Auto (not) darkening helmet!

Yes, I've owned a Lincoln Electric auto darkening welding helmet for the past decade or so. Generally I've been pretty happy with it. It has adjustable sensitivity (9-13) and has a photo cell array on the front, so that it doesn't rely on battery replacement to keep running.

Or so I thought. I dug it out recently for use with the brace of Arc Captain welders I rashly acquired and found that the "auto darkening" wasn't remotely happening. The upside of this was that I was able to rush out and get one of the new "True Colour" helmets from Yeswelder (Chinese) that claim to show colour (rather than dark green).

However, I'm pigged off that it stopped working. So what was going on here? If it has photo cells on the front, it shouldn't "run out of battery" should it? Well it turns out they are sneaky fuckers. Read on to find out....

Let's take a closer look. The adjustment knob simply pulls out and the pot can then be released:

And the cartridge is held in with a spring clip.

Quite a simple, self-contained assembly

There are a couple of holes either side of the photo cell array. These are (UV) light detectors, used to trigger the darkening.

But it's glued together. However, we have the technology...

And there you have it. 2 lithium coin cells, both dead as a fart, with a pair of solar cell arrays. It seems that these cartridges have a dual redundant circuit using 2 batteries, 2 photo cell arrays and presumably 2 largely independent control circuits. There's a reference to some sort of safety standard for welding helmets on the inside of the helmet. I couldn't be arsed to look up the details but I assume it requires some degree of failsafe operation.

Anyway, once these cells go flat it is evident that the whole cartridge ceases to operate. They didn't advertise that when I was buying the (expensive) helmet in the first instance but I expect they thought you'd simply go out and buy another one. Well it turned out they were wrong on that count.

The modern replacement for the cartridge is a mere $291 MSRP and the equivalent full helmet is about the same.

Those button cells are simply non-rechargeable lithium cells. Annoyingly:

• They are not the "normal"  CR2032 but a weird CR2335 ie 23mm diameter and 3.5mm thick. 
• There is no battery "holder" - these are soldered directly to the PCBA using solder tags. It's not obvious that I'd want to solder wires directly to these boys - even if they survive / don't burst, I doubt it would do them any good.

If you look carefully, you can see the UV detectors at the bottom of the photo. Also 2 sets of wires from the (redundant) photo cells.

The circuit must be pretty simple, as they have gone to the bother of grinding off the part numbers from the 2 ICs. I expect the small device is a comparator and the bigger device is probably just a CMOS or LS logic device. How pathetic, although it wouldn't take much detective work to figure out what they are from the tracking around them.

So I managed to find some similar-ish BR2330 cells at CPC Farnell for £1.55 each (plus ~£2.50 delivery, IIRC). These are simply CR2330 cells with solder tabs - but no insulation. Being slightly thinner and perhaps 20% lower capacity won't be a massive problem. Certainly, it'll be a massive improvement on what's currently in there.

Some polyester tape and tinned copper wire did the trick:

Then some more nasty yellow tape to hold the cartridge body together and the job is done. 

And yes it works, although the photo detectors on the front are clearly looking for UV light. Shining a high intensity LED floodlight at it does nothing, yet even a small arc from the MIG darkens the lens. There's then a few hundred milliseconds before the lens clears.