K2 River Rock headlamp mod - McR20, Seoul P4, Cheesy heatsink

LED Zeppelin

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I modded the K2 River Rock headlamp available at Target ($24.95) with a Seoul P4, McR20, and improved the heatsinking with a chunk of change.

If you're trying this at home, first disassemble the light by removing the straps. Work the straps through the gap at the center of each strap mount. Then you will find 2 phillips head screws on either side of the forehead pad. Remove these and the rear plate can be wiggled out, bottom first. Next the head can be removed (from the rear also). 4 screws hold the rear housing of the head. Remove these and the housing will separate. Inside you will find 2 shiny screws on the back of the heatsink. You need not remove these to pull out the guts. Only remove these if you wish to separate the heatsink from the reflector shroud and replace the emitter.

When reassembling, put a dab of lube on each screw. The heads are rather soft, and just threading them into the dry plastic is almost enough to strip them. With the lube they'll go in easy and you will be able to sense the proper torque to seal the housing gasket.

The stock beam of the K2 RR is very narrow, with uniform brightness and no spill. It appears deceivingly bright due to the small focus area, but it does have good throw. Most of my headlamp uses require spill, so other than for long range spotting the stock beam was not very useful.

Here are some shots of the stock K2 RR at right, and a Nuwai 1 watt side emitter headlamp at left. Both lights on hi, and exposure starting at 1/2.5 sec, F5.6, then stopped down to F7.1:


Initially I kept the original K2, and added a modified McR20. I also added 2 pennies to the rear of the heatsink, the maximum that could be added and still fit inside the housing.

I removed the optic and gasket, and siliconed a 22.8 mm UCL into the head. The McR20 had to be shortened until almost the entire first shouldered step was removed. The proper length, the McR20 was held in compression upon reassembly by the lens and K2, so no fixing was necessary. I carefully used a beltsander and finished the edge with a mill file by hand.

Here you see the removed optic and gasket, and the McR20 and UCL installed:


Next I shaped 2 pennies to epoxy to the rear of the heatsink. The inner penny sits inside the recess surrounding the emitter boss, and has cutouts to clear the leads. The outer penny simply has to clear the leads. These were epoxied to each other and the sink:


Here are some beamshots vs. a Nuwai 1 watt side emitter headlamp (at left), both lights on hi, and exposure starting at 1/2.5 sec, F5.6. Stopped down for subsequent shots to F7.1, then F9.0:


At about 1 m, the stock K2/optic beam measured 1150 lux. With the McR20, the lux went down marginally, 1075, but the spill and overall utility of the beam were greatly improved.

Nice improvement in the beam, and heatsinking, or so I thought.

After using the light for several days I found that on hi, after about 10 minutes, the rear of the plastic housing was getting very hot. With the 2 pennies very close to the rear cover, they were radiating heat to it. Imagine how hot the stock heatsink had been getting when insulated inside the head.

I decided to go the whole 9 yards and remod it with a Seoul (P4 USWOH) and readdress the heatsinking issue.

Disassembling the head again, and this time removing the shiny screws holding the heatsink to the plastic reflector shroud, I gained access to the emitter. It had black potting compound around the soldered leads which made a bit of a mess desoldering, but when heated the stuff pulled off easily.

I took the opportunity to measure current to the LED. Using partially drained cells (70% on my ZTS and 1.4V), the K2 was being driven at 480 mA on hi. After the swap the Seoul was driven at 400 mA. I'm not sure why the discrepancy.

I sanded 0.030" off the rear of the reflector for proper focus with the Seoul. Since I had already modified the McR20 to work with the K2, I used one of the Shoppe's copper disks to raise the Seoul to meet the reflector. Had I originally planned the Seoul swap, I wouldn't have needed the copper disk, I would simply have sanded 0.030" less off the front of the reflector. I put a light skim of epoxy at the base of the reflector to adhere to the emitter, and an epoxy bridge between the reflector and shroud at the notch in the shroud at the bottom. This will hold the reflector in position well enough until reassembly where they will be aided by compression of the parts.

I had to clearance the emitter opening in the reflector shroud to clear the round Seoul. You can see it here along with the installed Seoul, remodded McR20, and the removed K2:


I found that Chuck E Cheese tokens fit snuggly between the 4 screw posts inside the rear cover. I scribed the OD, and rough cut the hole with a scroll saw. I finished the opening with a proper radius half-round file to a friction fit. I filed one token flat on both sides, and the other on one side. The tokens turned out to be high quality coins, copper not anodized alum. Here you can see the prepped coins. One is nested inside the hole in the rear cover:


Here is the reassembled head ready to receive the tokens which will be epoxied to the pennies and to each other:


When the outer token was epoxied to the inner one, I used enough epoxy to squeeze out and fill the small gap between the tokens and the housing, effecting a weatherproof seal. The housing can still be disassembled, but now the LE will pull out with the rear cover as one unit. Here is the completed head:


Lux readings under my desk at about 20" were 2600 for the McR20 K2, and 6100 for the McR20 Seoul. These reading were taken on the same partially discharged cells.

Here are the beamshots on hi, again with the Nuwai at left and the same exposures starting at 1/2.5 sec, F5.6, and progressing to F7.1 then F9.0:


With the Cheesy heatsink, after a similar 10 minute runtime, Chuck E gets warm, but not hot, even less so than the plastic was getting with the 2 pennies behind it.

After my second effort, I'm quite happy with the headlamp. It's way bright on hi, and has a flashaholic-perfect beam with good throw and generous spill. And I'm no longer concerned about how hot it's getting whenever I'm using it on hi.

EDIT:

I made one final improvement to the light which was to drill a series of holes in the top and bottom of the frame to allow circulation over the heatsink. The clearance between the frame and head is very tight and restrictive to airflow when the head is upright, the position it's usually in on hi.

To do this I hand-held the frame and used a drill press since there's really no way to clamp it. If you try this use a medium speed that doesn't chatter or melt, and light pressure. The bit will grab the piece just when you break through so be prepared; it yanked it out of my hands a couple times. If you attempt it with a hand drill, reverse direction when you near breaking through. After drilling I deburred and chamfered the edges by hand with an Xacto.


EDIT #2:

After testing various combinations of cell types along with 1 or 2 dummy cells, I have come to the conclusion there is no boost or buck to the circuit at all. For proper heat management and acceptable output (on hi), the cell voltage needs to be in the 3.5 - 4.1 V range with the Seoul.

My recommendation to anyone modding one of these is to keep the K2, install a reflector, and do what you can to improve the heatsink. The increased output of the Seoul is not worth the difficulty achieving the proper drive current off a 3 X AA solution.

EDIT #2:
After using this headlamp for 4 months, it has become my favorite. I run it strictly on NiMH AAs, and have to use hi sparingly on fully charged cells, but after a few minutes the heat is manageable. The PWM flicker bothers me, but I use it mostly on hi outdoors so it's not an issue.
 
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LED Zeppelin

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Thanks PSM.

I do believe you were the first to transplant the McR20 into this headlamp. Thanks for the inspiration.
 

kevinm

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Jun 2, 2006
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Nice work, LED! Just one worry: you might end up with a Raiders of the Lost Ark style burn on your forehead. Could you leave it on for a while and tell us how hot it gets?

EDIT: Oh, now I see! The coin is on the other side of the headband.

Many thanks,
Kevin
 
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slaps

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Jul 31, 2005
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Seattle, WA
What a mod!!! Why can't manufacturers get it right themselves? Oh well, talented CPFer's can make it right. If you ever want to sell it.......
 

carbine15

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slaps said:
What a mod!!! Why can't manufacturers get it right themselves? Oh well, talented CPFer's can make it right. If you ever want to sell it.......

Exactly. Why can't they.
 

LED Zeppelin

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Kevin,

I did some testing with freshly charged Energizer 2500 mAh NiMH cells, fresh alkalines, and the same 60% 1.36V cells I have been using all along.

I had prepared a lengthy post with all the lux reading and time itervals composed as I did the testing and guess what, the server ate it.:hairpull:

So I'll just summarize my findings qualitatively.

I found that with the NiMH cells the headlamp started out very bright at about 2300 Lux @ 1m. After about 5 minutes the Lux was down over 25%, and Chuck E was hot enough that I couldn't keep my finger on him for more than 1 sec. The temp seemed stable after that up to 10 minutes.

I found a similar result with fresh alkalines, starting out at 2100 Lux. Temperature was similarly hot and stable after 5 minutes.

With my partially discharged alkaline cells, total 4.08 V, and testing 60%, the Lux started at 1400 and only went down to 1330 after 5 minutes. The temp was warm but I could keep my finger on Chuck E comfortably. After 10 minutes the temp was warm to very warm, but still not pull-your-finger-away hot.

What I think is going on is that the circuit is a boost, and since the Vf of the Seoul (low 3s) is much lower than the K2 (4+?), the Seoul is in direct drive on fresh cells. I'll use hi for short periods until the voltage drops, or use a dummy cell. I think my ideal setup will be 2 X NiMH AA and a dummy cell.

It appears my heatsink mod can cope with the heat given by the K2 or Seoul in regulation, but not overdriven as the Seoul is on 4.5 V. Or at least until the voltage drops to 4 and the Seoul is mildly overdriven.
 

LED Zeppelin

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I did some more testing with various cell combinations.

I made a couple dummy cells from alum gutter nails and rubber hose.

I tried one dummy and 2 X alkaline, then 2 X NiMH. No go, it was very dim to start.

One dummy cell and 2 Energizer AA Li primaries worked, but the light was a bit dimmer to start - 890 Lux @ about 1 m.

Then I tried 2 dummy cells and one 14500. The Lux reading @ about 1 m started out at 2,250. After 5 minutes the heatsink is very hot, and output has dropped to 1700 Lux.

I'm not a converter expert, but it seems that the circuit in the light may not be "regulated", but more like a muti-function direct drive. With a proper boost circuit the light should have been able to run off all the above combinations I believe.

So I now have a headlamp that only likes to run off partially discharged alkalines.

Next step is to gut the crappy electronics, and install a real converter with a 2-stage or multi-function. I'll have to examine the available interior space, switch options, and go another round with it. For the time being I'll use hi sparingly on fresh cells.

EDIT: Investigated the interior space to see if I could fit a Judco and a board, no go. I'll just have to baby it on fresh cells.
 
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