Aqualab

Celebrating the independent kiwi spirit of invention.

Fabrication: Four-mode Cree XR-E P4 Caving Headlamp

By Ian Mander 24-25 January 2010.

Need: acquired a new helmet

When picking up some new caving gear in April 2008 I found a helmet on sale, so I bought it, then started wondering about what lighting to use with it. The headlamp on my old helmet had two modes: 330 mA, which is way too bright in confined spaces; and 1 amp, which is horrendous if used for evil purposes (as I found out later when it was on loan). And so I made a new headlamp for my new helmet.

Features

It has three modes, spaced so that each higher level is roughly three times the current (and roughly the brightness as well) of the next lower setting. Switching is by a small three-mode push button, fitted flush in place of the original momentary switch which only provided electronic switching. Changing the switch required the fabrication of a new board for the battery holder terminals to contact.

Another feature is that while it normally works on three AAA cells – perfect for around camp – I made an external adapter that plugs into the headlamp in place of the normal AAA battery holder, simply by taking another battery holder and battery compartment cap, running a figure-8 wire (a length of speaker wire) through the cover and soldering the wires directly to the battery holder at the right points. The wire runs to the back of my helmet with an AA battery box attached there with a couple of cable ties, making for a removable battery pack. It makes for a very nicely balanced helmet, is not very heavy, and has great runtime. Swapping over back to the internal battery pack is simply by unplugging the adapter and inserting a (populated) AAA holder with the original cap.

I later added a momentary turbo mode using the original switch that came with it. Being small, it was easily installed in the top of the headlamp. All it does it supply power to the chip enable pins of the two unused AMC7135 chips on the driver board, turning the 330 mA regulation into 1 A if there's sufficient voltage. (With this particular LED there's not quite.)

Glow in the dark highlights add an interesting touch.

**Four-mode XR-E Headlamp Approximate Runtimes**
Mode	Lumens	Current	AAA (internal)	AA (external)
Low	7-12 lumens	25-40 mA resistor	20 hours	60 hours
Medium	22-34 lumens	80-130 mA resistor	7 hours	20 hours
High	75 lumens	330 mA AMC7135 regulated	2 hours	6 hours
Turbo (momentary)	up to 180 lumens (dependent on battery voltage)	1 A 3x AMC7135 semi-regulated	-	-

Total cost for donor body and parts was only about NZ$15 for the headlamp, and a few dollars extra for the external AA pack, plus a small amount extra for glow in the dark pigment, clear nail polish, and switch cap and o-rings.

Field tested – April 2008

Most of the time in the cave, being in a small passage, I used the medium setting. In larger rooms (or when wanting to see the animal behind the reflecting eyes at night) the high setting was great. There probably isn't quite enough voltage to run the turbo mode to a full 1 amp, but if the batteries are reasonably charged there's a very noticeable difference in brightness.

And it looks only slightly jury-rigged!

FWIW the red insulation tape is "just in case." After having my previous headlamp die mid-wriggle by being bashed to death I didn't want it to happen again. (FWIW the tape has now come off again.)

Further testing showed the need for better mud- and waterproofing, so I added a silicone glow in the dark switch cap over the button and sealed it with hot melt glue, and got some o-rings the right size to fit in the recesses around the front lens and battery cover.

With the insulation tape providing better bash protection for the front lens it has survived very well, with the only serious damage occurring when it was accidentally dropped by another caver while we were preparing to enter a cave (it tipped off the top of a bin of stuff, or something like that). One of the "hooks" that the battery cover clips into broke off, so it's had more insulation tape wrapped around it since then.

Celebrating the independent kiwi spirit of invention.

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Please don't buy stuff from a country in the middle of intimidating its neighbours.