This is Hacker Public Radio episode 3,781 from Monday the 30th of January 2023.
Today's show is entitled, The Jewel Thief.
It is the 20th show of Andrew Conway and is about 13 minutes long.
It carries a clean flag.
The summary is, using the Joel Thief to suck energy out of flat batteries.
Hello Hacker Public Radio people, this is McNallow also known as Andrew and today I'm
going to tell you about a delightful little circuit called The Jewel Thief.
The Joel Thief allows you to extract energy from an otherwise flat battery and more remarkably
than that, using a flat battery, so I'll say I'm using a AA battery, it's was 1.5 volts
when it started out its life, it's now down at around about 1 volt and even at 1 volt the
Joel Thief's circuit is more than capable of allowing you to light up a two volt LED or need
a string of LEDs of even higher voltage than that.
So the magic of this circuit is that it's so simple as well as being able to use a flat battery
to power an LED that requires a higher voltage and the secret of the circuit is very fast switching.
So the LED isn't in fact on all the time, but it's on enough of the time that you can't really
tell that it's flickering on and off. In fact, the circuit I've built as far as I can tell,
it's flickering on and off 100 kHz, which is 100 000 times a second and I'm not going to pick
that up with my eye certainly. So it only uses for components, the battery and for components,
so I have a flat battery, and it's worth noting that when you I say a battery is flat,
you will not be able to use it to power most things, but of course flat is relative.
When a battery is exhausted, it's exhausted for the particular task that you put it to, for example,
lighting up an LED torch, running a radio, running a Bluetooth mouth, Bluetooth mouse,
I've got that right in the end. Once the voltage goes too low, without some degree pro-curry trickery,
you can't use it anymore. But if what you really just want to do is light an LED,
there is a way to coax remaining energy on the battery. Indeed,
something like 50% of the energy in the battery is still there, it's just coming out to a low voltage,
so if you can, put it to another use, that would be great. So let me just describe the circuit.
So the four components are a resistor. The one I've got in front of me is a 10 kWh resistor,
but I used a 1 kWh resistor to start with, and that would work too. I've got a transistor and
NPN transistor, it's a BC-548, which is a totally bog-standard transistor, nothing fancy about
that at all. A yellow LED, I guess, is a work with almost any color of LED, and the fourth,
and most exciting component, is myself, one, and doctor. Now this is not an ordinary inductor.
An ordinary inductor is a set, which is just a coil of wire around a ferromagnetic core, usually.
This has a ferromagnetic core, it's in the shape of a torus, and most doctors will look like that.
But the clever thing about this inductor is it's got two windings. One that goes one way around the
torus, and another that goes the opposite way around the torus, and that's the key to how this
thing works. It's actually quite easy, a little fiddly, to widen one of these yourself. So what I did,
I had a dimmer switch for my kitchen light, it was a main voltage dimmer switch,
that eventually looked up in through some LED driver circuits to all the lights in my kitchen.
The switch on it became soft, so I had to replace it. But I don't like to throw things out,
so I kept it thinking, let me keep him in handy one day, and indeed it did, because when I opened
it up, I found it had an inductor. So I dismantled the inductor. Now a normal inductor just has,
as I say, one winding of wire around the ferrite core. This is a ferrite bead, so it's a quite large
actually, it's about centimeter across, and maybe a half a centimeter deep, and it's like a
donut of some ferromagnetic material. So I removed just unwound by hand, so I'll start watching
some television program, look to me a few minutes to unwind, about a meter worth of the wire
that was well done to the inductor. Then I took this wire and folded it over, so I have two
free ends, and the other end is now a fold, where the wire is folded in half. I anchored the two
free ends by wrapping it around the ferrite bead once, and then proceeded to thread through
the looped end, the folded end, repeatedly until I had windings that went all the way around
the torus. Let's say, done that, I cut the wire at the fold, so I now have four
ends, and using a multimeter, I actually, the next thing I did was I used a bit sandpaper
to scrape off the enamel, because it's all, the wire is insulated, it would work as an inductor,
if it was just bare copper wire, it's not, it's enameled wire. So at the ends, the four ends
I had, I used sandpaper to scrape off that enamel, so I could make a electrical contact, and then
what I did was I just joined two bits of wire that weren't connected to each other, and I used
a multimeter to figure out which two ends didn't really matter, it doesn't matter which two ends,
as long as they're not showing continuity in a multimeter, so I took those two ends and joined them
together, and that will be the north north, the positive end of my, the inductor I'll use in the
circuit. Now it's not an ordinary inductor, so it's got a common positive, and it's got two
negative ends, if you like, and so what you do is you take the positive end and you connect that to
your flat battery, so the positive terminal of your flat battery, one of the two negative ends,
one of them gets connected to a resistor, and then that resistor gets connected to the base of the
transistor, so the base of the transistor, if you imagine it, is like the tap that you can turn,
so you can turn the transistor on and off, which will be crucial in a moment, so the base of the
transistor controls where the current can flow through the rest of the transistor. The other
negative wire that comes out in the inductor gets connected to the collector of the transistor,
and the third and final leg of the transistor, the emitter gets connected to the negative terminal
of the battery, and you also then take your LED and connect the positive leg of the LED to the
collector, which is also connected to one of the ends of the negative end of the inductor, and
that's the positive, so it's the positive end of the LED, negative leg of the LED gets connected
to the emitter, and to zero volts, or the negative terminal of the battery. Look at the circuit diagram,
it'll be easier than that, but this is the crucial bit, so what happens is when you connect all this
up, current will flow from this flat battery, and it only needs a small amount, one volt is plenty,
through a resistor, one kilotone tank alone doesn't matter, as tiny little current will flow
into the base of the transistor, and then the transistor after a short delay will turn on.
When the transistor turns on, it will allow current to flow through the other winding, so one winding
of the inductor, remember, goes to the base of the transistor to open it up, the other winding
comes down through the to the collector, and at first it can't the current can't flow that path,
because the transistor is off, but once some current flows into the base through the other winding
in the inductor, then the current can flow through the transistor, and it barely very low resistance,
so very quickly, a much larger current will start to flow through the transistor, and a much larger
current will start to flow through the other winding of the inductor, and because the two winding
is the inductor, shear the same core, and they're winding opposite directions, what this does
is it stops the current flowing into the base of the transistor, so what happens? Well,
when that current stops, the transistor shuts off the tap, shuts, and then that main current
path suddenly closes off, and this is the key property of an inductor, if you rapidly change
the current flowing through an inductor, you get a spike in voltage, so the voltage, the
inductor equation is voltage equals inductance times rate of change of current with time, so if you
vary suddenly switch off the current, you'll get a spike in voltage, and so even when you're only
got one say a one volt battery, you can easily produce two volts, three volts, four volts are
potentially very large voltage is actually for a brief time, and that allows you to light
the LED, which is connected across the collector no matter of the transistor.
And as I said at the beginning, this happens extremely quickly. All of that takes place,
say 100,000 times a second, depends on the value of the inductor, I've got quite a high value of
inductor just because of what came out of the dimmer switch, and to a lesser extent it depends
on the resistor, but really the switching as far as I understand it is mostly depend on the transistor,
and the timings inside the transistor. Now the transistor that I'm using is not really designed
for switching, and if you look at status sheet, it doesn't tell you anything about the timings,
other transistors, MOSFETs, they do tell you stuff about timings, but this PC 548, no,
you don't get any information, because it's not really intended for that, it doesn't matter,
it just works. Now I have tried to understand the equations that governness and it's for such
a simple circuit with four components, it's deceptively simple, because the equations are actually quite
complex, so I actually went right back to Maxwell's equations, if you've done any physics you
might have encountered Maxwell's equations, and I was able to derive the inductor equation in this case,
and for those of you that are interested, it looks very much like the usual inductor equation,
so the voltage drop across the inductor is equal to the inductor times the rate of change
of the difference of the two currents that are flowing in the two's winding to the inductor.
But actually to try and model what's going on in the circuit using that equation is difficult,
because as I said earlier, it isn't the inductor in the resistor that is where the magic is
happening, well some of the magic's happening in the inductor, a lot of the magic is happening in the
transistor, and transistors are nonlinear devices, and if you don't have documentation and the
time delays involved, I don't see really how you can model the joule thief in any simple way,
you can certainly model it, but it's not simple, but it doesn't really matter, because if you want to
just light up an LED with a flat battery, well a joule thief is certainly the way to go.
Now if you would like to build your own, I highly recommend about half and half our video
by Clive Mitchell or Big Clive, as he's known in YouTube, he is the person who coined the term
joule thief, which is a lovely little pun, but the circuit itself is much older, it
I think maybe the first patterns are almost 90 years ago, but those components are very different
from the ones we would use today, and the ones I've got in front of me, and I think Clive references
a letter in the magazine that was published in 1999, but anyway, for those details and for a lovely
clear description and how to make it and how it works as well, do refer to Big Clive's video
which is a link I'll put in the short notes. I hope you've found that, uh, interesting. Bye bye.
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