, I decided not to convert the Norton to negative earth (that might be something for a future rebuild).  Instead, I made up a new headlight wiring harness with miniature relays for dipped and main beam so that I could isolate the negative earth LED system from the bike’s positive earth.  So far this has worked a treat but the life span of the mechanical relays is the issue.  I think they may last 5 or more years.  My initial version used mechanical relays because they are about a tenth of the cost of the solid state variety.  However, as soon as I can get some reasonably priced solid state ones, I’ll replace the mechanical relays.

Sidelights Dipped beam with sidelights Main beam with dipped beam and sidelights

Sidelights –> Dipped beam –> Main beam with dipped and sidelights (above)

The relays (rated at 5 Amps) are mounted on Vero board (similar in size to a H4 bulb connector) but the next version with higher current mechanical relays (rated at 10 Amps) will use a PCB for neatness.  The higher current relays will allow greater flexibility to swap between LED and the OEM halogen headlamp assemblies should the MOT dictate that!

Relay adaptor to use negative earth headlight on positive earth supply

H4 headlight connector and relay board together with the unlit headlamp system (above)

The circuit diagrammes with OEM and LED modifications are shown in Figures 1 and 2 respectively.


Note that when switched to high beam, the LED headlamp also continues to light the dipped beam so both beams illuminate the road for better coverage.  Overall, I’m very pleased with the result but appreciate that for some readers, this may be an unacceptable departure from the classic look of the OEM headlight!


© Nick Harwood – 8th April 2021



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