The Forever Light - a better Joule Thief to recycle your old batteries
The Forever Light - a better Joule Thief to recycle your old batteries

First things first!

I know stealing of ideas and designs is common practise these days.

Everyone can use this circuit for his own projects but if used in any way for commercial products I would like to be included in the earnings!

I am not greedy and offer my idea to everyone who might need them but I simply don't like seeing my circuits in a commercial product as stolen without being included.

Please understand this and consider doing the right thing.

If you like the idea but not paying me for a commercial use you can contact me and we can find other ways, like a good sized donation to a charity organisation or maker space for electronics.

Ok, now to the fun part :)

If you look for something to power a LED from a single battery you will sooner or later end up with a Joule Thief.

The classic design is linked at Wikipedia, a more modern versions only uses a single inductor - like 99% of all solar garden lights....

I have spend endless hours trying to make a better transformer, use better transistors, you name it and I tried it.

But no matter what I tried, no circuit performed on dead batteries without using between 25 and 40mA to get a white LED going properly.

So instead of finding better parts I started to focus on the actual workings behind this circuit and compared a number of different designs like the Supercharged Joule Thief and several version using additional transistors for a feedback regulation on the base of the first transistor - all that either wastes energy or makes it hard to start the circuit at voltages below 0.6V.

When I realised a properly optimised circuit produces a big negative voltage on the base of the transistor when the pulse to the LED goes out I decided to use this otherwise wasted engergy to feed it back to the LED as well :)

In this configuration the LED also provides a lame version of feedback regulation so the brightness does not change too much with the voltage.

What you need:

The parts as lited below.

A soldering iron or as an alternative a breadboard.

Basic skills in reading schematics.

A calm hand if you want to build in miniatur without a breadboard or circuit board.

I did not create a circuit board for etching as the parts count is quite low and not too many people will etch a board unless for SMD parts.

Parts list :

C1= 10µF 10V

C2= 10nF ceramic

R1= 50k Ohm

D1= 1N60P

Q1= 2SC1815 - but any audio transistor with a HFE better than 150 should do just fine, standard switching transistor like the BC557 might not work at all.

Transformer is wound on a standard inductor that was stipped of the original wires.

Winding pattern is a standard center tapped transformer.

The coil for the base has 6 windings, the coil on the collector 15 windings - wire diameter is not cruicial.

If you don't have the ferrite core of a small inductor a ferrite rod will do as well, you might have to add a few more windings on the collector coil though.

An air coil works fine too but you need around 30 turns on the base coil and 60-75 on the emitter coil, overall performance will be a bit lower due to the losses.

Ok how do I think this circuit works?

First it looks like there is no sense at all in it, but a closer inspection shows the voltage differencial between base and collector is the highest in the system -at least in a properly tuned circuit as the base voltage will drop far below zero.

I simply use this otherwise wasted energy as the ground for the supply of the LED.

In return I get a feedback on the base that makes sure the LED always runs at maximum brightness with a far better output than the standard circuits you might find.

C1 and D1 are not crucial in terms of getting light but they provide a more stable DC instead of pulses to the LED.

As a result the overall brightness is better without wasting energy as the excess that the LED does not use is stored in the capacitor.

R1 is only needed to start the circuit, once running it could be romoved as C2 form an oscillator with the coils.

The standard Joule Thief operates between 10 and 250kHz depening on the transformer and transistor, my Forever Light operates at around 1MHz.


This circuit is optimised for otherwise useless and "dead" batteries!

If you want to use it with fresh batteries (1.5-1.65V) you should add resistor of 10 to 100Ohm in series with the power supply.

Also depending on the type of LED you use it is a good idea to check the voltage on the LED and if higher than the rating to add a resistor for it as well.

I used a green 10mm LED rated at 3.4V and 20mA for my tests.

If you want to keep the parts count down you can decrease the number of windings on the coil to the collector down to a 1:1 ratio but this means you might not get enough brightness at voltages belows 0.5V.

Another option is to use two LED's in series as the circuit will still regulate the output accordingly.

I was able to use 5 LED's in series but like that not all the way down to 0.3V - you have to decide what suits you best.

Using this circuit without a load will destroy the transistor in most cases as the base voltage will shoot up into the regions of 20-30V - so never use it without a load. Some transistors don't mind this abuse some do.

In my tests I was able to reach almost full brightness at 0.4V @ 12mA, which is wuite a good result.

On a fresh battery this circuit should be able to run for a few weeks.

I only create the circuit on a bread board and it looks quite ugly, but I will build a miniture version over x-mas and upload some pics and a short video of the circuit in operation.

With a few modifications to an existing solar light this circuit can be used to get 2 or 3 days of running time inseat of 8 to 12 hours, I might update for this in the new year if someone is interested.

If you decide to try this circuit please share your results and feedback here!

And yes! I know the circuit is ugly and could need improovements but I created quickly with LTSplice and think it still quite easy to understand.

I also draw the circuit in way that allows to solder the parts together in an easy way as I prefer to work without a circuit board for such simple designs, you will see what I mean when I upload the images for a prototype that goes directly onto a battery.

Step 1: How to get going if you have little or no clue about electronics

As I know many people have concerns about identifying parts, soldering and testing electronics I will try to provide some help with the basics.

1. Finding a suitable tranformer core...

As I already mentioned a ferrite core is the best option.

Basically any ferrite core will do for the Forever Light unless you want to focus on high output power instead of very low power consumption - which was the main gaol here.

Check the image for some types I used for my prototypes:

The Forever Light - a better Joule Thief to recycle your old batteries
As you can see it does not matter too much and I use several different wire thicknesses ;)

Even an aircoil without any core material works if you add some more windings, but performance will be a bit reduced.

You find a lot of information on the web on how to wind such a transformer after removing the original wires from it, but I found most of just confusing to say it nicely.

To make it easy simply start by winding the coil for the base connection, direction does not matter.

Fix the end of the wire with some tape or solder it onto a terminal if you have a core like the one with the purple capacitor soldered on.

This first connection is for the base of the transistor.

Once you reach the number of turns fix the loose end - this is the other end of the base coil.

Take the wire again you continue winding in the same direction for the collector coil.

The first bit of this wire is the top connection of the collector coil that connects to the positive of the battery and the resistor between the two coils.

Once you finnished the turns the end of the wire connects to the collector of the transistor.

2. Identifying the transistor.

I recommend for everyone unfamiliar with electronics to simply go to a shop and ask for good audio transistor of the NPN kind with a HFE rating above 200 - they guys will know and have several types available, take the cheapest ;)

If you do know how to handle a multimeter you can salvage transistors from old electronics.

Easiest way of getting the data for the transistor is to check the datasheet for it online.

If you can't find any info on the pinout you can you a multimeter in diode testing mode to check what pin goes where.

Connect the positive probe to one of the three legs and the negative to another.

You want to find the pin on the positive probe that give a reading between 300 and 800 for both other pins on the negative probe.

Once identified this will be the base of the transistor - usually the pin next to it is the collector and the last the emitter.

If you don't get these reading you might have a PNP transistor, in this case you identify the base the same way as above but with the negative probe - as I won't modify the circuit at this stage for PNP transistors, simply disregards such a transistor and solder out a different one.

3. The diodes....

As many LED's have problems running at very high frequencies I added a Germanium diode, 1N60P, if you can't find one try the 1N4148 instead or if worse comes to worse any standard diode you can find.

Not all perform the same at high frequencies so you might have to test few diodes for performance in your circuit.

4. Capacitors.

The elctrolytic capacitor paralell to the LED is optional but if you do use one for increased brightness don't go higher than 4.7µF as it is not necessary at high frequencies.

The ceramic capacitor is more critical and depending on you transformer desing you might want to experiment with different values to get the best match for your transformer.

5. Soldering....

Soldering is no mystery, a hot iron and flux core solder will do it, a needle point soldering tip makes it easier.

Most of my prototype are soldered directly with no circuit board or similar.

But of course for testing purposes a bread board does help and there is always the old trick of drawing the schematics on a piece of cardboard, push the wires of all components through and solder the connecting wires on the back of the cardboard - do what suits your skills the best ;)

In case you want to use a breadboard I created a simple layout:

The Forever Light - a better Joule Thief to recycle your old batteries

Keep in mind it is only a layout and you have to check the parts before following it.

The transistor shown is BCE from the left to right, base, collector, emitter - if you inout differs please adjust accordingly.

The transformer is shown as two seperate coils for the ease of understanding.

The left coil is the base coil, the second the emitter coil, simply connect your transformer to match.

Breadboards are not always perfect in terms of connections, especially with thinner wire diameters, if in doubt use thicker wires or doubles check with a multimeter for proper conections.

Step 2: How to test the circuit

The Forever Light - a better Joule Thief to recycle your old batteries

That is what should happen when you connect the battery if all is connected correctly.

If the LED does not light up check if you don't any shorts from solder or if the transistor / transformer is wired incorrectly.

Once the LED is on you use a multimeter to read the mA drawn from the battery - you want it to be below 30mA!

If it is higher please add 47 or 100Ohm resistor between circuit and batter - you can see one in the pic above as I used a new battery for this one.

You can only read the voltage to the LED properly if you have an electrolytic cap as per the schematics as well, otherwise the high frequency pulses will give you a false reading.

Ideally the circuit should regulate itself to be not more than 0.2V above the max allowed voltage for the LED - it it is higher add a resistor in series (1-47Ohm should do) or if the voltage is much higher remove the capacitor.

The LED will handle short pulses at higher ratings at those frequencies.

Higher voltages than planned on the LED are a good indicator of a good tranformer and transisot combination.

Be aware that the type of LED affects the power consumption of the circuit!

For example using a white or blue 10mm LED rated for 2.4V @ 30mA will need more power than a 3mm green LED rated for 2.4V @ just 10mA.

So if extreme power efficiency is your goal opt for 2 or or three smaller LED's in series instead of a single LED that needs much more mA to run at full brightness.

The circuit should be able to light 3 or more LED's in series and as the power draw can't increase in series configuration you only need to get enough output voltage.

I tested this circuit with a AA battery thatfailed to work in my keyboard, it was down to 0.98V.

The LED was powered continously with this battery for almost 80 hours before the circuit ran out of juice.

Not too bad for a dead battery :)

Step 3: Possible ways to make good use of this circuit

Obviously the best use is in terms of recycling otherwise dead batteries as we all have our toys and remotes around the house.

Instead of throwing an old 1.5V battery into the recycling bin you can use for many days if not weeks as a supply for the Forever Light.

Upgrading old solar garden lights is another option as it allows for longer running times from a daily charge.

Simply add the circuit to the battery inside and re-wire the LED connection to it for 24/7 use.

If you want it just for overnight use you will have to cut the connection on the original lamp circuit that connects to the LED.

There are different types on the market but they all have a black blob on the board or a transistor like 4pin device.

Cut the connection from there that goes to the parts connected to the LED and re-wire the LED to the forever light.

The original circuit will charge the battery and switch the Forever Light on.

Night lights are another option, make them with D-cell batteries and don't worry about switching it off as the circuit will run for several months on a D-cell.

The act as an emergency light well when you encounter a power outage, place a few in a good location, and leave one switched on at all times, switch the others on when needed and don't worry about running out of juice before mains power is restored.

With more and more low power sensor entering the market the circuit can also be used in remote weather station to power the sensors and transmitter.

If a constant output voltage is required simply add a suitable zener diode for the LED but be aware that some will be lost this way.

Without the focus on efficiency and consumption it also makes a good emergency charger for your mobile phone in the same zener diode configuration but for sufficient output you might need a transistor that can handle more than the few mA of an audio transistor. Charge you phone from a single 1.5V battery, it won't give you a full charge but more than enough for some emergency calls.

You go fishing or hunting?

Add the circuit to your LED head lamp so have a lasting power supply.

Using 10mm LED's the brightness is more than sufficient for close range and if the dimensions of your light allow it you can add the circuit with a switch as an added low power alternative to the full power supplied by the orginal.

Options are endless and these were just some ideas I had on the way.

Enjoy building one and share your creations!


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