Caving Headlamp – 9036-A
By Ian Mander 8-13 April 2012, updated 14 May 2012.
The lamp housing is a Gainta Industries G102 waterproof aluminium enclosure from Surplustronics. 90 mm wide x 36 mm high x 30 mm deep. IP65 rated.
For general caving two Cree XM-L T5 neutral white LEDs are mounted at the ends of the enclosure. These use wide angle/diffusing TIR optics to provide a centre-weighted flood. The LEDs will need to be mounted to an aluminium angle attached to the sides (and end) of the enclosure. The bumpy front on the optics may hold mud and be hard to clean – this will need some testing.
A spot beam is provided using an aspheric lens with a Cree XP-G R4 neutral white LED positioned in the middle of the enclosure. The depth of the enclosure means the LED should be at about the right distance from the lens when the LED is mounted on the back of the enclosure – good for heat sinking.
The battery housing is a Naithawk waterproof plastic box from International Outdoor Store. Measured dimensions 132 mm wide x 82 mm high x 37 mm deep; internal 118 mm wide x 68 mm high x 31 mm deep. Claimed shipping weight 65 g. It includes egg crate foam. IP67 rated.
The housing is large enough for the following battery configurations.
The battery housing supplies power to the lamp housing with a 4 conductor cable (Surplustronics, no longer available), which allows the conductors to be paralleled for lower resistance, or used individually for separate battery packs. For example, three separate 4x AA packs with a shared ground, or a 3x Li-ion battery completely separate from a 4x AA battery.
The basic battery configurations with some specs. Protected Li-ion cells will have to be used in all situations except a single cell.
Different batteries are suitable for different drivers.
A battery with 2x Li-ion cells in series could be used with an AMC7135 driver but would have to drive two LEDs in series. It would rule out any other battery configuration, so it's not an option because being able to work with 4x AA is deemed essential thanks to the ubiquitous availability of AA batteries.
Using up to three Li-ion cells in parallel will give lots of capacity and resilience against battery voltage sag if needed, and with the option of 4x AA will provide the most versatile option.
If the AX2002 is going to be used it will have to be used for all the modes because it's too tricky to switch everything required to mix drivers.
3.77 V is too high for more than a few minutes runtime with a single Li-ion cell. It's even worse if the switch contact resistance is considered. Contact resistance of a typical micro switch (used as a momentary crossover switch for the turbo mode) is 0.05 Ω. At 5.0 A that will drop 0.25 V across it and produce a serious amount of heat. (Knew there was some reason turbo mode is momentary.) That takes the voltage the AX2002 requires to stay in regulation to over 4.0 V. The AX2002 is ruled out.
One significant advantage of the AMC7135 is that it can be switched with a very small current to the VDD pin with the LED permanently connected, thereby saving the 0.28 V drop a micro switch would have in turbo mode – very handy.
However, even having settled on AMC7135 drivers, choosing particular drivers is not so easy, as there are many available, and the AMC7135 can be switched so easily, including by MCU. With some clever switching (and maybe diodes) it might be possible to use just one driver for all modes.
The wide beam will have three general modes plus a momentary "turbo" mode. Medium will be the normal mode used while caving.
The two XM-Ls will be run at the same current and for most configurations are in parallel run by the same driver.
If using PWM a three mode switch would be wired off-on-off-on as per the spot beam. This would allow lots of "interesting" modes and provide switch redundancy. DX sku.7612 is the leading contender because 20 lm seems too low for low mode, and a jump from 5% to 30% is too great. I strongly prefer the modes to be spaced at roughly multiples of 3. However, 136 lm might be too high for medium. A driver with PWM modes of 10% and 30% would be slightly better.
Using continuous operation, the switch would be wired for the three separate modes to enable the resistor or the AMC7135 chips. This would provide just the three modes configured and no switch redundancy. The advantage is that the low mode could be set to anything desired and medium might be a little more efficient because it wouldn't be PWM.
To switch between the general modes and turbo, a micro switch is wired in a crossover arrangement; NC is the general modes, NO is turbo.
I'll assume DX sku.7612 is used. This greatly simplifies the driver situation. But what a lot of effort to get to this point!
There is little point in having more than one mode for the spot beam, so the choice of driver just depends on how well it will work with the battery. The two options for the spot beam driver are a multi-mode 4x AMC7135 at 1.4 A or an AX2002 at 1.5 A (using a 1 Ω resistor added in parallel to the 0.2 Ω resistor that comes with it).
Having only one mode means the spot beam will be switched using a three position switch wired off-on-off-on to provide a little redundancy for switch contacts going bad. The switch can apparently handle 1.5A OK but the AMC7135 driver could be switched with less than 1 mA – good because there won't be any power lost in the switch resistance.
Because the AX2002 will need about 3.86V to stay in regulation it cannot be used with a single Li-ion cell. The AMC7135 driver it is – DX sku.127685 seems ideal.
Wide beam general modes: 3x AMC7315 driver (DX sku.7612) providing PWM modes to two LEDs in parallel. Two Li-ion cells looks pretty good, and for around the same weight as 4x AA NiMH cells.
Wide beam turbo mode: Two 8x AMC7135 drivers providing a constant 2800 mA to each LED, 5.6 A total. The really low values for a single Li-ion cell serve as a warning not to use turbo with only one cell.
Spot beam: 4x AMC7135 driver providing a constant 1400mA.
Total is about $83 not including consumables. (This may have been reduced by the simple driver setup.)