FF’s personal headlamp

This may not be what most people think about when they hear the phrase “personal headlamp”. I don’t blame them, as I don’t consider myself to be “most people” very often at all. In particular when it comes to mad, rushed electronics projects involving lead-acid batteries and duct tape.

The back story of this lovely piece of kit is that I, years ago (before I got myself a pocket-sized Dealextreme LED torch) too often found myself walking around in the dark Finnish winter with but a cell phone to light my way. A late night with plenty of coffee, tape, creativity and small lead-acid batteries at hand spawned this:

The first revision. Mid-2008, I think. EXIF data is since lost, but I'm quite certain that the picture is from 2009.

It consisted of a 21W bulb inside of the rotating armature (from one of those silly LED lights with a crank), as well as the three-position switch and 7 Ah 12 V battery that are still there in its current incarnation. It had one level of brightness and it served me well for a good while. However, once the bulb went bust last winter due to an incident involving a big dog, air travel and snow, I wanted more for this year. I wanted an H4 bulb.

H4s are lovely due to their flexibility: They have two filaments, which can easily be hooked up to run in series, as the following “pseudo-schematic” demonstrates:

Schematic over how my H4 bulb is hooked up

In a car, pin 3 is almost always ground. When you switch between low and high beams, you turn on one filament at a time. The main difference between the filaments is that the low beam filament has a bit of metal beside it, which prevents light from getting reflected straight ahead and into oncoming traffic.

In my application, however, I abuse the fact that the filaments share a common ground for the purpose of running them both at once and in series. By applying a voltage across terminals 1 and 2, I’m effectively halving the power dissipation of the bulb with no external circuitry. Since pin 1 is always ground, I can then run the high beam filament by simply switching my voltage source from pin 1 to pin 3.

It isn’t a particularly good solution, as the bulb is vastly less efficient at lower power; I’d get more light out of the 25-ish Watt it consumes by using a circuit to PWM one filament at 50% duty cycle. However, if I was to care about efficiency, I wouldn’t be using an incandescent bulb to begin with.

Here’s how the hook-up looks in practise:

Black is GND, red is VCC, yellow is high power, orange is low power.

With that out of the way, let’s get onto the actual building of the headlamp in its most recent incarnation! Some painstaking abuse of my side-cutters resulted in this:

It looks like the light is supposed to go behind it, doesn't it?

The holder for the bulb is made out of an old computer fan grille.

Some abuse of my fingers then resulted in this:

I don't know what I'd do without my red copper wire.

Low power mode, where both filaments are activated.

High power mode, where only the lower filament is activated.

A "normal" exposure of the light being on, low.

A "normal" exposure of the light being on, high.

The bulb I used for this has an extremely cool colour temperature. The white balance I use for these photos is set for my 600W halogen flood light, which I consider to already be quite cool. This little 60W bulb still trumps it in that regard. I was actually fairly surprised at how white the bulb is, as the battery voltage drops to well below 12 V when using one filament.

So, that’s the story on my headlamp. It’s incredibly inefficient, not particularly durable, very heavy and ugly beyond belief, but it’s saved my butt more times than I can count. While my little LED torch is fine for most things, you can’t just put it down on a table and expect its little focused optics to light up a room properly. It also makes cars turn their high beams off for me at night, and I can even return the favour!

It looks a lot better in real life, as the filaments fade down over about a second after it's turned off.


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