SK-68 for bicyle tail light

Hello all

Have been wondering for some time about how to mod an SK-68 or clone for use as a bike tail light. Cheap, simple and I can "zoom" the beam pattern as needed. I'll need to fashion some kind of home-made mount and may have to tape the head to stop bumps and vibration altering the zoom, but otherwise I can't see a problem.

I always have plenty of AAs about so long runtimes are not required. Waterproofing should be adequate but if not a little silicone around the head would probably work.

I've seen a basic 3 mode light here I want the strobe option so single-mode is out.

and a red LED on 16mm star here

The problem seems to be that red LEDs have a much lower current and voltage draw than regular white versions.

I figure I will need to either replace the driver with a low current version such as this one or parallel some kind of resistor to across the LED to keep the current down.

I would only be planning on feeding it eneloops (or alkalines in a real pinch).

Any suggestions or comments from more experience contributers here would be welcome. I've done thread searches but haven't turned up much about using red emitters with regular drivers.


I’d look for a CREE red LED or maybe a Philips one (Luxeon K2 are common). The Osram’s arent that bright as far as I know.

I think this led should be OK with stock driver and probably brigter than osram :slight_smile:

Some SK68s come with o-rings and one can probably replace the springs of others with o-rings. With thick Silicone grease, I don’t think you would need anything else to hold the focus, and that part should be water tight unless you zoom it out while wet. Some of the other o-rings are sometimes missing.
The Osram page says 3W which seems high for a normal SK68 driver and alkaline AA cell or Ni metal hydride. Just the output is down.

Why would you use silicone grease? Is it not quite abrasive? I use vaseline.

Pure supposition here so please shoot me down in flames if I am being an absolute numpty, but would self adhesive red vehicle lens repair tape work if placed over the glass? Or would it mess with the output/colouration? Haven’t got a clue really but just an idea that sprang to mind.

It depends on what type o-rings you have. I think generally it is better for the o-ring and the grease to be different chemically to prevent the ring from soaking it up. Also that is what we used for vacuum systems. If you have replaced the original rings with silicone ones it is probably better to use vaseline. Wikipedia has some information on it.

Maybe you are thinking of silica or other inorganic silicon containing substances that are hard and therefore abrasive when in small particles. A silicone is an organic compound such as an oil or plastic that contains the element silicon combined in a certain way.

Thanks for the replies folks.

I kinda figured this thread had "sunk without a trace" but it's nice to hear your thoughts.

The LED from Intl-Outdoor in post #2 looks good.

I guess my main concern was overdriving the emitter due to the low voltage, but as I've since found out with further reading, the rated voltage should be pretty much irrelevant in this case as the driver will limit the current with the voltage just falling into line. Although there are no real specs published, I doubt the SK68 would drive the led above 700mA on a NiMH cell so I should be safe.

(Correct me if I've got that wrong though.)

Posts #5 and #6. I wanted to stay away from adding anything over the lens as I figured it would cut down the brightness too much. These are pretty low-output lights to begin with. I have experimented with some red cellophane inside the lens but with sufficient layers to make it look "red" the output is pretty dim. I think a proper red emitter would be preferable.

As to the silicone grease.... There may be better stuff around - I actually have some Nyogel that I could use. I won't be zooming it in and out in a downpour but a lot of water can get thrown about when riding in the rain. I had also considered using this as a warning light if I'm training in the canoe at night. I know the Sipiks are not entirely waterproof but they are cheap enough that if I destroy it I won't lose too much sleep.

Time to go shopping...

Ok guys, so I finally did some shopping and got a few goodies in.

I ordered two of these 3-mode Sipik clones

and two of these red XP-E emitters

I replaced the original XP-E emitter with the red version but found a strange problem. My 3-mode light had become a two-mode light. The strobe will not work.

This is very disappointing, since the strobe mode was the "blinking" attention-grabber I wanted on the back of the bike.

Tried the same process on the second light with the same result - no strobe.

Went back through my parts and tried soldering the red emitter to this driver which is a 5-mode 1-2AA driver. The combination turned out to be a two mode High/Lower result. Same driver with the white XP-E works fine in all 5 modes.

As an exercise in soldering it was useful but didn't get me the light I needed.

What is going on here? Is the problem related to the lower forward voltage required with the RED led as compared to the white?

Is there any way I can make this work?

Comments from some of you better clued up folks would be once again appreciated.


The simplest way the led can effect the driver is by its forward voltage. One should expect a red led to have lower forward voltage than a blue one, on physical principles.

I've read quite a few threads about swapping Red emitters into lights but they mostly relate to Li-ion cells and buck drivers.

I haven't found anyone "complaining" about the strobe no longer working on a boost driver.

I figured the lower forward voltage was probably responsible but it's a bit beyond my skill set at this time to work out the fix.