Introduction: The "Reverse Joule Thief" Battery Charger

Picture of The "Reverse Joule Thief" Battery Charger

Here is a totally different take on the Joule Thief (JT) circuit commonly found in garden lights. Instead of charging a 1.2v battery directly from the solar cell and converting the power to run a 3-volt LED, we'll be using the JT to convert the output from the solar cell and charging a Lithium battery first. Then when night falls, the battery is used to drive the LED directly.

 This method has some advantages: (1) the Lithium cell that was chosen here (and avialable for $2 here) has an output of 3-volts, which can drive a White LED directly; it also has a huge capacity (800mAH) and very low leakage. (2) The solar cell normally can only charge the NiCd battery in full, direct sunlight, but, with the JT circuit, it is able to deliver power to the Lithium cell even on overcast days.

Step 1: The 'Reversed' Layout.

Picture of The 'Reversed' Layout.

A look at the circuit will tell you this is not a run of the mill JT configuration. Most obvious will be the fact that there is only a single coil involved (the 220uH) - we are using a second transistor (Q2) and C1 to take over the timing requirements. This allows us to use a wider range of coil values, as well as operate over a larger voltage range.

Besides reversing the charge / discharge order, this circuit also reverses the location of the driver transistor and the coil, but wait, that's not all! The transistors all have reversed polarities, and even the output voltage is reversed!

No, it isn't an error! Diode D1, the LEDs and the charged battery all have their polarities  reversed! That's because this Joule Thief is configured as a voltage inverter. This arrangement was chosen due to its advantages for this kind of application.

To improve efficiency, the traditional JT relies on a fairly constant battery supply (over a millisecond or so) to give it a boost when it is delivering power. With the limited output from a Solar Cell, we have to store all its power in C2 and feed it into the Lithium in one big pulse, meaning the capacitor will be "empty" for the few critical millisecond, cancelling the 'kick' the normal JT requires to work well.

Our 'Reversed' JT circuit will work as a regular JT - without the 3v Lithium load, an input of 1.2v will light up the LEDs quite nicely. Not strictly necessary, the LEDs are there so you can SEE the system working, and also to prevent the battery overcharging.

Step 2: The Light

Picture of The Light

Because the complex stuff is done already, the light itself is quite simple. A BC327 transistor is used as the switch to turn on the LED. The 'inverter' circuit allows us to combine a 'dark' detector, as well as a over-charge limiter in one. Output from the Solar Cell and the charge on the Lithium is monitored through R3 and R4. If the Solar Cell sees no light, or if the Lithium goes over 3.6-volt (the safe charge limit for this cell), LED3, a 100mA unit turns on.
  Switch S2 is optional and allows you to run the LED at full power, otherwise R2 will only allow 20mA to the LED.

The second picture shows the two halves of the circuit together.

Step 3: Parts List

Picture of Parts List

Solar Cell. 2-volts with 100-ohm load

Q1,Q3 BC327 PNP. Can be any low-signal amp of sufficient current rating (>100mA)
Q2 BC337 NPN. Most will work but if you change Q1, Q2 or L1, you may need to adjust R1 for best performance (Try 3.3k to 15k)

D1 1N4148 or 1N914 or similar
LED1 Blue or White LED
LED3 100mA (1/2W) White LED

C1 220pF. Can be 150-500pF
C2 50-200uF

R1 10k-ohm
R2 330-ohm (use 470-ohm for longer run time)
R3 3.3k-ohm
R4 6.8k-ohm (use these values instead of the one on the schematic)
R5 100-ohm. Go as high as 220-ohm for lower brightness.

L1 100-500uH. Many home-made ones will work.


The second image shows the waveform measured at the top of the coil. The portion above the tag (2) is the charge stored in C2 fed into the coil. The sharp negative going pulseis the battery being charged.

Step 4: Testing the Solar Cell

Picture of Testing the Solar Cell

If you, like me, have collected Solar Cells from busted Garden Lights, it is important to choose one which has good performance.

A simple test is to take them out on a sunny day and measure their output with a Voltmeter that has 100-ohm resitor across the + and - leads. This puts a load on the Solar Cell and will give you some idea of how 'powerful' it is.

  In this picture, the meter shows almost 2 volts in a slight shade. Using Ohm's law, we know that 20mA (2-volt / 100-ohms) is also flowing from the cell, and this is the values we will use to design our circuit.

Step 5: Some Calculations

Picture of Some Calculations

To get an idea of how well our system can perform, we need to figure out how much power we can get, which dictates how much power we can use up.

Using the numbers of 2-volt and 20mA from before, we now know that we can get 40milli-watts by multiplying Volt with Amp. Over the course of an 8-hour sunny day, we should be able to get 320mWH of power from our system.

With a 100mA LED running full steam, it will draw (3.3-volt x 100mA), or 330 milli-Watts! Divide this number into the 320mWH we get from sun-power, it means we'll use it up in less than an hour! Of course the battery will continue to supply current from its reserve, but that is power that would require extra days to replenish.

Step 6: Variations

Picture of Variations

This circuit performs the same function, but switches the input's polarity so that the output is more 'normal' looking -- positive at the top and negative as ground. It also shows the use of 2N390x transistors and 3.7v Lithium-ion cells, which can also be 3 NiMH batteries in series.

The values of R4 and R5 are important: not only is Q3 tasked with switching on the light after dark, it is also responsible for switching the LED off when the battery drops below 2.9v, as well as to prevent over-charging. In fact, it will delay switching on the light if the battery has not received a full charge during the day.


kc8hps (author)2014-04-26


For murky cells I use auto clear coat rubbing compound. it brightens them right up. wash the dry (I set mine in the sun). I then gave the cells a quick coat of clear spray paint to keep the shine, Im not sure if that has an effect on the cell efficiency.

Hope that helps. Bryan

iamdarkyoshi (author)2013-05-01

to anyone with "murky" solar panels, clean them and put a piece of clear packing tape over it. it turned it crystal clear for me and i saw a .5v increase in its output!

coherent23 (author)2010-08-13

If you wind your own inductors/transformers using parts and methods described in other JT instructables, and using larger wire, I think you can significantly reduce the resistance. I think this may be one key to my desired 0.5V 0.5A cell JT-esque device. And perhaps FETs instead of bipolar transistors, too. Maybe. - Chris whY

qs (author)coherent232010-08-13

One major hurdle with using a single cell is, as you suggested, the (in-)efficiency.

Power is proportional to I2 * R so, with any given inductor, the power loss is four times higher for every doubling of the current. Thus to get 3v from 0.5 v, the current draw will be at least 6x, making the losses 36 times higher. Also, getting the proper bias for any semiconductor unit at these voltages will be problematic, to say the least.

jimengn01 (author)qs2011-10-01

I have a project I need some help with. I am new to this forum and can get lost for hours ready all this cool stuff. I need a PCB board made with a subsiquent schematic that ressembles the joule thief, Would you be interested ? I will pay !!

Dave Kruschke (author)jimengn012012-06-24

Go to
Here you will find a "Joule Thief" circuit that is new and so simple that you may not have to worry about a PCB board...

coherent23 (author)2010-08-13

Oh, and it might be worth considering swapping out the overload-protect LEDs with a zener and FET charge-regulator, at little additional time/expense. Can probably grab 'em off a scrounged board. Older boards often even have the schematic symbol for each part, making it easier to ID zeners, transistors, etc.But I still give kudos on this build just the way it is. Haven't built it yet, but it looks sound to me. -Chris whY

qs (author)coherent232010-08-13

In constructing this circuit, I was trying to specify parts which most interested experimenters would have on-hand. Hence the LEDs rather than anything more fancy.

The idea of a zener is a good one. In fact that is what's being done in an adaptation of this circuit used in an MIT project in rural Africa, you can read more here.

nigel cox (author)qs2011-12-18

Good evening, have just finished building your circuit, have been looking for a more powerfull led light set up for a long time, can only get the main light to come on dimly but does come on gradually when covering the solar panel, have probably made a mistake with the transistors, will be picking my friends brains as to the position of the pins, using a mobile phone battery of 3.6 volts, do you think this would be alright as I know you can have a certain amount of pecentage error in most circuits, also will this circuit charge up in dull winter weather, thanks for a very interesting article, regards Doc Cox.

ramatern (author)2011-09-04

Oscillating output for a charger... is this thing functioning as a pulse charger? Now, if I could get this in a size for lead-acid batteries...

.Unknown. (author)2010-05-29

So, is this safe for the battery? What if it were a 2800mah 3.7V li ion battery?

qs (author).Unknown.2010-05-30

Once the battery reaches about 3.3v, the LEDs will turn on to make sure the battery doesn't overcharge. A 3.6v Li-Ion at full charge is 4.2v, so the circuit as shown will charge it to about 80%. By replacing LED1 and LED 2 with a 4.8v zener, you will charge the Li-Ion to 100%.

middlenamefrank (author)qs2011-08-23

Actually, though the link you posted earlier isn't explicit, that looks like a LiFEPO (Lithium Iron Phosphate) battery rather than a generic "Li-IoN" battery (which can refer to a variety of exact chemistries, but not LiFEPO).

A LiFEPO battery has a nominal cell voltage of 3.0V, and an open-cell, fully-charged voltage of around 3.6, as compared to Li-Ion which is typically 3.7 and 4.2 for those conditions. It also has about half the capacity of the Li-Ion batteries, but on the plus side it's a lot cheaper, more rugged and has much longer life (it may last 1000 cycles under circumstances where Li-Ion would only last 200).

So your 3.3V maximum charge voltage is a good maximum, since at that voltage the LiFEPO battery is really nearly fully charged. Don't increase the maximum voltage any more on that battery; although LiFEPO is less likely to behave catastrophically than Li-Ion, you really just don't want to abuse lithium batteries.

.Unknown. (author)qs2010-06-06

Can I charge 2 li ion batteries with this? Or do I need a better power source?

dasimpson1981 (author)2010-12-23

from what you have said using 100ohms to do the maths that gives you 0.02amps
that would be 2ma

qs (author)dasimpson19812010-12-23

No, there are 1000-milliAmps in an amp, so 0.02amp is 20mA.

dasimpson1981 (author)qs2010-12-23

i stand corrected lol

dasimpson1981 (author)2010-12-23

i think you should of used 10ohms

jimk3038 (author)2010-05-03

Great instructable!

Just the other day I bought a solar lamp at the hardware store for $3.50 bucks. I bought it just so I could rip it apart. Blows me away - only $3.50.

I'm guessing there is no reason I couldn't use a pair of 1.2V NiMH instead of the Lithium cell. I've got a bunch of AA cells.

I'm hoping to keep a Zigbee radio running instead of the LED. The radio uses 200uA while sleeping. When transmitting / receiving the current jump up to 35mA. However, the active time is only for a few seconds. The radio then goes to sleep for a programmed amount of time (15 minutes).

OK, my question, is there anything special about the inductor? Will any old 200uH inductor work? I found a couple at digikey - here is an example:

I had a hard time seeing your inductor in your pics. Could you put a lasso around the inductor to call it out?

Thanks again for a great instructable.

Nerdz (author)jimk30382010-05-03

Since the joule thief is nothing more than a boost converter, the value of the inductor doesnt. The value of L just determines the frequency and amount of ripple in the output. So you can probably get away with using a home-made inductor using a ferrite core from a old power supply with a few turns of wire (say like..10+ is a good start)...or if you shack sells a assortment of can pick and chose one that works from there..its like 2.50 or something for the pack

qs (author)Nerdz2010-05-03

In this case, the circuit will accept pretty much any inductor between 100 and 500uH, because the timing is done by a transistor.

In traditional JT circuits where 2 coils are used, it does become a whole lot more critical - as anyone who's tried to power up a 1-watt LED from an AA (and a single transistor) will readily attest to.

You're right in that the inductor determines the frequency - but it also affects the duty cycle which affects brightness and efficiency.

rcisneros (author)qs2010-08-31

I wonder if you could reverse this circuit? Everything but the inductor is in one component. To the left is the LED and the Blue/Black wires come from the 2v 20ma solar cell. Red/black wires go to the 1.2 600mah nicd.

rcisneros (author)rcisneros2010-09-01

It's real name is QX52521F. It cost 0.01 yuan. I'm looking for it the states with no luck. If anyone can read Chinese,

qs (author)rcisneros2010-09-01

The device you described can be used here by (1) connecting the Solar cell and the 100uF cap to the BATTERY input and (2) adding the over-V limiting LEDs and the diode at the LED OUTPUT and then to the Lithium battery.

Here's a ROUGH English version of the web page courtesy of Microsoft Translator:

Tung Tai Shing electronics

Professional IC IC, Triodes ....

Solar lawn lights LED driver IC QX52521F TO-94 applies to section battery programme originally proxy, long-term supply ...


QX52521 is a specially designed for solar-powered LED lighting design of ASIC. It consists of switching driver circuits, light switches, circuit, over-discharge protection circuit, internal integrated Schottky diodes, etc. Only one external inductance can be composed of the solar light fixtures. QX52521 uses patented technology to enable undervoltage shutdown when the LED lights flicker-free.


Working voltage: 0.9V-1.5V

Output current: 3mA-100mA

Patent over discharge protection: turn-off flicker-free

Built-in integrated optically controlled switch

Internal Schottky Diode

External components only need an inductance

High efficiency

TO-94, DIP-8 package

Welcome to contact us ...It should be usable by attaching the solar cell to the battery inputs, and the lithium through a diode (extra) on the output side.

Here is a *ROUGH* translation of the webpage courtesy of Microsoft Translator:

Tung Tai Shing electronics

Professional IC IC, Triodes ....

Solar lawn lights LED driver IC QX52521F TO-94 applies to section battery programme originally proxy, long-term supply ...


QX52521 is a specially designed for solar-powered LED lighting design of ASIC. It consists of switching driver circuits, light switches, circuit, over-discharge protection circuit, internal integrated Schottky diodes, etc. Only one external inductance can be composed of the solar light fixtures. QX52521 uses patented technology to enable undervoltage shutdown when the LED lights flicker-free.


Working voltage: 0.9V-1.5V

Output current: 3mA-100mA

Patent over discharge protection: turn-off flicker-free

Built-in integrated optically controlled switch

Internal Schottky Diode

External components only need an inductance

High efficiency

TO-94, DIP-8 package

Welcome to contact us ...

rcisneros (author)qs2010-09-01

1. Thanks for the response. 2. If I find this stateside, I'll post the info asap. 3. I'm a little confused. Got the add a 100uF cap to the solar cell and connect them to the battery input. The second part is where I'm confused. Which diode would you say is a good choice?

qs (author)rcisneros2010-09-03

I would connect as the diagram below. The output diode is not too critical - any diode capable of 150mA (like the 1N4148) will be OK.

A possible hack is to remove the ground connection of the zener and route that to the "Solar+" pin through a 1k resistor. This would 'fool' the device into turning itself off when the battery is fully charged.

Sorry for the image quality - I used an iPad to draw the diagram.

rcisneros (author)qs2010-09-07

How does the size of the inductor effect the circuit? Would a larger inductor drain the battery faster but give a larger out, brighter light? Or does it effect duty cycle?

qs (author)rcisneros2010-09-07

Hmmm, did you mean a physically larger inductor (with the same value) or a higher inductance value?

If it is the first, then larger inductors tend to have lower DC resistance (thicker wires) and higher saturation point, which help in the efficiency.

The value of the inductor changes (inversely) the operating frequency and possibly the duty ratio. In general, larger inductances put less stress on the drivers but will require more turns of wire, which means higher resistance. It is usually recommended to use the lowest inductance / highest frequency, subject to the components' operating limits and watching out for possible interference to radios and TV.

rcisneros (author)qs2010-09-03

Wow. That sounds great. I'll post the results as soon as I build the thing. QS you rock btw.

rcisneros (author)qs2010-09-07

Hey I found a supplier that will sell them to me for 10 cents or 20 cents a piece. I'll have to go with the 20 cents because the 10cents people have a minimum of 1000 pieces policy. If you don't have access to this IC, I can send you a couple for you to play with as a thank you for your time and advice.

qs (author)jimk30382010-05-03

L1 lassoed in steps 1 & 3.

The combination of the Red and White LEDs is to limit the voltage for 3.6v Lithiums and NiCd packs. Combine different LEDs and diodes for LED1 and LED2 to get regulation for different voltages.


jimk3038 (author)qs2010-05-03

Thanks for showing the inductor in your pictures. I guess I forgot they could look like resistors.

I'm not sure I understand your comment about using the LEDs / diodes for regulation. If there were no LEDs or diodes would the circuit simply keep the 3V battery trickle charged? After becoming fully charged, the battery would simply waste the extra energy by becoming warm.

My Zigbee radio can handle 2 to 3.6 volts. So, if anything, maybe just a 3.6V zener diode to clamp the output from ever going over 3.6V. You see, in my application, I don't want to share any energy with an LED.

Does that make sense?

Thanks again,

qs (author)jimk30382010-05-03

You're right about NiCd's "using  up" the excess charge. Lithiums, with their much lower internal leakage need more care.

I had the LEDs in as a precaution, since back-EMF can pop the transistors and then found myself 'tuning' the circuit (via R1 & C1) by watching how brightly they glowed. I guess a zener there will do a better job, but I DO like my lights!

coherent23 (author)2010-08-13

I like your reasoning of using a LiION battery and direct-charging it. I also _LOVE_ this ste, even though I would not recommend some of the more dangerous projects (unlike your rather safe one) to amateurs. Some of them scare me, and I have experience with Tesla coils, and knowledge (in person, and with copious explanation, but no personal builds yet) of micropulsed very-high powered "can-crushers" and theoretical instruction on things like railguns. But this here is the kind of circuit I would recommend to any person who can understand enough about it to avoid catching the LiION on fire. :) I like to see novice to intermediate level projects on here with real-world utility and simple builds, using scrounged or super-cheap components. Bravo! GREAT build! I will most likely be using this idea with my two freebie garden lights and surplus cellphone batteries, but slightly modified for other uses and the parts I have. But, since I can't stop getting sidetracked with 4 more browser tabs every time I open a page, I will ask if you (author or readers) have something like this but for another type of solar cell. There's probably one on here, and/or I could probably knock some of the designs around enough to work with it, but I came here looking for the "pre-invented wheel" so to speak. I was given a solar cell that can generate 0.5V at 0.5A, which is a substantial amount of power - 250mW - as tested for voltage with no load other than my meter's 10 or 20Mohm voltage circuit, and current with a direct short through the 10A low-resistance - probably an ohm or so - circuit on the meter), but at a very low voltage. (The meter was a freebie too, needed repair, and I don't have a spec sheet on it.) I have experience with many types of electronics, but not with ultra-low voltage. Is there a JT-type of circuit I can power with this cell? (Although I have even outdone and defied true engineers a couple of times with my hacks they said were impossible, "I are jus' a technishian, sir," as I was instructed by my professor in trade-school 20 years ago to say. And I'll design bicycles for fish if I'm so-inclined, but don't like inventing the wheels. :) Anyone? Anyone? Bueller? (Someday I'll have a solar-powered catapult that throws cars, like that crazy British nobleman has, but solar. Yeah. That's it. That's the ticket. I also want a Tilt-A-Whirl(tm) ride combined with 12-gauge skeet-shooting, and Wile E. Coyote's ACME Portable Hole. I need Trump's money and Wile E's ACME catalog. That may just be my life's dream. How did Wile E. make his money for all that stuff? I'm unemployable by conventional standards. And I'll catch that darn bird, someday.:) (comment deleted and reposted after correcting typos.) -Chris whY

rmfungaro (author)2010-06-15

I have a 6v solar cell, removing the joule thief circuit i can use this circuit to charge 3.6v Lithium batteries?

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