Design Analysis of a Petzl ACTIK CORE Headlamp

Hi! It’s been a while. :-)

Since my last post here five years ago I’ve been around a fair amount, played with new toys, built few things. One thing that recently impressed me though is this little mid range Petzl headlamp.

I’ve been using it for trail running in the night for the past two years, including some fairly long adventures. As an embedded enthusiast I got more interested in the design of the little thing, so I took it apart to study it, and found many non obvious things. Let’s try to highlight some of them here!

Main body

The body of the headlamp is the classic Petzl TIKKA. Apparently Petzl is selling lamps based on this design since the 2000s with a few smaller variations on the body and strap design (this one is fairly basic) on higher end models.

It’s a compact and lightweight design and there’s a 30 degree angle between the back that sits flat on the forehead and the lenses plane, which makes it look a bit goofy in my opinion but also seems to place the beam just about straight on line of sight on the highest position. Well done Petzl!

The body tilts downwards on two pivot points on the bottom, and has a ratcheting built into it that gives five positions, pointing almost vertically down on the lowest one.

The electronics sits on a middle piece screwed to the main body and everything is locked by the battery compartment, the lenses are on a separate single mold.

To state the obvious, there are three LEDs on this one, two high power white ones and a low power red one.

Power source

The original TIKKA was running on 3 AAA batteries, which largely dictates the size of the whole unit. Current models run on rechargeable lithium batteries, and the battery is a separate unit that includes both protection and the charging circuit via micro-USB.

I think that the reason why the charging circuit is not part of the main lamp electronic is both to simplify construction (one less port to open and protect from dust/water ingress) and to avoid problems with mixed chemistry charging (preventing the charger from trying to charge non rechargeable batteries). That said, this setup works great in practice, it’s easy to charge batteries in parallel, and it gives the option to run the lamp on non rechargeable batteries for those long over-night events.

The battery compartment is not sealed, but the main electronic part is. In any case I’ve used this under pouring rain plenty of times and never noticed any water ingress, just don’t submerge it.

Power output

The power control for the ACTIK is trivial, there are three MOSFET (the ones marked G31) to control each of the three LEDs. There’s a pair of limiting resistors for the two high power 3535 white LEDs, and apparently a separate one for the red one.

The white LEDs are PWM controlled and have three (white) power levels: a low power mode on a single LED, a medium power (guessing 50% PWM) and a full power one (100% PWM).

The light power drops with the battery voltage following the discharge curve of the Lithium cell. In practice, I found it to be noticeable only when the battery is almost completely out of charge and about to switch to medium power.

Anyway, these are the power numbers from the light at various charge levels, full power mode:

• Full charge: 4.2V 1.2A 5W
• Half charge: 3.7V 0.8A 3W
• Almost flat: 3V .27A 0.8W

Which makes sense with a ~3V drop on the LEDs and 0.7 Ohm on the limiting side. There’s no heatsink and the LEDs seem to dissipate on the battery assembly.

Petzl offers a constant power mode on more high end models (Reactik and Nao), this is a photo from one of those (thanks FCC):

The way that seems to work (guessing from the layout) is with a pair of transistors (the SOT marked 053) driven linearly by a dual op-amp (the ST QFN16 part next to them) for constant current feedback. In practice, it’s like replacing the fixed current limiting resistors with two variable resistors.

The constant current design is neat and I would love to see it on lower end lamps. In Petzl’s defense though that circuit seems to take quite some space on the board, and I haven’t been able to find any existing high power constant current sink on the market.

That said, both designs are horribly inefficient. I realize that’s not much space for a 1A DC/DC power converter, but that’s what I’d love to see on top end models to squeeze a bit more time out of the battery.

The lens system

This is what really, really intrigued me on the design of this lamp.

At a first approximation, it’s a headlamp with two LEDs. Two is better than one, period. Twice the LEDs, twice the power, half the battery life, easy, next paragraph.

Well, actually there seems to be quite a bit more going on there. The beam produced by the lamp is quite special, and the optical magic that seems to be behind this appears to be a clever combination of three (!!) TIR lenses design squeezed in two optics.

First, I don’t know anything about any of this, but these guys at LEDiL do, and wrote a nice guide about TIR lenses applications, so go read that to see the light. (haha, you see what I did here?)

Now let’s see what we have here, starting from the middle light, the one above the ACTIK text:

The lens is an horizontal oval. It produces a fairly uniform wide beam on the horizontal axis, by means of those vertical grooves on the lens surface. The beam looks a little like this:

In practice, it throws some light power on the left and right side of the lens axis.

Now the left one is the really interesting one, as there’s a bit going on here:

The construction of the lens seems to be this: roughly two thirds of it is flat polished, the remaining third is horizontally grooved, but the features are not a symmetric triangle, and more like a sawtooth. This is the resulting beam:

What’s going on here is that the flat part of the lens gives a focused circular beam straight on the lens axis, taking two thirds of the light power, while the horizontal groove is creating a vertical oval with the rest of the light, but the groove design is offsetting the beam towards the bottom.

The resulting beam of the two lights is the following:

Which we can break down in the three sub-beams in this horrible picture:

• The horizontal oval in blue
• The spot light in red
• The vertical oval in green

What does it mean in practice? Looking down the trail in the middle of a forest, the spot beam gives a nice deep point of vision to see where we are going, the vertical beam is showing us the nearby rock that we are about to trip on, while the horizontal beam gives some peripheral vision which helps getting the sense of speed, and also seeing the bear that is about to start chasing us.

To achieve the same with just a wide angle lens, we would have to waste a significant amount of power throwing light where we don’t need it: in the sky, and on the corner of our field of vision.

I love this beam shaped design! Good job Petzl!

Electronics and assembly

The electronics on this is maybe the least interesting part. The LEDs are on a usual MOSFET + resistor setup, the micro running the show seems to be a PIC (12F1572 3x3mm MSOP?) with its own linear regulator. The remaining IC may be a power supervisor or some other support stuff.

The interesting thing to note about the construction is the flat cable connecting the two boards. To maximize the battery space and keep the unit compact, the two boards are mounted on different planes: the lower one is flat and the LED carrier is angled. The way Petzl went for manufacturing though, was to make a single board (single design, single BOM, single assembly line), connect the boards with a flexible cable, and integrating point to snap the two boards apart for final assembly. Clever bit of design for mass production!

That’s it, maybe way too much enthusiasm for a headlamp, but I’ve learned a lot out of it, and felt like sharing. Thanks for reading!