Meet TJ-100

Meet TJ-100, or just TJ to friends. He’s a happy little robot, but with a very clever ‘party trick’ - he can bring out the last remaining bits of electricity form batteries that would be normally considered flat. His eyes then shine out brightly!

And how does this party trick work? It’s a ‘Joule Thief’ circuit. You’ll find many references online; the history, examples of construction and the theory of how this ‘trick’ works. This is not a new discovery, one of the earliest references to the basic concept is from the 1930s

Origin Story

Having quite a few ‘flat’ batteries around thanks to the numerous kids toys, finding this circuit sparked my interest. Initial attempts to build this circuit weren’t productive. Not being able to find a decent inductor to do the windings, plus having only a very old collection of LEDs that are not efficient where, I believe, primary reasons for failure.

Move forward to mid-2019, and this I found this article

I knew I had an old transformer in the ‘junk parts bin’, and the feedback circuit where intriguing (plus also some newer LEDs where available). This was successful, and the circuit worked as expected, though I admit that the feedback part didn’t seem to work that well. But we had light! Hence I decided to drop the feedback element, the downside being never connect a full charged battery.

Breadboard circuits are all well and good, but I wanted to ‘make something’ out of this circuit, to try and put all those old batteries to use.

On looking at articles I’d seen several pieces of art made from old components, plus one Joule Thief in the shape of a person holding the battery. Two things then came together, I also spotted examples of using off-cuts of wood, bolts, nuts to make robot models. They are very effective, plus easy to construct. Joule thief circuit, plus offcuts of wood = TJ-100.

The use of the bulky transformer was an issue, how to work that in? But then another stroke of luck – in the parts bin was a Maplin metal detector project (30years old?) with the perfect ferrite rod with copper winding. One my shelf as well is a coil of enamelled copy wire on a wooden reel that is probably 60+ years old (this was from an uncle’s family, and I was interested in it purely for aesthetic reasons). But with enough copper wire, to make another winding.


All the parts where in place! Electronic and otherwise


I don’t know the exact number of turns on the original ferrite rod; the second winding I did directly on top. The wire is slightly thicker but done in the same direction. From the articles online, there appears to be large variation in the sizes and number of windings without obvious changes in performance.


Two white LEDs put in series, these are new LEDs, forward volatage is 3v, with 20mA forward current.

Capacitor, Resitor, Transistor

Nothing special about the capacitor or resistor. The transistor is a BC109. Again this was from the parts bin and the choice is I believe not critical. The precise nature of how the circuit works is influenced by the behaviour of the transistor, but this is a level of detail beyond my abilities.

Battery Holder

Didn’t really have a single battery holder in the parts box, plus it would nice to be able to use both AAA, AA, C and coin cells. But then hit on the idea of using a couple of bolts, washers and some very small magnets. Put together these acts as very easy and quick battery holders.

How does it work?

Will leave a detailed discussion to other articles, but the basic operation is for the transistor to be turned on by as the base is connected via the 1k resistor to +ve of the battery. The second coil will build up a large magnetic field, this will inhibit the current flowing to the base of the transistor, hence switching it off. However, there is still some magnetic field in existence, this will collapse and in doing so induce a current that flows through the LED.

This process repeats, somewhere between 5kHz and 50kHz. Hence to the human eye the light is continuous.


Using wood from an old dining table, the robot body was quite easy to build. There was already a hollow notch cut out that just happened to fit the ferrite rod and coil. I opted for the freewire construction method, by choosing a ‘junk’ style of construction means that the you can get away with a few imperfections!

I used a few old eyelets, and some solid copper wire. Which is difficult to solder without additional flux, and sanding the surface first.


I haven't quantified the performance; it works with a bright light from a very flat battery. Functioning in a model was my objective. It's hard to give an idea of relative effect, but by the image flare you can get an idea how bright the LEDs are. And from the battery meter you can see how low the battery was.

The End

There we go, one project ticked off and done 🙂