This is an implementation of Lionel Sear’s Wall-Clock Joule Thief circuit, found here: https://rimstar.org/science_electronics_projects/joule_thief_powering_clock_ls.htm and described by him in this YouTube Video. And featured in this Hackaday Blog.

The circuit itself makes sense and should work in theory. I’m tired of replacing batteries in my wall-clocks so often and figured I’d give it a go.

The reference designators on this board are the same as in the original circuit at the above link. I did, however, use BC847 transistors instead of the BF546B specified on his original circuit. You should be able to use any similar NPN transistor. For C1 and C2, I opted for tantalum parts to make a smaller footprint. Those are Case-A 1206 size. Their ESR is slightly lower than electrolytics would be, but I believe will be a negligible change. For the diodes, I selected MBR0530 parts since I had them on-hand and they are low voltage-drop schottky type. You should be able to use any similar diode there, such as a LL4148. The resistors are 0805 footprint and the potentiometer is a Bourns 3362 series part.

The toroid I selected was a Magnetics, Inc. ZJ-42206-TC part, found on eBay. I selected that one because of its high permeability and small size (and inexpensive eBay price). Similar spec parts would be the following from Mouser (623-5975001801, 871-B64290L0638X035, and 871-B64290L0638X830).

The toroid mounts on the backside of the board. And assuming I’ve interpreted the toroid windings correctly, the four winding connection points are labeled 1 through 4 with the start-winding connection on “A” and the end-winding connection on “B”. In other words winding 1’s start would connect to “1A” and its ending would connect to “1B”. And so on for the remaining three windings.

UPDATE: This circuit does indeed work, at least electrically. And it does suck the last bit of juice out of a “dead” battery, ending at around 0.5 volts (0.523volts on my first test), just as Lionel said it would. However, it does NOT prolong battery life in general. The big design flaw in Lionel’s circuit seems to revolve around the output voltage limiter circuit, which turns off the Joule Thief by shunting the input through a 1K resistor via an NPN transistor.

This means the circuit draws approximately 1mA continuously. Lionel himself explained that 1mA draw in his video, found at the above links. Unfortunately, a continuous 1mA load is quite large for any small battery circuit that’s expected to run for a prolonged period.

Present testing of this circuit on several different wall-clocks and several identical circuit assemblies has shown an average of about 2-½ months life from a new AA battery with this Joule Thief. That, compared to a minimum of six months to a year with just the battery itself in the same wall-clock without the Joule Thief.

So feel free to build and experiment with this circuit, but don’t expect any “free-energy” nor prolonged battery life from it. I’m planning to investigate some circuit changes to replace that shunt regulation with a switch style regulation and use a PNP transistor to turn on the Joule Thief when charge is needed rather than the NPN shunting it off when it’s not.