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A high-power LED torch using a single AA-battery

A high-power LED torch using a single AA-battery
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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.
 
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Step 1The circuit and layout.

The circuit and layout.
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  • BoostAA Ckt.jpg
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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.
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93 comments
1-40 of 93next »
Jun 3, 2011. 8:15 PMHavoc481 says:
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.
Jan 31, 2011. 11:36 AMShalendar says:
I'm having trouble finding 25mA 5mm 120000mcd white LEDs on any reputable site.
Feb 7, 2011. 9:36 PMShalendar says:
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.
Apr 10, 2009. 4:47 AMRusdy says:
I wonder how efficient this circuit is (i.e. volts and current input and output)
Apr 11, 2009. 7:51 PMRusdy says:
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.
May 16, 2009. 7:36 AMSagar Gondaliya says:
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
May 16, 2009. 1:41 PMSagar Gondaliya says:
im mainly having problems with the hack-lite one. thnx
May 17, 2009. 6:26 AMSagar Gondaliya says:
yea it is. thnx
Aug 11, 2010. 4:30 AMzorba02008 says:
thank you for reply
Aug 10, 2010. 3:11 AMzorba02008 says:
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
Aug 4, 2010. 8:54 AMzorba02008 says:
i finished my one and add something i think it is very important (reflector) like in the images.......
Jul 10, 2010. 5:50 AMnewzy says:
@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 more.it will b helpfull.. @qs thanx for ckt..it 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 ..
Jul 10, 2010. 8:41 PMnewzy says:
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 theoretically...it will be helpfull 4 me..thanx 4 ur cooperation..
Dec 19, 2009. 4:17 PMjoinaqd says:
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?
Oct 25, 2009. 8:46 PMSandivar says:
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?)
Jul 29, 2009. 7:00 PMcarpeteknus says:
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
Jul 29, 2009. 8:54 PMsign-up says:
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.
Jul 21, 2009. 1:23 PMBiotele says:
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?
Jul 21, 2009. 2:54 PMBiotele says:
what is the function of D1? Is it to clamp the voltage at C2 to a certain level?
Jul 5, 2009. 8:09 AMDavidl3 says:
When you said "25mA 5mm 120000 mcd white LEDs," didn't you mean "25mA 5mm 12000 mcd white LEDs?"
Feb 24, 2009. 8:50 PMimakethings says:
what is the difference between PNP and NPN
May 15, 2009. 8:03 PMtanmanknex says:
a simple explanation is that they are built differently and are thus wired up slightly different.
May 11, 2009. 7:03 PMralegg says:
Could you list out which parts go to certain points in your electrical diagram? I am not very good with circuitry but I am trying, I have all the parts on your part list but your diagram has listed a 1N4148 diode, is this necessary b/c it's not on your list? Thanks
Apr 24, 2009. 5:26 PMML159 says:
When I try to build the circuit the light just doesn't seem to light... I have to admit that I do not have every part specified and used different parts, is that the main source of my problem? I am using a bread board to test it and so far I have managed to get other "joule thief's" to work perfectly fine. I have been to you website and I have to say it is pretty amazing, kudos. I tried building those but i managed to get only one to work very poorly. Yet again I did not use the exact parts specified so that might be the problem. I am open to any suggestions as to why it doesn't work... thanks.
Apr 27, 2009. 4:11 PMML159 says:
Thanks for your help! I think I am starting to get it to work.
Feb 27, 2009. 4:01 PMdog812 says:
Hey i was wondering.. Would a circuit like this work for my applications with my LED hula hoops? I would like to make them less expensive for people.. using a single AA or 2 AA in a drop in style holder would be awesome.. Please message me i would love to ask you some questions..
1-40 of 93next »

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