How I Made My Caving Lamp
I finished making my caving lamp (in May 2008) and thought I
would record what I did as I went along. I would definitely
have appreciated finding a site with all the relevant details on
when I was planning the light. So I figured if I couldn't find
such a site, why not embrace my techno geekery even further and
write one myself! Then other people making a lamp
for the first time at least have a bit of a starting point! At
some point I'll also get round to putting links in to show where
I got the various bits and bobs from.
- Here's a list of what I ended up using to make the light, in no
- An old Oldham headset
- Four 3.7V 2600mAh 18650 lithium ion cells for each
battery pack (I made two).
- Self amalgamating tape
- Electrical tape
- A lithium ion battery charger
- Powerpoles: 2 for each battery pack, 2 for the charger
and 2 for the headset and also a crimping tool.
- The following types of sticky stuff:
- Arctic Alumina Thermal Epoxy (heat conducting glue)
- Silicon sealant
- Some sort of semi flexbile waterproof plastic glue...
- Some wire
- 2 Cree XR-E R2 LEDS (at the time of writing these are
the brightest ones...I think)
- A bflex driver from TaskLED.
- Some silicon grease.
- A selection of clickable shut Lakeland Plastic boxes.
- A small length of bungee.
- Soldering gubbins: solder, iron, wire strippers,
a multimeter is useful.
- A Carclo lens (I chose an 8 degree one for my spot
- A piece of copper pipe.
- Some heat sink of some sort.
- Some conformal coating for protecting circuit boards.
- Some sort of power tool like a Dremel or a JCB or
- A Mr Kipling Cherry Bakewell.
- A fair bit of patience.
- An old Oldham headset
- This is just a bit about justifying my choice of batteries etc and working out expected run times and that sort of stuff.
- Batteries don't have a constant voltage - it gradually
drops. This means that the current through the LED will gradually
fall and it will get dimmer as the battery drains. This is annoying.
- A driver circuit gives out a constant current to the LED.
- There are two types: boost drivers are used when the voltage
of the battery pack is lower than the required voltage of the
LEDs. Buck drivers are used when the battery voltage is higher than
the required LED voltage. There are some that are both buck and
boost (useful as the voltage of the batteries drop below the
required voltage) but I'm told these are inefficient.
- It's a buck driver with the required voltage ranges.
- It has a low voltage sensor to protect my batteries.
- It has a temperature sensor to protect my LEDs.
- It's really clever: there's a chip in it and you connect it
to a switch to give you loads of control over your LED current etc.
BatteriesI chose Lithium Ion batteries because they seem to hold more energy per unit weight than NiMH or NiCad. The drawbacks are that they need looking after more: you can't run them until they are completely flat and you have to be careful charging them. But since I was intending to use a driver circuit with a cut-off voltage that I can choose and I was going to buy a special charger I figured these difficulties didn't actually matter. I decided that using 4 cells per battery pack was a good tradeoff between weight and amount of battery life.
LEDsAt the time of writing the 'best' LED seemed to be the Cree R2 although it only seems to be available from Australia. At 1A each LED gives of something like 270 lumens which doesn't mean anything to me but is pretty bright when you try it! My main worry was heatsinking. My light sepcifications are very similar to the Scurion in which the whole headset acts as a heatsink. To get round this I decided to mount the LEDs on a copper plate and connect this to a heatsink running through to the outside of the lamp which the connects to the helmet mounting bracket. At 1A, I suspect this still won't be enough heatsinking but the driver circuit I'm using has a temperature sensor which lowers the current when it gets too hot so my LEDs are still protected. In addition I'd only be running at 1A for relatively short periods underground.
The Driver CircuitI knew nothing about these before making the lamp so here's a quick intro, as I understand it:
Expected Run-TimesDisclaimer: these calculations might be wrong. Each battery pack contains 2.6Ah at 4*3.7 = 14.8V (although in fact when fully charged they are more like 4.2V per cell. so these estimates are conservative). Running at 1A the battery can deliver 14.8W for 2.6hours and therefore contains 38.5Wh of energy. If I run my LEDs on full power, according to the datasheet they will want about 3.7V each = 7.4V. I make a conservative estimate that the driver is 85% efficient and so the LEDs will draw: 7.4*1/0.85=8.7W. This means they will run for about 4.5 hours. The lowest current the driver can deliver is 40mA. At such a low current the LEDs only need about 2.5V each. This gives a runtime of about 163 hours or something stupid. But I've yet to test if they actually produce enough light to see by at this current! A more sensible current for caving on might be 200mA. At this current the LEDs need about 3V. This gives a runtime of about 27 hours.
|A battery pack.|
heatsink poking out the back.
of the headset.