Harbor Freight sells an auto-darkening welding hood (Item# 46092) for about $50 -- depending on the current sale. It is cheaply made (of course), but not ridiculously so, and has proved to be worthwhile to me. However, I had a very unusual problem that I could not find documented anywhere else on the 'net. Hopefully my blog post will help others in my situation.
For the first month I owned my TIG welder, I only welded aluminum. That is the main reason I bought the machine, and I was practicing quite a bit. Eventually I got curious about stainless steel, and switched the machine over to DC and tried some welds. I'll write more about welding stainless in another post, but the first thing that I noticed was that my welding hood flickered badly during the welds. Strangely, at low power arcs (10-20 amps), it was dark all of the time, but when I cranked it up to 150A, the lens would be become clear, and only ocassionally flicker into the dark mode. At the time, I didn't make the connection between using DC mode on the machine and the hood flickering. It had been a week since I last used the machine, and there seemed to be no reason why DC would cause the hood to fail. I also noticed that aiming the hood at a normal fluorescent light would cause it to darken. I thought the thing must be broken.
I took the hood apart and looked at its circuit board: 3 opamp ICs, 2 logic gate ICs, an oscillator, and two ICs that had their numbers scratched off. The board itself is multi-layered with an apparent ground plane on the bottom, so determining the wiring between chips is damn near impossible. I was expecting to find a soldering problem, dead battery, dead solar cell, etc, but all testable components were good. After connecting my oscilloscope and learning a little about how auto-darkening hoods work, I decided to return the thing. I figured one of those unmarked ICs might have fried. Harbor Freight gave me a new hood and tried to sell me a 2-year warranty. When I politely declined, the saleperson smirked and said "I'll sell you another one in 6 months." Gotta love Harbor Freight.
I really expected the new hood to work properly, but alas it flickered in exactly the same manner as the first hood. By this time, I had noticed that the hood worked flawlesly for AC welding, was darkened by fluorescent lights, and even was darkened by me waving my fingers in front of the sensor. It was NOT darkened by pointing it at the sun, or a DC welding arc. So, my conclusion was that the hood only responds to high-frequency changes in light -- not high-intensity changes. Apparently, the DC arc on my inverter welder is so stable, it does not have enough ripple to cause the hood to trigger. The solution: I set the machine to "pulse" at 200 Hz, with the background current %90 of the peak current. The current pulsing is so slight, that it doesn't really affect the weld, but it is enough to reliably trigger the hood.