Tuesday, October 26, 2010

Caving Headlamp

The Plan
I decided I might need a caving headlamp. It needed to be waterproof - as in able to withstand immersion - and able to withstand a beating, so I thought I'll have to make one myself. Also I didn't want to spend a gazzilion dollars which is what would have happened if I had to buy one off the shelf.

The LEDs
I had a couple of old white 3W Luxeon Star LEDs lying around so I thought in the interests of reducing waste I should use them even though the more recent models available now are more efficient.

Current Regulators
The best way to run high powered LEDs efficiently is with a buck regulator set up for current regulation.Modules to do this used to be exorbitantly expensive despite the regulator IC's on them being fairly cheap. Luckily there are some nicely made Chinese units available at www.dealextreme.com. I bought the buck regulator circuits that were configured for 800mA and changed the sense resistor to set the max current to 690mA as required for the 3W Luxeons. I also lifted the feedback pin and inserted a 1kOhm resister to allow me to switch in resistance to Vout thereby dimming the LED by reducing the current.

I decided to upgrade my rechargeable batteries from clapped out NiMH AA cells to some nice 18650 lithiums. I soldered leads to each cell and covered them with heatshrink to protect them. The leads are terminated with miniature locking connectors from South Island Components.
I also bought a fancy do-everything charger from dealextreme. This is a very nice unit which can be programmed to do multiple charge/discharge cycles and record the energy in and out - useful for assessing the condition of suspect cells.

LED Configuration and Dimming
The left LED is set up with a DPDT (on-off-on) switch for medium, off, full. It has a spot beam lens for navigational use and illuminating distant features. The right LED is on all the time that the batteries are plugged in. This is a safety feature to avoid bumping the switch and ending up in total darkness unexpectedly. It is controlled by a SPDT (on-off-on) switch for low, full medium power. The currents for the low, and medium settings are set with some small surface mount 50KOhm trimmer pots between the feedback pin of the regulator and Vout.

Lamp Enclosure
The lamp enclosure is made from a block of aluminium milled out to fit the LEDS and circuitry. The Leds are thermally connected to the back of the case so it acts as a heatsink. The front of the case is 5mm thick polycarbonate (aka Lexan), which should be strong enough to stop a bullet should I happen to encounter one. I used some silicone sealant on the front of the case to seal it when putting it together.

Battery Case
The battery case needs to be water proof and rugged. I found just the thing from Witz. Unfortunately, in New Zealand they only seem to be available at a high price from Highbeam. Still, they do seem to be worth it. I sealed the cable entry points of the case and lamp with plastic cable glands from Jaycar. To house and protect the battery case I made a cover from neoprene scraps. This is fixed to the helmet with a nylon strap and a buckle.

Wednesday, October 6, 2010

Working out parallel resistor combinations

Have you ever tried to figure out which two or three resistors to combine in parallel to get a desired resistance? Usually you have a limited stock of components to choose from too. Here is a simple little application written in C# for .net that calculates the best values to use from a given set.

Download Program

Download Source Code

Sorry about the poor design and sloppy nature of the coding. I whipped it up in rather a hurry. Maybe one day I'll have a spare life and get around to improving it.