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Battery-less LED flashlight

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I would like to build a decently bright LED flashlight that uses a capacitor instead of a battery to power the LED. I'd like to use some sort of magnet in a tube of windings to generate the current - also may consider a little solar panel. I know they sell these "forever flashlights" for around $20, but I'm a handy, do-it-yourself kind of guy and would like the satisfaction of doing it on my own -- except for I know crap about electronics. Ok, so I know a little.

I just thought capacitors took a little trickle charge until they reached capacity and then zapped out all the voltage like a little lightning bolt, but I guess I was wrong.

So, any ideas, tips, pointers, schematics or ridicule will be accepted.

thanks -
My cousin has one of those flahslights that you shake, causng a magnet to pass through coils and charging a battery. I may have to 'borrow' it and see how it works.

I just thought capacitors took a little trickle charge until they reached capacity and then zapped out all the voltage like a little lightning bolt, but I guess I was wrong

They are capable of this, but do not necessarily behave this way. They will only discharge as fast as you allow them to. Note also, that capacitors have "leakage current", which will cause them to self-discharge (after a long time), without any load.

There are basically 2 problems here - 1) Capacitors have a very low energy density, compared to batteries, and unless you have a VERY large capacitor, your light will not last very long (maybe 1 minute with a big cap). 2) As the capacitor discharges, the voltage drops logarithmically - a battery keeps fairly constant voltage until is nearly empty.

I think the solar panel idea is better.
hey, i think that the solar panel is the best idea, if you combine it with some rechargable batteries to store the energy.
i face a little 'problem': if you only use the solar panel and the LED, then you get light from the panel and power the led, wich as i see, you dont have to use the flashlight as you have light to power the panel.
the idea with having some magnets that shake and produce energy is also good, considering that you moove the flashlight while you use it.
i think that if you combine this with a solar panel, some rechargable batteries, and the magnet stuff you can get something interesting. you could use a PWM circuit, or to drive the leds, or maybe a special led driver.
as for the caps, you can't power a led for a long time by using a cap. maybe if you have a cap of 100mF, yes, or even larger. but you might want to combine it with a switching suply so that you can get a constant output voltage.
The best schematic for a LED flashlight that I've seen so far was published by
in the november 2002 edition.Sadly I am not allowed to upload the article due to copyright restrictions. You could try asking them for a copy though.

What I can say though,

Some of best LEDS you can buy are made by a company called Nichia (japan) ref NSPW 500BS [6400mCd!]

Maxim make a couple of ic's for driving leds
**broken link removed**
**broken link removed**

You may also want to take a look at battery chargers for Lithium-ion cells as they are very suitable for this application.

Build your own solar cells

I have a strong issue with the ShakeLight and its contemporaries. Shaking is just a REALLY bad way of converting motion into electricity. If you want a decent hand powered light, you really need to be using a crank generator. Shaking is really just silly. If you want a nice technical breakdown of the ShakeLight, check here.

With that said, The ShakeLight uses a whopping 1 Farad, 5.5v capacitor to hold its charge. The ForeverFlashlight uses a 1.5F cap. Doesn't sound so high... until you try to find some. However Digikey does have that and bigger in electric double layer or Aerogel styles. Personally, I'd get as big a cap as possible to store maximum charge.

As far as the light and your power source go, I've considered picking up one of those squeeze crank lights, commonly called Dynamo lights, for around $10. It will give you a good generator to base you design on. I would expect that the first thing I'd do is change it from a squeezing action to a cranking (round) action.

And lastly, you can get nice cheap Nichia LEDs from **broken link removed**. I've bought from him and he has good stuff.
Components and such

Ok -- I've found a 9000 MCD super bright 10mm LED for under $3...and I think that's what I'd like to use. Sounds like a good deal anyhow....and it looks pretty darn bright.

I found a super cap for $1.49 - 5.5v but it's only 0.1 Farad....which according to what everyone is telling me isn't NEARLY enough!

I'm beginning to think that the solar panel/nicad or lithium ion battery is a better idea than the shaky idea.

So, how do I figure out how long the battery will run the LED and how long will the solar panel take to charge the battery??? The LED is 3.6v to 4v at 25mA. For $5 I can get 2 3.6v 1100mA Lithium Ion batteries. So then I assume that on one fully charged battery the LED would run for 1100/25 = 44 hours??? Is that right or am I screwing this up. I have no problem figuring out Amp Hours for bigger batteries...but this seems different.

Also, do I really need an IC to control the led? How do I figure out what size resistor to use to drop the 1100mA to 25mA?

See, I told you I was mostly clueless! I used to be into electronics quite a bit more when I was in Jr. High, but most the things I built with my Radio Shack books didn't work so I became frustrated.

So, I'm trying to figure this out from the point of view of the LED and battery above with a solar panel.

Thanks for all the help so far!
Well there's a few formulas for calculating power consumption, unfortunately I can't find any.

To figure out what kind of resistor you need get all the info you can on the LED then figure out the following formula.

R= (Vs-Vl)/I

Vs = supply voltage (Your Battery Voltage)
Vl = LED voltage (The Max Voltage of Your LED)
I = LED current (In your case 25mA)

R will be in KOhms, so don't try to find a .05 Ohm resistor (You can search forever, there is no such thing) just means 50 Ohms. :D

So say when you get it all done, the supply voltage is 6, your LED is 4v, and you MA is 25, well then you answer would be .08KOhms, or 80 Ohms.


I found a super cap for $1.49 - 5.5v but it's only 0.1 Farad....which according to what everyone is telling me isn't NEARLY enough!

OMFG Slap whoever told you that's not enough, with that, if you had screwed up your project and sent too much power to it you could kill yourself.

.1 Farad is roughly equal to 100,000uF and that's about 4545 times greater than the power of low grade stun guns. I mean sure it's not carrying the 10,000v a stungun does, but that could still have seriously hurt you.

For me, just beat the living F out of the guy who told you that.
Now if you insist on using a capacitor as a power source, find out what you would need by downloading a schematic program like the multism2001 electronics workbench demo (only good for 15 days, but hey, it's better than popping some of your electronics) and find out what kind of power your solar cell or other power supply will be outputting, calculate the formula above, insert the proper resistor, configure everything, and then test capacitors, if it blinks, go up, if it stays on, good, if it doesn't light up at all, go down, if nothing works, you probably assembled it wrong, keep trying until you get it right.

However what I want to know is why you are so averse to using a battery, I mean if it's because you want it supercompact, use a lith coin cell battery, you can get your flashlight under 5cm long, and 1cm wide.
Also if you want to save some power, or are concerned with it, try to use a dimmer led, and put a reflector, and a condenser lense on it, a lower power led will allow it to run longer.

The reflector will double the apparent brightness, a properly built reflector with a condenser lense can make an 1000MCD light look like a 4000MCD light.

But do not get a super powerful condenser lense, as if improperly handled can damage the user (when properly reflected a super powerful condenser lense in direct sunlight can make wood scorch, and eventually burn)
welll, if you come to think about it the power disipated on a led is P=U*I so P=4*0.025=0.1W
if you multiply power and time you get the energy utilized for the process.
now, the energy stored in the cap it W=C*U^2*0.5 wich means that for a 0.1F cap charged at 5 volts you have : W=0.1*5*5*0.5=1.25Joules.
if you will be using a led driver with an efficency of 90% then the time it will power the led is t=W/P=1.25/0.1=12.5 seconds of full power, assuming that the driver can get all of the energy out of the cap till it drops to 0 Volts. but in practice you will not get this situations, you will probably use a simple resistor connected in series with the led. so the brigtheness will descrease. about using lenses, well, i dont think this helps, for a flashlight, because i assume that you don't wnt the light focused on a 5cm spot when you light it from a few meters away.
if you were to think that using a highter voltage will be better, because the energy stored in a cap is proportional to the square of the voltage, also with the capacistance.
so, by haviin a 100uF cap charged at 200V you store an energy of 8J compared with the 1.25 from the 0.1F cap. but if you can get a 1F, or 1.5F cap it would be better.
i would go for the ieda of using some AAAA batteries, Ni-MH, with a solar panel. the building of the flashlight will be important if you try to do this, because you will have to make the flashlight small, yet have a big surface for the solar panels.
I was in DSE yesterday - I noticed that they had a hand-powered flashlight on sale - you have to squeeze the handle repeatedly to make it light - it was a really pathetic excuse for a flashlight. To get any decent sort of light, you have to squeeze it so fast that you can't keep it steady.
the freeplay plus which also has a flashlight built in has the best design I found so far... some of the crank flashlight/radios I found just charge a battery but this one has the option to charge a battery or to run straight off of the slowly turning spring that you wound up.... so the wound up spring acts as a battery/cap and can hold a "charge" for a long time... well.. sooner or later if you leave it wound up it'll lose some of its spring...

**broken link removed**

I get the feeling that I'm going to need a schematic or a list of components from a project someone else has already built. I downloaded that Multisim program...but I don't think I know what the hell I'm doing.

I put in a battery - 7.2V, an LED at 4V and 25mA. Then I hooked up a multimeter to the LED and measured the amperage. If I use a 330 Ohm resistor the amperage through the LED is 24mA -- this is what I want right??!! I don't want too many amps running through the LED because that is what will fry it, right???

I knew I should've taken that electronics course in college!!!
:D OK -- 7.2v battery, 4v 25mA LED 7.2 - 4 = 3.2 3.2/25 = 0.128 therefore I need a 128Ohm resistor to make everything work out. Hey, I think I've got that much figured out!

But now what do I need a LED driver or IC for??? Can't I just run everything this way and all will be good???
But now what do I need a LED driver or IC for???

LED drivers can increase the efficiency of your torch, typically by using switched-capacitor techniques. You can lose a lot of power in your resistor, and this is one method of overcoming that.
The use of an ic enables you to use a smaller battery with a lower operating/charging voltage, and still drive more than one led.
Less weight to shake or less solar cells to make / buy.

Variable brightness control to conserve power, auto switch off when not in use, switch on with vibration (eg Fliklite)
Never be left in the dark again.
well, thats what you can do with a led driver.
the thing i like it the most is that it can operate more leds in series making sure they have the same current trough them and cand drive them from a low power suply, most led drivers can operate from 2 1.2V rechargable batteries.
efficency is quite good.....i think that it is better than using your 128R reistor.

by the way, has anyone heard of these new solar panels made from alge wich have about 20-305 efficeny? i heard they are still experimenting on them, but they say that have double the efficency......
bow i would want to get my hands on some of that........
Hi everybody, im really glad i found this site, im not too smart into electronics, so i apologize for dumb questions. But im really interested in building one of these. here are some comments

-12v batteries could be bought at radioshack and they are really small
- a case can simply be fashioned with 2 rubber tips and some tacks

- instead of a rod and having a ring magnet slid back and forth, you could use spherical magnets and put them into a container ( im hoping to use a bic pen ) then wrap some wire over the container

- you could order power neodyminium magnets cheap from
- and i get my leds from

- you can get 8,000 mcd reds or any other colors for a rather cheap price

i just have a question on how the copper coil is suppose to work. Can i just wrap some magenetic wire over this bic pen design? does the wire have to be insulated? I've seen pictures of those forever flashlights and their coil doesnt really seem to insulated. Any gauge suggestions? i would like to keep this small, in order to fit inside a glowstick..

and im still confused on the capacitor, what should be the minimum to power a 2.6 v led?
Hi everybody, im really glad i found this site, im not too smart into electronics, so i apologize for dumb questions.

The Only Dumb Question is the one not asked, remember that, if you don't speak up, you'll never know.

Okay, so you say you want to build a magnetic flashlight eh ?

Well, some of your ideas are on the ball, some are not.

The 12v battery may be a bit too powerful, re-read the earlier posts to find out what battery may be best. I'd think 3-6v would do.

If you plan on using a pic pen as your case, I'd say go with something a lot smaller battery wise, probably 2 lithium watch batteries, or maybe 2 hearing aide batteries.

Now the magnetic recharger needs to have 2 poles that stay relatively stable, and get a constant charge, so a spherical magnet may not be the best idea, I'd just say stick with a rod magnet, another thing you must remember, is that those wires will be conducting electricity, nothing too high in volts/amps to hurt, but enough to get hot, not hot enough to melt plastic, but enough to make you drop the flashlight, so I'd make them inside the pen.

So my best idea would be to get the rod magnet, and get a nail about 1mm larger in diameter than the magnet, so it can move back and forth freely.

Tightly wrap the wiring using a thin wire, most likely enamel coated copper to generate more electricity, not necessarily a higher voltage, but the more coils the magnet passes through the longer electricity is generated.

As for the rest, it's all up to you, so you have the final decision.
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