Introduction: A High-power LED Torch Using a Single AA-battery

Picture of A High-power LED Torch Using a Single AA-battery

This high-efficiency design can power LEDs at 100mA of current and is not much bigger than the battery that powers it. The unique circuit uses 2 commonly available transistors (BC327 and BC337) in an oscillator to boost the 1.2 or 1.5 volts from a rechargeable (nickel) or alkaline battery to the 3+ volts necessary to light a LED. No fancy ferrite inductor is needed as we will be using the metal jacket around the battery as part of our coil.

Step 1: The Circuit and Layout.

Picture of The Circuit and Layout.

The circuit uses a BC327 (PNP) and a BC337 (NPN) transistor. They are connected as a blocking oscillator at about 100kHz through the 150pF capacitor. Each cycle starts with the coil being charged, then this charge, plus the voltage of the battery, about 3.5v, is applied across the LED. The 1N4148 diode takes this voltage and stores it in the .1uF cap, which is used to drive the transistors, resulting in a much more powerful and efficient circuit.

The 22uH coil consists of 35 turns of #28AWG wire over the battery, using the metal in the shell to multiply the inductance.

A simple diagram shows the placement of the parts.

Step 2: The Parts List.

Picture of The Parts List.

The LEDs are standard 25mA 5mm 120000mcd white LEDs from eBay. The rest are available from Radio Shack or surplus at places like

BC327 PNP transistor
BC337 NPN transistor
1N4148 Low-signal diode or similar (Virtually any silicon diode)
0.1uF ceramic capacitor (can be anything larger)
150pF ceramic capacitor
12uF 6v tantalum cap (can be anything over 5uF)
100K resistor (controls brightness: 470k for 50mA; 68K for 120mA)
4-ft of #28 or #30 AWG wire wrap
Perforated copper boards, pre-trimmed as shown

Step 3: Making the Tube 'body'

Picture of Making the Tube 'body'

Cut a strip of paper 11" long, 1/4" wider than the length of the battery. Apply stick glue as you wrap it around the battery. This will be stiffen up the tube.

Use the widest battery you have as the template so that the tube is loose enough for the battery to be removed and replaced.

Set aside to dry while you assemble the circuit board.

Step 4: The Assembly

Picture of The Assembly

Start by placing the 4 LEDs. Make sure the shorter (-) leads are on the outside. You will need to make sure they are all flat against the perfboard. Solder the shorter leads (only) to keep it secure.

Then 'fold' the longer leads over to its neighbor so that all the + leads are connected. Note that only 3 sides are connected and ends in a 'hook' which extends a bit out from the board. This will be where the coil is connected.

Continue with the BC337 and the BC327 transistors and the other components. Note that one lead (the 'E') of the BC337 and all connetions to B- are also left pointing up. See the images below. The red sleeving is used to prevent shorting - from a connection error... (:P)

When all is soldered and checked, slip the smaller perf-board, copper-side up. This is the level all the B- connections are made. You should be able to wrap the leads so they can touch each other. DO NOT solder them yet!

Step 5: A Quick Test...

Picture of A Quick Test...

If you have a 22uH inductor handy, you can test your circuit by connecting one end of the coil to B+ and the other end to the 'hook' left in Step (4).

Touch B- to the wires at the center of the small perfboard and the LEDs should all light up. SUCCESS!

On to the the final assembly bits now.

Step 6: Final Assembly

Picture of Final Assembly

Crease and push in one end of the paper tube as much as you can to add support. Then make 2 pin-holes at the base of the crease. Push the negative end of the 12uF capacitor through one of the hole.

Solder this wire to the wire cluster in the middle of the perfboard before pushing it out the other pin-hole.

Start the coil by soldering the stripped end to the 'hook' on the main perfboard and start winding around the tube over the battery and the 12uF capacitor. Solder the other end of the coil to the + side of the capacitor.

The wire of the capacitor is long enough to reach around to the positive of the battery - that is your on-switch. Push it to touch the battery and you get LIGHT!

Do NOT look into the light - it will be bright!

Step 7: What's Next?

Picture of What's Next?

I hope you have lots of fun making these lights.

Here are a couple of different combinations: 4-LEDs or 1 120mA LED. The single LED is simple enough that I soldered the components directly onto the leads of the LED. Sealed in 3/4" heat-shrink tubing.

You will find much more about LED circuits at my Website: Give me a visit and let me know what you are building this week!


msharri11 (author)2017-12-18

A addendum to the previous posting. I've tried substituting higher value resistors to the PNP base. The LED(s) brighten just a bit but nothing to write home about.

qs (author)msharri112017-12-18

NTE components tend to be much lower performers than their "equivalents". For example, if you're using the NTE-298, it only has less than half the gain of the "real" thing! To offset that you can try (1) reducing the 100k to 75k of 82k, or (2) reducing the coil to 28 or 30 turns. Change ONE element and test.

msharri11 (author)qs2017-12-19

Researching "equivalents" is an exercise in insanity!!! For the BC337/BC327 pair you constructed your circuit with cross to NTE123AP/NTE159 which is what I used. BC337/BC327 supposedly cross to BC637/BC627 but with NTE they cross to NTE297/NTE298, and don't get me started with BC337 crossing to 2N5818 which cross to a NTE128/NTE129 pair. With transistors, WHAT BRAND IS THE REAL THING and which cross reference is the correct one???!!!

Inductors have me somewhat guessing. The transistor pair's switching, I'm led to believe, continually build and collapse the inductor's magnetic field leading to an increase in voltage. Won't decreasing the number of turns decrease the magnetic field strength and thereby decrease the voltage output?? Is decreasing the number of turns simply decreasing the inductor's resistance and the resulting decrease in inductance irrevalent?? Inquiring minds want to know! :-)

qs (author)msharri112018-01-02

Most general purpose transistors (BC3x7, 2N2222 etc) are available through sales channels as well as eBay. Then you would be sure of getting the correct performing components.

Yes, larger inductors do build up a greater charge, but it also takes time to do so. Lessening the inductor may yield a lower charge, but it does it at a higher frequency, so the resulting light may be greater.

msharri11 (author)2017-12-18

Not sure if you're still responding to questions about this circuit. If you are, the circuit functions as designed but I can't get near the current from it as you seem to be getting. All components have identical values using NTE brand equivalent parts. The choke has 35 turns of 26 gauge (had no 28 or 30 on hand) around a label-less AA NiCd cell. I only use the NiCd case for the inductor. I've powered it with either a NiMH or alkaline cell. The LED's are 25mA max, 2.4Vf max. Each LED added to the circuit just dims them all slightly. Even one LED isn't all that bright. I can't accurately measure the amperage thru the LED(s) as the oscillating current causes the VOM to read an average. Thoughts?

DuchoD (author)2017-04-21

Say, If I use a different resistor for example like 1 ohm and connect a 3.5V 1watt led diode do You think it will work??

qs (author)DuchoD2017-04-27

No -- the resister is there to help the transistors work, and doesn't have a direct function in the LED's brightness. TO use a 1W LED, the transistors will also need to be changed to handle the current to 'charge' the coil.

Also, most AA batteries will not support the 1000 mA draw to power the light.

nullsoff (author)2015-11-22

What happens to your website?
It's not accessible anymore.

diywannaber (author)2014-11-01

Which component in your schematic determines the voltage across the LED. For example, instead of a white LED if I wanted to power a red LED what would I have to change. Thanks.

qs (author)diywannaber2014-11-02

One way is to increase the # of turns by 10 to 45, but that would make for quite a large winding!

Alternatively, the charge on the coil is approximately proportional to (V^2 * I) so if your V is 2.5 instead of 3.3, then adding two or three more LEDs in parallel would compensate for the difference.

alias198 (author)2014-10-03

I have finally made it to work, can anybody suggest me how to increase the power of led, using bc337 intensity of light not as much as 3xAA battery torch i have, will using 2sc2500 make it powerful? I have used 50k resistor instead of 150k

alias198 (author)2014-09-25

hello sir i have used every thing u have mentioned but still can't make it to work pls pls pls pls help

qs (author)alias1982014-09-25

If your parts are correct, then the problem would be your assembly, so some photos and explanation of what you have done would be very helpful here!

Also, read through the other comments below and maybe you'll see something that would help pinpoint the problem.

alias198 (author)qs2014-09-26

Thank you for your reply, I have uploaded 3 images

1) Schematics on breadboard layout (top)

2) components image.

3) layout on breadboard.

i have tried changing the diode, led and battery polarity but nothing happening,

but there is faint light spark if i short collector and base of BC337.

Pls pls pls reply,

qs (author)alias1982014-09-26

Using your layout and the specified components, I have no problem getting the circuit to work. This would suggest that some of the components have been damaged in prior tests. In particular, I would try a different 337 because if you ran the circuit without a LED attached the back-EMF from the coil would be enough to 'pop' it. Also check/change the diode for the same reason.

alias198 (author)qs2014-09-27

Done everything but nothing happening, also tried your other circuit still not working, i think some of the value i am using is not correct. I don't have access to l-c meter that's why i am unable to say if my cap and inductor is of correct value.

alias198 (author)alias1982014-09-27

if i short collector and emitter (BC337) there is light for a sec.

qs (author)alias1982014-09-27

All this does is duplicate the action of the 337 -- by shorting the leads, you are charging up the coil, which discharges to light the LED when you removed the short. At least you know the coil is not open.

From your pictures, the coil, R and caps appear to be correct. Couldn't see the numbers on the transistors or diode but one or more are likely to be defective.

Havoc481 (author)2011-06-03

hay quick question i don't care if you think its stupid but what would happen if you where to apply 3.5 to 4v to this circuit what would the out put voltage be.

qs (author)Havoc4812011-06-04

It will be zero.

The additional drive at the base of the transistor will not allow it to turn off and it becomes a short circuit, quickly draining the battery until it is destroyed.

Shalendar (author)2011-01-31

I'm having trouble finding 25mA 5mm 120000mcd white LEDs on any reputable site.

qs (author)Shalendar2011-02-07

Any 25mA LED will work. The mcd rating is just how bright the 'spot' is.

Shalendar (author)qs2011-02-07

the 25mA leds i have didnt work. My guess is that the voltage was wrong. The LEDs i have are 2v. I'm pretty sure that the LEDs you used are at least 3v. I'll have to see if I can get higher volt LEDs and try it again.

zorba02008 (author)2010-08-11

thank you for reply

zorba02008 (author)2010-08-10

What will happen if we replace the 1.5 v battery by 3 v battery ? can i use a mobile battery instead of one AA battery (rated 3.6 650ma) ? How can inductance of coil affects the efficiency of this circuit (low or high)? thanks for your efforts

qs (author)zorba020082010-08-10

For Lithium (3.6v) operation, please see my response to Newzy.

zorba02008 (author)2010-08-04

i finished my one and add something i think it is very important (reflector) like in the images.......

newzy (author)2010-07-10

@mchenson could you plz post all the details of the component that u used in your ckt..becoz u have not used c3 and value of c2 is much will b helpfull.. @qs thanx for is a nice 1..what is output voltage and current of this ckt??and if i use primary battery then 4 how many continuous hour will it last??can i add more led and 2 aa battery instead of 1??what modifications i have to made??? thanx a lot ..

qs (author)newzy2010-07-10

With Two 1-1/2volt alkaline batteries, the KISS design would be best: Direct connect the LEDs to the batteries. As long as they are White, Blue or Violet LEDs, their Vf is at or higher than the 3-volts supplied by the batteries. This ensures that the batteries are used 100% efficiently. It may not be cute or exotic, but it is the best solution.

If you are squeamish, a 22-ohm resistor in series with the LED will reduce the current (and efficiency) a bit to avoid overdriving the LED.

With 3000maH primary batteries, a single LED (@25mA) will run for (3000 / 25) = 120 hours.


newzy (author)qs2010-07-10

hii qs thanx 4 the reply..if u dont mind then can u explain me about theoretical calculation of this ckt??/like how to calculate voltage and current acroos o/p will be helpfull 4 me..thanx 4 ur cooperation..

joinaqd (author)2009-12-19

i have 2 questions:

1.) What colors are the band markings?

2.) If i have ferrite, how many turns would be needed to run this thing?

Sandivar (author)2009-10-25

Great circuit, and I look forward to building it just as soon as I can buy the required parts.

However, I do have a question:  You use a 22 uH inductor to test the circuit; why wouldn't you just use the same 22uH inductor wired into the circuit and skip wrapping the wire around the battery?

(And where can I buy some of those types of inductors, by the way?)

qs (author)Sandivar2009-10-26

Any inductor of the correct value will work in this circuit, but inductors are not nearly as common or cheap as other electronic components - that is why I try to come up with home-made solutions for them.

One source I've used is eBay - there are a few Asian sources, like this one which has them at about 10c apiece.

carpeteknus (author)2009-07-29

Excelent ! lined up for my next weekend project :) Do you know how long will a new alkaline battery last? my rough math says around 20 hours for a 2000mA h(2000mAh/(4x25mA) ), but i'm not sure that's the complete discharge calculation

sign-up (author)carpeteknus2009-07-29

Remember that the LEDs are run at 3.3v, while the battery is only 1.5v

Using the power calculation:
Input = 2500mAH * 1.5v = 3,750mWH
Output = 4 * 3.3v * 25ma = 330mW

So, to get operation hours: Input / Output = 11.4 hours. Of course there is also a penalty in the conversion (in-)efficiency we have to factor in, about 75% by my observations, which lowers the operation time to about 8-1/2 hr.

Biotele (author)2009-07-21

This is awesome ! You use the battery in place of the the ferrite. Does it have to be BC337/BC327, or could it be any type of NPN/PNP pair like 3904/3906?

qs (author)Biotele2009-07-21

To optimize performance, it really needs to have the components specified. However, that said, the real key is the NPN transistor. It has to have the power handling, gain-bandwidth and a low enough Vce for the kind of current draw we need. Ideally it should have been a 2SC965 or similar device designed for disposible flash-camera use, which shares most of the same requirements as our project, including a 1.5-volt power source.

Biotele (author)qs2009-07-21

what is the function of D1? Is it to clamp the voltage at C2 to a certain level?

qs (author)Biotele2009-07-21

The diode rectifies the output voltage of about 3V to more than double the drive to the transistors, like a supercharger on a car engine. It also keeps C2 from discharging when Q2 (the BC337) conducts.

Davidl3 (author)2009-07-05

When you said "25mA 5mm 120000 mcd white LEDs," didn't you mean "25mA 5mm 12000 mcd white LEDs?"

Rusdy (author)2009-04-10

I wonder how efficient this circuit is (i.e. volts and current input and output)

qs (author)Rusdy2009-04-10

With a new alkaline at 1.6v, it's 78% efficient, but drops to about 65% with a 1.2v Nicad - it has to do with the Vce drop across the transistors at 200mA.

Rusdy (author)qs2009-04-11

I was a wee bit pessimistic when I saw the circuit. But after testing it myself in PSPICE, I stand corrected. It is pretty damn efficient for such a simple blocking oscillator circuit (as they're not really efficient). Those components choice, do give efficient conversion. At 1.2 V, PSPICE simulation does give around 40-50% efficient, and it's consistent down to 1V. The transistor does play a big factor here though (as you commented), definitely need high gain in high frequency, your typical hobbyst transistor (such as BC547/557) won't give the same efficiency (and less brightness as well for the LED). I wish I saw your circuit first, before I ventured out to LM2623 from National Semiconductor. It definitely gives 70-80% efficiency down to 0.9V. Unfortunately, it only comes in a surface mount (for a very good reason, with upto 2MHz switching frequency), so it's pretty expensive to work with.

qs (author)Rusdy2009-04-11

I've been experimenting with the flyback circuit since the first Joule Thieves went around the web. You can see my efforts here on my website.

Normally I use low Vce transistors like the 2SD965 which really helps in the efficiency, but, in designing this circuit here, I wanted one that can be duplicated by the average hobbyist without access to the fancier equipment and components.

I'm glad you like it.

Sagar Gondaliya (author)qs2009-05-16

your ideas are amazing. i was just wondering, i dont have a very good monitor so i cant read your schematics on the webite. could you maybe post some computer generated ones so i could read them? thnx very much in advance -a fellow maker

qs (author)Sagar Gondaliya2009-05-16

Tell me the ones you are having trouble with and I can try and send you better images.

Also, some of the circuits on my website link to larger ones too.

Sagar Gondaliya (author)qs2009-05-16

im mainly having problems with the hack-lite one. thnx

qs (author)Sagar Gondaliya2009-05-16

Here's a bigger image. Or you can try this link which is accessible from the smaller pic on my website as well.

Let me know if this is helpful.

Sagar Gondaliya (author)qs2009-05-17

yea it is. thnx

imakethings (author)2009-02-24

what is the difference between PNP and NPN

About This Instructable




More by qs:Joule Thief LED circuitsA Triple Channel Musicator - the TriM...Musicator Jr - Mk 2
Add instructable to: