polaris

Polaris - an LED backup light

As many cavers, I first bought a Petzl Duo as backup lamp for my helmet. And, as almost all Duos I know of, it failed after some time. The contacts inside had corroded and it wasn't possible to revive it permanently - the plastic case was just not waterproof. Most of the time, it worked at least at a Mono, but sometimes as Zero.
This was the reason why I constructed the Polaris lamp. The basic idea was to use the almost perfect combination of LiTCL batteries with white LEDs (see my LED lamp page for more info). From my experience with my 4 LED experimental lamp, I knew that even a single LED would be better than nothing, four just ok and ten a real lamp. With more LEDs, the lamp gets more expensive, bulky and there is a problem with the heat building up in the LED array. 

All features in an overview:

  • CNC-machined aluminium case
  • O-ring seal for the plastic window
  • waterproof switch, additional O-ring in the knob
  • LED array with ten white LEDs
  • Rotary switch with six positions (off.....max)
  • Powered by a single LiTCL LSH20 battery

 The LED power is controlled by simply adding a resistor in series with the LED array. Now this is really primitive, but it's not so bad after all:

Runtime
 

Setting

LED current [mA]

run-time [hours]

run-time [days]

Efficiency

1

194

67

2,8

100%

2

48

240

11

84%

3

30

433

18

80%

4

16

812

33

74%

5

7

1857

77

69%

6

0

The advantages are:

  • 100% efficiency (no loss) in max. setting
  • very simple, rugged and compact design
  • works until battery is completely empty
  • "always on" mode is possible (always find your helmet in the dark): runtime 20 years @75uA (this is still visible light!)

 The disadvantages:

  • efficiency might be better in low settings, although this is not easy at lower current levels
  • no "high power"-mode - the current is limited by the LED characteristics

 Have a look at Henry Schneiker's ActionLight for a lamp design with a really sophisticated power supply and a larger LED array.

The LiTCL battery

The battery is an important part of this lamp. Look at the description in the LED-Lamp page. The LSH20 battery has a capacity of 13Ah, weighs only 100g (4AA alkaline: 90g) and has a shelf life of 10 years. When you look at the burn times, it is quiet likely that it lasts a couple of years when you use it as a backup lamp. Most people are a bit shocked about the price of the battery - 38 DM is real money, but think how long it lasts. When I am driving to a caving trip to Switzerland, this costs me about 150 DM only for fuel - so the battery costs are absolutely irrelevant.

Pictures

All pictures are reduced to 800x600, click to enlarge.
This is the aluminium case itself - I replaced the knob later with an selfmade one.
 

Top view into the lamp - you can see the SMD resistors that are soldered at the switch terminals.

 

 

The complete lamp with the new knob. The knob has another O-ring seal that allows you to fill the inside with petroleum jelly (or something similar) for additional protection.

 

The base (you can see it's the same NC program as the window) is a bit too heavy - I only hat 4mm aluminium handy when I made it...