Celebrating the independent kiwi spirit of invention.

Caving Headlamp – Housings

By Ian Mander 24-27 January 2010, updated 26-30 October 2010, 5 & 8 November 2010, 6 April 2012.

Two housings will be needed – one for the lamp and one for the battery pack. Both should be waterproof, since I occasionally go swimming in caves (sometimes unintentionally) where my helmet goes underwater.

Easily removable housings are strongly preferable because the weight of the lamp and battery pack are not needed if rock climbing or abseiling outdoors during the day.

Safety note: When bolting housings or brackets to helmets, always ensure as little as possible protrudes on the inside of the helmet. You don't need a bolt-shaped hole in your forehead or a screw embedded in the back of your head if your helmet takes a knock. (Hope for the best, plan for the worst.)

Lamp housing

Enclosure

For the lamp enclosure, aluminium is definitely the preferred material. It has good thermal conductivity, is robust, and is reasonably light weight.

Various outlets sell aluminium project boxes of reasonable sizes at reasonable prices. They come with a neoprene gasket to seal them to IP65 – described in the boxes' specs as "hoseproof" – although they could possibly be improved with the use of silicone grease (US$2.70 from DX). They sound ideal for surviving waterfalls in caves or being cleaned off under a tap afterward.

The second smallest box at 64 mm x 58 mm x 35 mm ($8.50 at Surplustronics, $11.90 at Jaycar) looks like it has the right amount of internal space, allowing just enough room for LEDs and switches and drivers. It would be a similar size to several high-spec caving headlamp head units.

The lamp housing shouldn't need to be opened often (if ever), so the bolts can be securely tightened.

Glass insert lens

The front glass should be as thick as possible to resist bash damage; I just don't trust the available lenses that are only 1.25 mm thick, because I know how easy it is to bash one's headlamp on a sticking out rock. 30 mm ø x 3.2 mm thick (US$1.98 from DX) is the thickest I've found, although it has a strange coating which should be easily removed if it proves to be undesirable. Four XP-Gs will comfortably fit behind it if using 10 mm square optics.

I found some large grommets which may be able to seal the glass, but sadly are only for a 1.6 mm enclosure (28.5 mm, $3.80 from Para Rubber). Being aluminium, the edges of the enclosure hole could easily be beveled to reduce thickness by 0.7 mm each side. When the glass arrives I'll find out if it's possible to get the 30 mm ø glass into the 28.5 mm grommet, or if I need to get the 31.5 mm version. With the larger one, maybe just a layer or two of rubber from a bicycle inner tube would be enough to provide a press-fit, and might improve bash resistance because of the flexible mount.

Alternatively, perhaps lead-light window techniques could be used to mount the glass. Either way, a suitably sized round hole will have to be cut in the aluminium lid.

Transparent front window

An alternative to using a glass lens inserted in the aluminium lid, the whole lid could be replaced with an acrylic or polycarbonate sheet. Acrylic is slightly harder than polycarbonate but chips reasonably easily. Polycarbonate is very tough, and is generally said to be 200 times stronger than glass.

This would allow a glow in the dark o-ring to be used to seal the sheet, but the hard part is fitting the o-ring into the back of the window. Melting an o-ring channel does not adequately work.

Luminous button caps

Simple buttons can (I believe) be reasonably waterproofed using luminous button caps inserted with sealant in a tight fitting hole in the headlamp housing. Extra sealant could also be added around the edge of the cap inside the housing.

A UV LED behind the button caps makes them glow brightly, but would only be usable with some switches such as single micro switches that don't cover the whole back of the luminous cap like the 3-mode pushbutton switches do. I'll be using double micro switches. Also, the caps are sufficiently translucent that any LED or even stray light from inside might make them glow reasonably well.

Lamp housing hinge

I haven't sorted this out yet, but the aluminium housings have handy bolt holes that should allow some sort of hinge to be mounted on the back. The base will be bolted or riveted to the helmet.

A quick release bolt pin might be nice if it doesn't leave dangerous protrusions on the front of the helmet. Hinged hinge flaps?

Thermal management

Good heat sinking back to the aluminium housing will be needed because the highest mode will be over 20 W. A finned heat sink on the back of the lamp housing might be a good idea – I have a low profile one in stock.

If using a round glass lens, the LEDs will be positioned around 20 mm from the back wall - a lot of space to have heat conducted across it. A finned heat sink (US$4.70 from DX) cut in pieces, trimmed to size, and reassembled with the fins interleaving (with plenty of heat sink goo or glue) should conduct heat very well and be height adjustable. Alternatively a wide strip of aluminium attached to the top and bottom of the box would also position the LEDs in the right place and might also conduct enough heat for most modes to be run continuously.

If using a plastic window, the LEDs can be at the edges of the box, so they can be mounted on aluminium angle attached to the sides of the box.

Rejected options

90 mm x 36 mm x 30 mm ($9.50 at Surplustronics) is the better option for a small lamp housing; a multimode switch could be located on each end, each switch controlling one or two LEDs.

51 mm x 51 mm x 32mm is another option ($9.90 at Jaycar) but has unknown water resistance. It might be a good option for a different configuration, with a single LED and large reflector (eg, 41 mm ø US$3.68 from DX), but the 64 mm x 58 mm box is cheaper and would offer a bit of driver room.

Battery housing

Enclosure

A plastic container is attractive because thermal conductivity isn't required, yet robustness and minimal weight are important. Having an insulating material is probably a good thing, as it might simplify things for whatever is holding the batteries inside the housing.

A solution has been found in an inexpensive Naithawk waterproof plastic box (US$4.57 from Intl-Outdoor Store) with "egg carton" foam which will do a nice job of cushioning batteries and other items carried in the box. With dimensions of 125mm (length) * 77mm (width) * 35mm (height) it's still a fair bit bigger than required for just a single battery. Without allowing for any intrusions (such as a cable gland) it can fit up to twelve AA cells at once, or three 18650 cells and four AA cells at the same time – giving a potential runtime of 61 hours at 75 lumens.

The seal as installed has a gap in it that allows the box to leak, but with the seal reinstalled without the gap it seems to be quite effective, not bubbling air out or sucking water in when the box is squeezed.

They also ~~have~~ had a more colourful option (US$2.95) without the foam.

Rejected option – internal lighting

Because the waterproof box is solid black plastic, internal lighting is no longer an option. Single 5mm warm white LEDs mounted in the sides of the battery box, directed sideways, might still be a feasible idea, but introduce possible ingress points for water.

A wide angle red or warm white LED in the battery housing would be a very nice touch, the warm white LED in particular being very comforting in tight squeezes where I've found it can get dark very quickly once I take my helmet off. Having light coming out the back of the helmet would be very useful if the headlamp is pointing away from me down a narrow hole, up behind me, or pointed straight into a wall or mud puddle. Quite simply, it helps to prevent claustrophobia to be able to see. A transparent or translucent box would obviously be best for this, so no extra holes would need to be cut, but replacing an opaque lid with clear acrylic would also work, giving no extra holes to seal. The LED would only need to be run at a few milliamps.

Having something glowing inside the battery box would also make it easier to find my helmet if the front light fails while I'm not wearing it. That happened once when I didn't have my helmet strap done up. We live and learn: We had stopped for lunch, so I unbuckled my helmet then a hardcore caver said "Ooh, look at all the pretty glow worms!" So I looked up and my helmet dropped off, going out when it bounced off a rock. Because of the glow in the dark highlights I'd added to the headlamp it wasn't any problem finding it, and it started working again with a minor adjustment (I bashed something back into place on the headlamp).

Rejected option – metal

The same 64 mm x 58 mm x 35 mm aluminium enclosure as used for the lamp housing at one point looked like the best option for the battery pack. It is just (and only just) large enough for a hardwired 4xAA square battery holder ($1.50 from Surplustronics, IF they have them in stock). A bit of hollowing out is needed on the ends of the enclosure so the battery holder can be inserted and removed easily, and on the bottom and/or inside the lid to allow room for the lid to fit on easily (3mm thick top and bottom; 29 mm available height, 30 mm required).

The aluminium enclosure lid could be replaced with a piece of thick acrylic, which would provide a lovely clear window – very grunge industrial – and would be much easier to hollow out than aluminium if more height is needed for the battery pack.

Using my normal configuration of cable ties to hold the battery pack on the helmet will allow for battery changes without having to unscrew the enclosure lid, and allow for reasonably easy access to connect/disconnect the battery pack.

Rejected option – plastic

A 200 mL Klip It container made by Sistema looked almost ideal but unfortunately serious doubt was raised about the quality of the seal when a container leaked on a rafting trip (Sept 2010). Testing of the seal without silicone grease showed this box would not be reliable, especially when subjected to rough treatment in a cave. No testing has been done with silicone grease.

The Klip It container is more than twice the size of what I want and will need some foam rubber inside to stop its contents from banging around. I could fit a 4xAA holder and a 6xAA holder in one container – 10 cells total, providing over 13 volts and potentially over 36 hours of runtime at 85 lumens. That configuration would be rather heavy though. The plus side is they ~~have~~ had movie character boxes. (There's even Buzz Lightyear!)

Because it would be easy to get into, perhaps I could use the extra space for small items which I'd want to have on me in an emergency, not in my caving bag. For example:

Space blanket or survival bag.
Key ring torch.
Whistle.
Muesli bar or lollies.

An alternative is to investigate various waterproof project boxes from electronics outlets or plumbing or electrical fittings (example). The disadvantage is that they probably wouldn't be easily openable... although the specs for some sealed cases (example) say there's a hinge kit available.

Power and cabling

Cable

A circular cable is preferred, as it should be much easier to seal with an off-the-shelf grommet or cable gland. It would be best to have as few holes in the housings as possible, for wall area economy and on the basis that the fewer holes there are the less likely something is to leak. Round mains power cables are readily available, although tend to have quite a large diameter for high quality cables. Oval cables that are smaller are available but have a larger resistance.

A four wire round mains cable ($2 per metre from Surplustronics) looks like the best option. Doubling up the 1.0 mm² wires will mean 2.0 mm² conductor cross section area, which will give around 0.04 V total voltage drop in the cable (round trip) on turbo mode.

The cable should be cut long enough that the finished headlamp can be mounted on a variety of sizes of helmet.

Electrical connections

On/off switching of the headlamp is via the connection/disconnection of the battery pack to the headlamp cable with a 9 V battery clip ($0.60 from Surplustronics, up to $1.50 for a shrouded type from Jaycar). There is not enough room in the enclosure for a 9 V clip on the battery holder itself, and to use two 9 V clips as a master switch will need trimming of the clips. To protect the battery pack from shorting when not being used, a dummy battery clip will be housed inside the enclosure. If there was more room (for string) it would be tied to the end of the cable so it can't get lost.

It may be possible to reduce the height required by the battery holder by removing some plastic from the intervening layer of plastic. Reducing the length of the battery holder, if possible, would mean hollowing out the ends of the enclosure wouldn't be needed.

Perhaps there are special low-profile 4xAA battery holders available with the cells offset or touching each other instead of with a layer of plastic between that would circumvent the need to hollow out the top and bottom of the case. But it should be possible to make a low profile battery holder by altering a 4xAA battery holder without weakening it too much. Using a couple of 2xAA battery holders offset from each other with the cells facing inward so they can overlap might weaken them too much because some of the "vertical" bracing would have to be removed for the AA cells to "mesh" between each other.

For larger battery housings, a "tap-on" connector could be used when/if battery packs are to be paralleled. Because the battery packs would be connected in parallel (no diodes for battery balancing; that's just a waste of voltage) any spares, when not used, should have covers on them – things could get very nasty if they contacted the wrong way. Cover clips should also be on any spare populated battery holders in my caving bag.

Cable waterproofing

After a lot of searching I eventually found some 6.35 mm rubber grommets that can fit a 3.2 mm enclosure. ($1.20 from Para Rubber.) The cable has an OD of 8 mm, which is rather difficult to get through a 6.35 mm hole even when the grommet is not yet installed in an enclosure. There's no room inside the enclosures to bend the sheath around anywhere, so it's fine that only minimal length of the sheath can be pulled through the grommet – I won't have to enlarge the hole at all. The sheath should be trimmed back from the end of the cable before insertion into the grommet to give enough wire to work with inside the enclosures.

Rejected options

If it's sufficiently round (not square thanks to a thin sheath), a 4-core alarm cable might be an inexpensive option ($0.84/m), with the wires doubled up. However, the resistance would probably be undesirably high on turbo mode; 0.23 V would be dropped in the wire, which is 1.36 W of heat dissipated in the wire! That would warm the wire up a bit, and if using thin wire is a good reason to keep it only a temporary mode. That voltage drop would impact slightly on turbo mode runtime because of the missing voltage, and because it adds almost 6% to the total power consumption the runtime would also be reduced.

A two wire oval cable ($1.50 per metre from Surplustronics) has better wires at 0.5 mm² so would give 0.16 V drop on turbo mode and just under 1 W of power dissipated. A two wire round cable ($2.51 per metre from Bunnings Warehouse) has slightly bigger wires at 0.75 mm² so would give 0.105 V drop on turbo mode.

Waterproof cable glands with IP68 rating are available in various sizes (eg, 4-8 mm 2-pack, $6.90 from Jaycar). With the new Naithawk battery box I'm no longer concerned at how much internal and wall space a cable gland would take up. The cable glands don't look small, but space is not at a premium in the walls and inside the battery box.

Note: Many of the cables available from Surplustronics are copper-clad aluminium (CCA), but are not always clearly marked. If the cable seems really cheap, it probably is CCA, not oxygen-free copper. CCA is subject to corrosion in the lang term (we're talking using it in caves, remember), and has twice the cross-section area for the same resistance as an oxygen-free copper cable – needlessly big and costs just as much for the same resistance.

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