This is one of those Joule Thiefs that uses an old germanium 2N404. But I got them from a German seller on eBay, and I have a suspicion that they were made within the last decade or so, probably by one of the makers in the former soviet bloc countries. I’ve measured true 2N404s and they don’t have high gain like these do.
The LED is white but looks bluish in this picture. It’s still bright even though it’s running off a depleted AA cell, with only 0.87V left in it. The resistor behind the coil is 1k. By the time the LED dims to nothing, the AA cell will be below 0.2V, probably 0.1V or maybe less. That’s DEAD!
What would you consider to be the best for a down to 0.2V project? 2N404?
Yes. Or any germanium transistor. Or else a JFET.