Building a glue stick flashlight

Building an LED flashlight is simple, right? Take a battery, connect it to an LED by way of a resistor. Alright wise guy, now make one that steps up the voltage for multiple LEDs and don’t use a boost-converter IC to do so.

[fede.tft] shares a flashlight built inside of a  used glue stick case. It’s the perfect size for one AA battery (we’re always on the lookout for good battery cases), and a shape that we’re familiar with as a flashlight. The problem is that he wants two white LEDs but with just one AA cell he’s never going to have more that 1.5V available. He licked that problem, getting to 7.2V by designing his own step-up converter using one transistor, an inductor, and three passive components. To get the inductor he needs, a stock part is disassembled and rewound to suit. Maybe you just end up with a flashlight when all is said and done, but then again, the Sistine Chapel is just some paintings on a ceiling.

29 thoughts on “Building a glue stick flashlight

  1. @andres: yep. it’s a step-up circuit with transformer feedback just like the joule thief.
    This one however has a capacitor and a diode added, and i don’t know, whether i like that: at this low a voltage the extra diode eats up probably 30% of the energy. i’d rather go without a capacitor and let the leds flicker (at a frequency above 100hz).

  2. @ultramagnus: i take the bait.
    yes, your suggestion would improve it, but only if you add a sensing resistor after the leds and use its voltage at an adc-input to measure the current, so you an adjust your pwm-signal.
    however, if one doesn’t want to implement a current feedback a microchip would be overkill and i can’t imagine why anyone with half a brain would use a microchip in that way

  3. You could also use a Schottky diode for a low forward drop.

    This reminds me of the episode of Get Smart! where Maxwell Smart had a tape recorder disguised as a camera and a camera disguised as a tape recorder.

  4. @addictronics, @localroger:
    germanium would be an improvement, other than price, i don’t know why you don’t find it in any circuits. schottky usually is the recommendation you find in the example circuits from chip manufacturer.
    my 30% guess was off by the way. looks like it is closer to 10%: with 20ma average, the leds would use 128mw (6.4*.02) and the diode 14mw (.7*.02). with only one led, or parallel leds, the diode would waste 18%. -another good reason to use leds in series and go to higher voltage.

  5. Actually, I’ve chosen the the BAT42 diode because it is schottky, in an attempt to increase efficiency (together with wiring the LEDs in series). However, I measured the voltage drop, and it is more close to 0.7V than to 0.3, don’t know why, probably because of the peak current.

    Also, I didn’t know of the joule thief, I think it’s possible to make a mix of the two circuits by removing the output diode and capacitor, probably increasing efficiency but at the cost of a (fast, ~100KHz) blinking led.

  6. Aren’t there a great many nicer enclosures for led lights? I mean yeah it fits an AA maybe but surely there are spiffier looking things that do too?
    Still, I know it can be harder than it seems to get a thing like that nice and working, so kudos anyway :)

  7. I did this with a Benzedrex inhaler. It only takes an AAA battery, an LED, two pieces of wire and a staple to do everything. I should go find it so I can share it because I was really lucky to stumble upon such a simple design.

    I think this design here is better and probably more reliable, but more complicated than the cheaply improvised one I had.

  8. @peter: i think you would have problems getting enough power out of a single AA to really drive a cree. the maximum current even a boost circuit can get out of a battery is the short-circuit current, which is limited by the internal resistance. as battery voltage goes down, that resistance goes up. if you want to build a circuit that really uses the battery down to 0.3v or 0.5v, it won’t support a cree anymore (a box full of empty batteries in parallel however probably would)

  9. @fede.tft: it might be the higher voltage or it might be your meter. i measured parts before, only to find out that my cheap probe added 0.4v (contact resistance probably)

  10. @pete: i found most store-bought lights to be quite awful. the ones with power led (even brand-name) often use a resistor instead of a microchip driver, or use a driver that isn’t properly matched.
    the ones with multi 5mm led all wire the leds in parallel and just let the battery decide, what the current should be. the current changes between 60ma and just a couple ma.

  11. The Sistine Chapel is more than just paint on a ceiling- its also an army of Swiss Guards just waiting to swoop in a steal your DSLR when you try to take a photograph! I admit though, I’m jealous of their threads.

    As for the actual hack at hand, I like the design above all else- but imagine what you could do with an Elmer’s glue bottle.

  12. I was looking at those pictures from the paintings in the sistene chapel and it struck me that the style of many of the art is very anime-like, weird, never noticed that before.

  13. I actually just completed winding my own inductor and assembling this circuit, its sitting on my nightstand now working like a charm. I am going to build a case out of some form of wood tomorrow, similar to the flash drive housing I made out of wormy chesnut. Any suggestions for lumber type? I will probably stain/varnish if it turns out right.

  14. You guys think too hard about this. Buy some 1/2Watt rated about 140, 000mcds.
    i hated the stupid headlamp I bought at a homeimprovement store and it was pitiful It had this dumb flashing mode. If you like this check out a few basic transistor based oscilator circuits or you could buy a 555 timer and add 2 or three compontents. Look into astable bistable monostable vibrator circuits. I think they use them in dildos, turn signal lights, alarms, all kinds of retarded and critical applications. Their older than me twice my age roughly. I scraped the headlamps LEDs kept the head band and the reflector.
    Get a mini glue gun, a drill press or (a hand drill will do but it is easier to get these ultra bright lamps in straight). Use the 10mm type 1/2 or 1 Watt, I like the 1/2s and just use more of them. I fit 5 in my headlamp (slightly larger fixture than your standard maglight. (For a mag you can get them damn bright but a bit trickier mechanically that is.)
    One 9 volt battery slapped on top of the lamp or inside whatever flashlight your using.(I moded a mini mag and stuck with the AAs and it’s bright and the batteries last a lot longer) You can make a cute case for it if you want, maybe a subminiature SPST switch. I don’t use a resistor and it does not need a silly circuit to drive it. ( volts power these lights brighter than any consumer grade LED lamps I have owned or seen. ( volt batterries are more practically suites for their hook up simplicity and shape. EBAY Ultra bright LEDS 100mm. BUy some 1/2Ws and Some 1Ws and experiment a bit. Also get some adhesive backed plastic or paper printale sheets with CAD or illustrator drawings you can mark any holes or special design ideas you want to implement. You can use a Potentiometer as a rheostat to control the brightness but I don’t need that option. This would be nice for retrofitting a reading or drafting lamp with LEDs.

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