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I always love to make something by using used stuff.I found plenty of used battery in my drawer so I thought how to reuse it .The best idea come to my mind is to making a powerful flash light .During summer in eastern part of INDIA , we face storm and power cut on every evening .So I think my flash light will be very helpful during this time.First I thought to hack a flash light for the casing but incidentally I found a gems chocolate casing in my nephew's (4 years old) hand.It is really matched my requirement and it is very handy also.

Most of the parts I used in this project are collected from scrap electronics.My flash light is more efficient than a normal flash light and can be run even at a very low battery voltage.

Uses :

1.Camping light

2.Searching light

3.Emergency Light

A dead battery which is discarded for normal use have still plenty of juice to light up the leds. The working of this light is based on the JOULE THIEF circuit.Before making the flash light first I will explain the details of how to make joule thief circuit,how a joule thief circuit works , and the basics of a coil (inductor) as well the ferrite core.

If you are not interested to know about the basics of ferrite core and coils just skip it.

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1. Gems Chocolate cassing

2. 4 LEDS

3.Transistor (2N2222 or 2N3904 )

4.Resistor (1K )

5.Ferrite Core


5. Perforated Board

6.AA dead battery

7. AA battery holder

8.Push Button switch

9. Aluminium foil


1. Glue Gun

2. Wire cutter/stripper ( or LEATHERMAN )

3.Tin Snips/cutter

4. Soldering Iron


Step 2: How to Make a Joules Thief Circuit

Make a Joule Thief circuit :

Take two plastic insulated wires of different color and hold them together.

Twist both the wires at one end.

Keep the two wires together and wrap them around the toroid until you have covered the whole toroid.

Left about 3 to 4 cm of wire free on either side for further connection.

Remove the plastic insulation on all four of the wires as in the photo.

Note : The trickiest part of the joule thief circuit is winding of the ferrite core.So I made a video for wind the toroid core.You can go through it.

Test the circuit before soldering

Take one wire of a specific colour (orange) on the one side of the toroid and one wire of the other colour (blue)on the other side of the toroid and place them together on the bread board. These two wires forms the top of the toroid as in the circuit diagram and is connected to the positive terminal of the battery.This is done for connecting two coils in opposite dot polarity (see dots in schematics).

Connect transistor,resistor and battery as per schematics.

Magic Of Joule Thief Circuit :

First check to light up a led by using a single battery. Sorry :( it will not work as led require a certain minimum voltage to turn on.A normal LED requires 1.85V and a power LED require 3.5V or more to turn on.

Then test it by using two battery. Happy..!!! it works.

At last check by using a single 1.5V battery (AA) and the Joule Thief circuit. In this case the LED will turn on even if a

single battery is used.This is the magic behind the joule thief circuit.It can even works when the battery voltage is very low ( < 0.8 V)

Step 3: Basics of a Coils :

An Inductor is a passive component which consists of a wire loop or coil..When a current flows through it, energy is stored temporarily in a magnetic field in the coill

Stored Energy (W)=1/2*L*I^2

L is inductance of the coil in Henery

I is current flowing through the coil in Ampere

When the current flowing through an inductor changes, the fluctuating magnetic field induces a voltage in the conductor, according to Faraday’s law of electromagnetic induction.This opposes the change in current that created it.

Some of the energy in the field around a coil can be induced ( transferred ) to the second near by coil.This is principle of the transformer.The input side is called the primary and out put side is called the secondary.see the above picture.

Step 4: How a Joule Thief Circuit Works ?

In the previous step you understand how to make the joule thief circuit.But how a 1.5V battery is able to light up a LED which require around 1.85V to glow.

In this step I am going to explain how the battery is able to drive the LED.

Before starting you should know few things about the transistor.

Working Of a transistor as a switch :

Refer the above picture I have shown transistor as a switch.In normal condition the collector to emitter path is open,

but when a positive voltage (Vbe) given to the base the path it will be closed. A small current change in the base produces a large change in collector to emitter current (amplification).

Joule thief Circuit :


Initially current coming from battery have two paths to go,

i ) one is through the resistance R and coil -1(secondary)

ii) another is through the coil-2 (primary)

But it will flow through the resistance as the transistor switch is open.

when current comes to base B it will try to open the collector and emitter path.


As the collector to emitter path open current begin to starts flowing through the coil-2 and collector to emitter.A small base current produce large current in the collector to emitter path.The current ramp up (rise) in the coil-2 with a slope V/L.As I have told earlier a changing current produces a voltage according to the Faraday's law of EMI.


The two coils are connected in opposing direction so a opposite polarity voltage is produces in the coil-1 due to transformer action.But now the voltage in coil-1 and battery are in series,so a large voltage is produce across the base and emitter junction which open the collector to emitter path more and more.when current flows through the coil magnetic energy is stored in the inductor. After a certain time the ferrite core gets saturated,so it will produce no more increasing magnetic field and the current in the coil-2 is lost, depriving the transistor of base drive and the collector to emitter path gets open.Now the stored energy in the inductor try to discharge,as the transistor is open it will flows through the LED.Now the voltage across the LED is more than the forward break down voltage .So it will glow.

When the stored energy in the inductor is back to zero, the whole sequence repeats.The switching of the transistor takes places with a very high frequency.Due to our persistence of vision we can not see when the LED is off.

When the circuit in operation I used my DSO nano to see the wave form.I placed the probe across collector and emitter terminal of the transistor. You can see that the transistor switching at around 20.3 KHZ and peak to peak voltage is 5.86V (which can able to to turn on a power led).

Step 5: Know About the Ferrite Core

Why a ferrite toroid core ??

Ferrite toroids offer high magnetic efficiency as there is no air gap and the cross-sectional area is uniform.

Why it is comes in different colors ??

The color is basically a coating above the ferrite core which given for ease winding and to improve insulation strength.

Where ferrite toroid can be found ??

You can get it from old electronics power supply like computer SMPS. A very small size ferrite core is also found in side the CFL.

Ferrite is also a magnetic core which are made by mixing iron oxide (Fe2O3) with oxides of one or more metals such as manganese, zinc, nickel, or magnesium .It is used for its properties of high magnetic permeability coupled with low electrical conductivity.Due to low electrical conductivity

It prevent eddy current to flow. The ferrite core have low coercivity. Due to low coercivity ,during the high frequency of operation reversal of magnetization inside the core takes place very easily without dissipating much energy .So hysteresis losses reduces drastically.

As Ferrite core have comparatively low losses(both eddy current and hysteresis loss) at high frequencies, they are widely used in the cores of high frequency inductors and SMPS

What is Permeability and Core Saturation :

Let take a common example to understand the permeability.Take a dry sponge and add few drops of water over it

.It will be immediately disappeared because the void space in the sponge absorb it.Add more and more water to it, a time will come when no more water is absorbed.This happens as the sponge is saturate.The water absorbing capability is similar to permeability.

The ability of a magnetic material to pass magnetic flux lines through it is called magnetic permeability.Just like sponge the magnetic core also saturate.Permeability is depends on material composition and geometry.


Make a circular outline over the perforated board by using pencil or marker.

Cut it by using a tin snips/cutter


Insert 4 leds close to each other and make a square shape.

Tricks : Insert led in such way that all the long leg (anode) are close to each other.This makes very easy during soldering.

Solder one by one, connect all the anodes and cathodes together(parallel connection).see the above pics for soldering.Trim all the legs of led.Make terminals by extending the solder for further connection (As I marked + and - ).


As I dont have single AA battery holder I used a double AA battery holder.For using a single battery you have to modify something.Short a wire between the empty side terminal.Now you can use a single battery.


Bend the transistor leg as shown in picture.

Insert the two terminal near to the + and - mark.(Collector goes to positive and emitter goes to negative).

Solder the 1K resistor to the base (middle pin) of the transistor.Use any insulating tap or heat shrink tube to insulate the bare part.

Now pour sufficient glue over the entire soldering and place the ferrite toroid over it.

The toroid have 4 wire with orrange and blue at each end.Now solder one end (ending ) of the orange wire with the 1K resistor.

The other end (beginning) and he blue wire ending end (opposite) connected together and solder to any free space in the board.Now solder a red wire to this point.This is goes to the switch.

The starting end of the blue wire is solder to the collector terminal of transistor.

Solder a black wire to the emitter which will goes to negative terminal of battery


Make a hole by using drill for inserting the wire from the pcb.

Insert the wires through the hole and hard glue on the cap of the casing.

Fix the pcb and wrap a tap on the periphery of ferrite core.


Make a hole just 2.5cm bellow the cap of the casing.I used hobby knife for making it.

Solder the red wire from the pcb to the one terminal of push switch

Solder another red wire (2 to 2cm ) to the other terminal of the switch.

Pour sufficient glue surround the switch to fix it.


Now solder the red wire from the switch to the positive terminal of the battery and the black wire from the pcb to the negative terminal of the battery.

Insert the battery and wires in to the casing carefully.

Now test it .Hey its glowing !!!!

Prepare a circular aluminum foil for reflection of the light.Place it over the PCB.

Congratulation now your gems dead battery flash right is ready to serve you.

At last I found difficulty in removing the battery holder for changing battery.So I made a hole on bottom of the casing.This solved my problem.Just push through the hole by using a screw driver,the entire battery holder will eject on the top side.

If every one try to reuse scraps at our home we can make a better world.So don't throw the scraps, recycle it wisely and live happily.

Thanks for reading my instructables.

I have written this instructables to gather more knowledge on joule thief circuit.I have written the working of it what ever I learned.If any one have better and depth knowledge on it please share it.

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18 Discussions


4 years ago

Hi, great project. due to extreme lack of electronics suppliers and no card for online buying I've somehow managed to solder the circuit without a breadboard. it looks a mess but works!!.

playing around I found it could power a few kids novelty flashing light toys. rather than two or three batts they now run off 1.

and thank you for the explanations too (=)


5 years ago

that was a great explanation of how a torroid, transistor etc work. It answered a lot of my questions. Thanks for putting this up

One question-is the frequency due to the transistor used or the windings/size of the torroid? I'm really wondering what the effect is of having more or less windings


Reply 5 years ago on Introduction

The number of turns around the toroid shouldn't change anything unless the two wires are wound unequally (think transformer).

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Reply 5 years ago on Introduction

The frequency can be changed by using a potentiometer instead of a fixed resistor.I think the applied voltage and number of turn also affect but I never checked it.


Reply 5 years ago

duh!! Didn't consider the resistor. Thanks for the reply


5 years ago on Introduction

Nice application of the joule thief. Also, great explanation of it!


5 years ago

really helpfull article on transistors