Quantsuff reminded me of his Hak Light in an email. I haven’t visited his web page in awhile so my memory needed refreshing. I checked it out, and I thought about another use for his circuit. At the dollar store, they sell (when they’re not out of stock) the CR2032 or CR2025 coin cells on a card for a dollar. I forget how many there are, maybe 5. But they’re dirt cheap compared to Radio Shack where you pay $5 apiece. I know that I could run a single LED with one coin cell. But with his circuit I could put three of the coin cells in series and get a much longer battery life. And it would act as a touch switch too. The coin cells are good for about 5 milliamps. Three times 3 volts times 5 mA is 45 MW, and at 95% efficiency that gives 40 MW, which is plenty for a single white LED.
Also, three coin cells are much smaller and lighter than a 9 volt battery. They’re cheaper too, but the dollar store sells two 9 volt zinc carbon heavy duty batteries for about a dollar or a dollar and a half. If I wanted to stay at the same size as a 9 volt battery, I would use 2 AAA cells.
Back to QS’s circuit. This is a small, cheap and simple circuit that is much more efficient than a single resistor. He shows two circuits and the one I’m talking about is the one that uses a choke, not a coil. It makes a lot of sense to use the circuit because it saves about one quarter of the battery power that would otherwise be wasted with the resistor. I think an adequate choice for a transistor for Q2 would be the SS8050. They’re only about five dollars for a bag of 100, and they can handle 1 amp or more, which should be enough.
This circuit has a weak point, it has no short circuit protection. Over the last few years I have collected a few cell phone chargers that plug into the car’s cigarette lighter outlet. These have died, and at least one of them burned up inside so badly that the values of the resistors could not be read. The cigarette lighter plug has a fuse inside, but the circuit sacrificed itself to protect the fuse!
Another circuit that I built A few years ago was the one from Roman Black’s website. It’s a 2 transistor circuit similar to this one. I found out while experimenting that it, too did not like a short circuit on the output. The short circuit stops the circuit from oscillating and the full input voltage may be applied to the output which can cause excessive voltage and/or current, and can burn up the circuit and whatever is connected to the output, too. This shouldn’t be a problem as long as just LEDs are being used. But the circuit really needs overcurrent protection to prevent it from destroying itself.