Joule Thief LED Driver

[Bird603568] sent in this sweet little LED hack that’ll drive a white LED from just 1.5 volts. The circuit consists of a pair of coils wound on a ferrite core, a resistor and a NPN transistor. He notes that the circuit still functions even at .35v. The version pictured is even small enough to fit inside a normal flashlight bulb base.

30 thoughts on “Joule Thief LED Driver

  1. what a coincidence. looking around for a good description of this sort of circuit and it appears on hackaday two days later.

    also, i wonder why my google searches didn’t pull up the page linked by woop. very helpful. thanks guys.

  2. yeah talking electronics is a small Australian company. they make kits and books which used to be sold at dick smith electronics, but no longer unfortunately. they still sell kits in aus.
    the site is full of info and circuits.

  3. @mike please correct me if i’m worng anybody, but i think this would actually lower the efficency of an LED throwie, as i think they typically run off 3v coin cells
    so are quite capable of driving the LED directly, and running them through this circuit would reduce the efficency because the power has to be transfered to the LED through a transformer, which creates losses.
    But saying that i guess it could be used for making a white LED throwie because i dont think you can drive white ones straight of 3v.
    Neways pretty sweet!

  4. Another use for a little voltage pump like this would be for those cheap solar garden lights. The “charging circuit” is usually a simple blocking diode to the AA-sized NiCad battery, and that wastes output from the solar cell on days with less than peak direct sunlight. (The battery is only going to charge until it reaches the same voltage as the cell minus the drop through the diode.)

    My garden lights have space for a second battery (or replace both with a cordless phone 3.6v pack). Adjust the output voltage to match the batteries.

    Efficient solar-charged weatherproof enclosures ought to be useful for loads of flea-power projects. Add a jellybean CPU, sensors and low-power RF data transmission, and it’s a back-yard localizer. Or just replace the LED with a laser diode and smiley screen from a cheap pointer, and do some guerilla advertising (banned in Boston).

  5. You could adapt it to work with the 3v from a lithium battery, but it wouldn’t be worth the trouble. the trick with throwies are they are supposed to be cheap and easy. Also, pretty kid-safe. Your talking about soldering, which is most certainly not kid safe.

  6. would it be possible to use a solar cell like androidcat mentioned, and use it to power a small low power microcontroller with an ADC onboard to measure the ouput of the charge pump an adjust it with the microcontroler to produce a constant voltage source for battery charging? (i think i remember something similar featured on HAD for driving blue leds but cant find it.) that could be great for making a more perminant throwie, maybe blinking out a stream of morse code? (been playing with the idea for a while)

  7. Does anyone have the specs for the transformer for simulation?

    I’m trying to understand this better by simulating it, but can’t get it resonating. I’ve tried some reasonable values for all the components but no luck.

  8. wolf: this would be a bad way to power a microcontroller. There would be high frequency glitches on its Vcc line due to the constant switching of the transistor. It only works for a light because your eye can’t see the high speed pulsing.

  9. I wonder if it is possible to use an electret or an RF antenna that collects enough energy to power an LED using a circuit like this. I’m thinking about a light that requires no battery and runs 24/7 off of energy from the environment outside of the spectrum that’s used in solar cell collecters.

  10. you could make a cheap led throwie using a zinc battery instead of a lithium batt
    I wonder if it could be made completely SMD small enough to fit in a mini maglight. the ones which run off AAA’s

  11. The circuit pictured above is actually a simplified form of a boost converter. The LED should be replaced with a diode and capacitor to ground (10uF tantalum worked well for me – both components are in series. The output is taken between the diode and the cap. With 1.5V input and using the specs provided on the site, I was able to get about 26.8 volts out (this is open circuit voltage) and using a 2n3904 instead of the BC series transistor listed. With an LED on the output, the voltage drops to about 2V (the narrow gague wire limits the current the circuit can drive). Without the modifications that I’ve mentioned, I was getting about 2.67 V open circuit with 1.5V input. The scoped output showed pretty high voltage spikes (higher than I could measure on the scope) so I don’t know what the instantaneous peak is until I can find a 10x probe. I’m building another one tomorrow out of heavier gauge wire and an IRFZ40 to see if I can drive a heavier load (like a DC motor)..

  12. I’m using LTspice, it’s designed to do switching power supplies so I’m not expecting a problem there. For the excitation, I’m driving it with a pulse with a fast rise time (1ns to 40ns).

    I’m at the point now where I can get it ringing a bit, the capacitor below the diode helped. But still a significant current through the diode. Any thoughts? Has anyone simulated this? If so, how’d you set it up?

  13. i’ve done some experimenting with this circuit, trying a couple of dozen coils and some oddball transistors and such. what i have posted isn’t ready for prime time, but if you’re interested, add my nick to your search to find it.

    bottom line: add the cap and diode medix suggested. Just solder a 2-10 uF tantalum SMD across the led (the right way, please) and use a Schottky; it’s worth it. Oh, and with a germanium transistor you can boost a voltage as low as 0.150V to a more useful level.

    @mike You won’t increase efficiency but have an outside chance of increasing the runtime of a lithium battery if you use enough LEDs in series so their added voltage drops are greater than the 3v lithium. But, you can use this circuit if your throwies use lower-voltage batts.

    @hitch You got it. Many white LEDs will work fine off of 3v lithiums, so don’t use this circuit.

    @ben jackson the core must be ferrite (many of the colored toroids are iron powder), unless your transistor will work at fairly high frequency. Also, you need to find the right resistor value for the inductor and transistor used.

    @androidcat @hitch please submit those hacks for posting here. try zener (or better) regulation. this thing seems to act like a ‘constant power’ source. whatever power your component selection draws from a given voltage (less its inefficiency) it will try to deliver it all to the load. it can and will fry things, believe it or not.

    @nate a zener and a nice fat cap should smooth things nicely.

    @MindTrap this circuit prefers a very low-impedance (low voltage, relatively high current) source. most rf will be fairly high impedance; you may be better off with a rectifier and a supercap. i’ve read reports of supercaps getting charged this way to well above the useful input voltage of this circuit.

    @medix i’ve found transistors rated to handle at least 5x the desired current are necessary to minimize the circuit’s input impedance. a low-threshold mosfet is very efficient, but its 4-ohm on-resistance results in an average 30-ohm impedance and seriously limits the current (at 1.2v!) the circuit can draw.

    @Snoop i used a simple online java applet, but had to increase the inductor size to around 5 mH (0.005 H) to get oscillation rate slow enough for the software to handle. actual inductors i’ve used successfully (with the right transistor/resistor combo) range from

  14. [Actual inductors i’ve used] range from less than 5 uH to 3000 uH, but 300-500 uH works best. (these measurements are of one coil, half of the total windings.)

    [Again my apologies. it was barfing on the less than symbol. doh! my bad.]

  15. Waggy

    I love the work you have done with these circuits. I am haivng difficulty reading some of your wiring diagrams on your website. Anyway I could ge you to re-draw one of them on a napkin and snap a photo of it?

    Otherwise excellent work!


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