Circuit design to run LEDs on AA battery

Discussion in 'The Projects Forum' started by stASH, Aug 29, 2010.

1. stASH Thread Starter New Member

Aug 29, 2010
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I am attempting to design a circuit that will light 8 various colored LEDs with a single AA or AAA battery. I have a very basic understanding of circuits and would have no trouble with this if I could use 2 or 3 batteries. How do a create a simple circuit that will boost the 1.5 volts of the single battery to say 3.6 volts? I also have physical space constraints: the circuit must fit inside tubing with a 1/2" inside diameter.

Also, what would be done differently if I were to use a rechargeable battery with 1.2 volts instead?

Thanks for the help, I was getting a bit confused trying to figure this out myself.

Dec 5, 2009
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3. stASH Thread Starter New Member

Aug 29, 2010
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Thanks. How do I determine the output voltage of the Joule Thief? I suspect it will change based on the number of windings around the ferrite bead and the value of the resistor. Also, does the Joule Thief output a constant voltage?

I understand about forward voltage and how to determine the correct resistor for the LEDs.

4. Wendy Moderator

Mar 24, 2008
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It doesn't matter, the joule theif interacts with the LED so things work out pretty close to optimum.

5. BMorse AAC Fanatic!

Sep 26, 2009
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I have this Joule Thief circuit that is IMO, better, since you do not have to wind your own inductor..... and I have used it to power 9 white LED's off of one AAA battery....

B. Morse

6. Audioguru Expert

Dec 20, 2007
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Most cheap solar garden lights use a single 1.2V Ni-Cad battery and a simple voltage stepup circuit to light a red, blue, green or white LED or a color-changing LED.

7. stASH Thread Starter New Member

Aug 29, 2010
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Thanks for all the input. This seems like it'll be fairly simple to accomplish. One more question (for now), I have some slow changing LEDs that I'd like to include in the circuit. They have the circuit built in that controls them. They always start the color changing cycle on red when power is applied. If the joule theif essentially is blinking the LED at a very high rate, will the color changing LED be stuck on red?

8. SgtWookie Expert

Jul 17, 2007
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Very likely it would get stuck.

Take any of the circuits, and add a Schottky diode between the LEDs and the circuit output, and a 1uF or larger cap across the LED string. If your current is quite low (<40mA average, 80mA max) you could use 1N914/1N4148 switching diodes.

Last edited: Aug 30, 2010
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9. Audioguru Expert

Dec 20, 2007
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I converted some white solar garden lights to the multi-color kind by adding a diode and capacitor so that the multi-color LED gets a smooth DC voltage.

10. stASH Thread Starter New Member

Aug 29, 2010
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Ok, I still have some questions about utilizing the joule thief. If I have an LED with forward voltage of 2.2 and another with 3.5 in parallel on the same circuit, how do I know which resistor to use on the 2.2 volt LED without knowing the output voltage of the joule thief? Also I definitely want the LEDs to be very bright. How do I ensure that they are drawing, say 25 mA?
Well, if I'm running 8 LEDs at 25 mA, isn't the total current 200mA? Different diode then? This is getting interesting. The trouble I get myself into for my girl...

11. SgtWookie Expert

Jul 17, 2007
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Hmm. You may have difficulty running 8 LEDs in series. You may need to run 2 strings of 4 LEDs in parallel, which would require 50mA. However, you could use two 1N4148/1N914 diodes, one per string, and one cap per string.

Basically, the size of the inductor determines the operating frequency. The larger the inductor, the lower the frequency, and the larger the cap(s) you'll need across the LEDs to keep the current flow through the LEDs relatively constant.

12. tom66 Senior Member

May 9, 2009
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You could also use a 1N5820 or similar Schottky diode. This device works up to 1 amp continuous current, and because of its small 0.3V (thereabouts) forward drop, you will get a brighter LED and the battery may last longer.

13. Audioguru Expert

Dec 20, 2007
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I think the output of a Joule Thief is only a few mA and keeps dropping as the battery runs down. A current-limiting resistor is not needed because the max current is already limited.
Two different LEDs would each need a current-limiting resistor that will reduce the already low output current then each LED will be dim.

14. stASH Thread Starter New Member

Aug 29, 2010
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I was actually thinking of running all 8 in parallel on one string. The battery, circuit components, and string of LEDs will be in a section of tubing with 1/2" inside diameter.
So, is it even possible to get the LEDs to run brightly on 25mA using the Joule Thief?

15. stASH Thread Starter New Member

Aug 29, 2010
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Oh, and thank you all for the input. I'm getting a little confused, and I'm wishing I remembered everything I learned in Electrical Theory while getting my engineering degree. I moved in the direction of water resources so haven't applied this portion of knowledge to anything in at least 15 years. Anyway, with all of your help, I'm feeling pretty good about getting this thing to work.

16. Audioguru Expert

Dec 20, 2007
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Many "bright" LEDs are simply old dim ones in a case that focusses the beam into a narrow angle. Many applications for LEDs require a bright wide-angle beam of light.

My solar garden lights have an average LED current of only 10mA.

17. stASH Thread Starter New Member

Aug 29, 2010
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Hmmm, I think i'm gonna do a little testing on the LEDs to see how varying current affects their brightness. Maybe the lower current from the joule thief will still be adequate. Although, I want really bright, not just adequate.

18. Audioguru Expert

Dec 20, 2007
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In my microphone VU meter (it is always turned on) I have ultra-bright wide-angle LEDs at 25mA and they are very bright.
In my LED chasers I have ultra-bright wide angle LEDs pulsed for only 25ms at 26mA and they are very bright.
In my two night lights I have Luxeon Super-Flux LEDs (4 pins each) at 53mA each and they are very hot and very bright.

19. stASH Thread Starter New Member

Aug 29, 2010
13
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Um, I'm not really sure how that information helps me.

20. retched AAC Fanatic!

Dec 5, 2009
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You can get very bright light out of an LED with surprisingly little current.

AND some LEDs can get very hot with "little" current.