# 50 IR LEDs powered by 9v battery??

Discussion in 'LEDs and Optoelectronics' started by jeepinjeff, Aug 10, 2011.

1. ### jeepinjeff

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Aug 10, 2011
I need help creating an LED circuit. I'm trying to make an infrared flood light using 50 IR LEDs. The 'light' is to be powered by a single 9 volt battery and last approximately 6 hours before draining a battery. The LEDs in question use a forward voltage of 1.3 volts, have a 1.2 volt drop and draw 50 mA each. The data sheet can be seen here LINK.

There is currently a light in production that uses these same specs, so I know it can be done, I just can't figure out the proper configuration and resistors to use. Any help would be GREATLY appreciated.

Thanks!

2. ### daddles

443
3
Jun 10, 2011
It can't be done. A 9 volt battery is one of the most pathetically anemic batteries available; it has a capacity of approximately 500 mA*hours. You're saying the LEDs have a drop of 1.2 V at 50 mA. Put six of those in series (that gives you 7.2 volts, or an optimistic estimate of end-of-life voltage of the battery). Put 8 of those strings in parallel to power 48 LEDs. Now, you'll be drawing 0.4 A from the battery. It probably will last only about 30 minutes at best.

If some commercial device is doing it, then they are switching things at a lower duty cycle to make the battery last longer.

3. ### jeepinjeff

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Aug 10, 2011
I know it sounds weird, but...

I understand that this all sounds weird and impossible, but I contacted the creator of the item and asked "What are the specs of the IR LEDs you use for your 50 LED IR Flood Light? I am having trouble understanding how you can get 6 hours of run time from 50 LEDs using only a single 9v battery."

The item in question can be seen here --> LINK

I got a reply from the maker stating "The Flood lights actually last longer than that. I am not sure want you are wanting to know here. These are our own design and patent pending. I can not give out specs but they do last over 6 hours and the 25 infrared illuminator last over 12 hours of constant use."

So, I replied asking "Do these run on a single 9v battery or some combination of C or D cell batteries?" To which he replied "The lights run off of 1 9v and that is it."

So, ultimately, the reason why I came here to ask you all for help is because this all sounds nearly impossible to achieve and I figured that your knowledge would be able to shed some light on how they are accomplishing such a feat.

Thanks!

4. ### BobK

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Jan 5, 2010
Well, they don't say what kind of 9V battery.

Bob

5. ### Laplace

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Apr 4, 2010
In the first place, you would not run 50 mA through the LED -- that is an absolute maximum rating. A current of 20 mA is more reasonable. Also the data sheet shows a maximum forward voltage of 1.6 V, with a typical value of 1.3 volts at 20 mA. So at most you could string 5 LEDs in series (5x1.6=8.0) leaving 1 volt to add a 20 mA current source. Ten LED strings would draw 200 mA. Or.....use a forward current of just 5 mA -- LEDs are more efficient at lower current. Then the total current for 10 strings would be only 50 mA, and with 50 LEDs there would still be plenty of light. For the 20 mA current source I would use a high gain transistor such as the MPSA18 with an emitter resistor of 33 ohm (recommend 1% precision resistors here) and drive the base with a 1.2 V bandgap reference such as the LT1004-1.2 which can be used as the common reference voltage for all strings. Adjust the value of the emitter resistor as necessary to change the current. If you are only using a total current of 50 mA then a 9 V battery could last quite a long time.

6. ### daddles

443
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Jun 10, 2011
If they say it lasts 6 hours, then they're probably drawing a current of around 50-75 mA from the battery.

Since you want to know what they're doing, you need to open the device up and measure what's going on and make a circuit diagram. Put a scope on the LEDs and see if they're being switched. Use an optical detector and measure the actual output power they're getting. They're not doing anything magical -- and I would be surprised if they have anything actually patentable. I can guarantee they're playing within the rules of physics.

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Heh. Assuredly.

8. ### MagicMatt

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Jun 15, 2011
I believe the LEDs are lit in a cycle, not all simultaneously. I can't be sure on that, but I have seen a device that I *think* was the Phantom Lites 25LED model. It was a while ago. I remember it didn't work with the CCTV camera, as it gave a sort of shimmering/flickering light and it took us ages to work out why. Also, the idea of it reaching 50ft was laughable... 5ft was more realistic.

9. ### Laplace

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Apr 4, 2010
What is the design advantage to multiplexing a flood light? I just don't see it. Is there some principle of physics at work there of which I'm totally unaware?

10. ### poor mystic

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Apr 8, 2011

Laplace asks good questions. Multiplexing a floodlight raises the expense of the installation and nothing else.
Some LEDs do work better when pulsed. I had thought that this pulsing had to be at several kHz though.

11. ### MagicMatt

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Jun 15, 2011
It makes about as much sense as using 50 LEDs to start with, when one good quality one would probably do the same.

Example - I have a flashlight with a single high powered LED (Cree Q5). Fantastic light. I get challenged by people LED shower-heads running off 3 heavy D-cells who take one look at my little light and think it'll be pathetic. I turn it on and blow them away, as it's so bright you can't even see their torch is on.

Personally I think manufacturers do all sorts of stupid things so they can make fantastical claims as to the power of their products.

Whatever way you look at it, a 9V cell holds 500mA. To last 6 hours, it has to be pulling under 80mA off the cell. If it's driving all those LEDs at once, it must be providing no more than 1.5mA to each LED, which I'd be amazed if it even illuminated.

Multiplexing can even out the light a bit if you're using LEDs with video cameras. Driving LEDs alternately gives better light than the same LEDs simultaneously at the same duty cycles - less flicker, though there is still some flicker..

12. ### poor mystic

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Apr 8, 2011

A tribe whose warriors assert dominance through the brightness of their led torches. We must alert the anthropologists!

13. ### MagicMatt

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Jun 15, 2011
They lurk here - http://www.candlepowerforums.com/vb/forum.php - some very knowledgeable people on that forum!

In my case, it's more a case of wanting to see where I'm going walking through woodland in the dark, without wanting to carry lots of extra weight. Anyone who wants to carry an extra kg or two in torch and batteries is mad IMHO - I'd rather carry an extra 100g in my pocket and not really notice it's there until I need to use it.

14. ### poor mystic

1,074
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Apr 8, 2011

So these enthusiasts "lurk", do they? Hidden in the depth of shadow cast by torches is powerful as these, they might lurk in great numbers. I've never seen the attraction of it myself.
I think it was 5 years ago by now that I read of a Californian company making LED torches that would set fire to paper in front of them. I didn't know of any other uses for these devices, but now I understand.

15. ### MagicMatt

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Jun 15, 2011
That's the really silly end of the market. Same with the overly powerful laser pointers. I agree there's a point where it ceases to be useful and becomes a novelty item.

I still recommend everyone buys a high powered LED torch that runs off a single AAA battery. Everyone I know who has one finds it useful, be it for just finding something they dropped down the back/ under somewhere, seeing around the home if the power goes out, that sort of thing. They fit on your keyring, no more intrusive than a car key. Fantastic little things, but as bright as the standard 2D maglite.

Going back to the original topic, the main use for IR lighting that I've found is illumination for CCTV. The system I put in was at an observatory, where obviously we don't want light pollution from security lights! With the IR system, the CCTV can see everything happening on site, and nobody really notices them (though they do need turning off for astronomical imaging). The lights I fitted were multi-LED as there wasn't a more compact option available at the time, but these are units that run off 24V power supplies, and they use a lot of juice. They light to 50m properly, and a single 9V cell would power them for possibly a minute or two, if you could ramp up the voltage without any loss (impossible of course). These are big, heavy lamps, and not out of choice.

16. ### donkey

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Feb 26, 2011
i just recently bought a torch for night security work..... i am still unsure if 2500 lumens is enough... i mean yes it looks like daylight for 300meters.... but it doesn't show whats on the opposite side of the wall like in cartoons.....
http://www.jaycar.com.au/productView.asp?ID=ST3361&keywords=st-3361&form=KEYWORD
its funny to see other guards pull out there mag lights and hits me in the eyes, then i get my torch tell them to stand still, run 200 meters up the road and still blind them from an indirect flash.... but thats just me lol

anyway to the topic at hand i found this for you http://www.jaycar.com.au/productResults.asp?keywords=kg-9068&keyform=KEYWORD&SUBMIT.x=47&SUBMIT.y=8
it may be of help and they have the optional plugpacks and boxes to suit, some soldering required

Last edited: Aug 15, 2011
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