Introduction: "Joule Thief" Circuit in a Flashlight Bulb? Yes!!!

Picture of "Joule Thief" Circuit in a Flashlight Bulb? Yes!!!

Investigating a Paradox:
Recently, I saw an "energy saving" LED flashlight for sale and it only used one 1.5 volt battery. I purchased this light and took it apart expecting to find a battery, bulb, switch and some kind of circuit on a board that would boost 1.5 volts to the higher voltage needed by any LED, especially a bright white-blue LED. I expected something similar to circuits used in my Instructable, "Joule Thief" circuits, crude to modern OR my Instructable, "Joule Thief" - no IC and no Transformer. I looked everywhere but only found a bulb, switch and a battery. This didn't seemed possible at first but then I wondered, could the "Joule Thief" electronics be entirely located in the flashlight bulb? Well, this possibility ruled out the use of any clunky, bulky toroid type of "transformer" used in the more popular "Joule Thief" circuits. It was time to try and cut open this bulb.

Step 1:

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Well, isn't this amazing! Here is some kind of "Joule Thief" circuit so small it fits in the shell of a standard flashlight bulb. Especially notice the tiny and simple inductor along with the lack of any "toroid." The bulb had 1.5 volts and 0.2 watts stamped on it's metal shell. The brightness rating is 9 Lumens while the number of expected hours of use is 52 hours (In the past, I've seen 8 or 9 hours of use listed for incandescent bulbs).

Step 2:

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And now the test. Did I wreck this LED circuit when I cut it out from it's casing? Here we have a direct hook up to a 1.5 volt battery. When the final connection was made, it lit up brightly. What a move forward this design is!!! The hardware isn't super friendly for Instructable People but it is not impossible either. I haven't "decoded" the circuit board yet as the layering looks tricky. And I just wasn't ready yet to risk destroying this cool light. Maybe I'll do this later or maybe someone else will do it...


Dave Kruschke made it! (author)2016-09-04

mguima has a point. One can use the LED leads themselves as sort of a platform for the ic. The regular sized ic is shown used here but a SOT SMT device has also been used by another person, using very careful soldering...

mguima (author)2016-08-31

How to build a simple JT in a flashlight lamp:

If you want the JT to be small, you should not use a PCB.

Dave Kruschke (author)2015-12-11

Two or more batteries wired in parallel (plus to plus, minus to minus) would make this last longer. I've never thought about making the circuit work more "efficiently" or using more than 1.5 volts to power this circuit. Let us know if you come up with something that works for you...

RonE14 (author)2015-12-11

Any ideas how to make this ZXSC380 circuit work efficiently with 2 batteries instead of 1? I would like to make it last even longer. And has anyone tested how long it lasts?

dragontirex (author)2015-09-10

This is exactly what I've found in a Varta one D cell flashlight, I think that it puts out less than 10 lumens but still good enough. Well, it did until I connected the supply voltage backwards and fried that little IC in there, good thing I had a 40mA one cell led driver (PR4402), just soldered it in and worked like a charm... except at lower brightens. I do like were they are going with this kind of bulbs though. Thanks for sharing. Cheers :)

SugarCluster (author)2015-01-31

The green resistor thing is called inductor, and it works just like toroid transformers (which are also called inductors).

So basically it's more compact, but it's still a joule thief.

(To drive EL panels in some watches, they use voltage booster which uses tiny inductor and high frequency circuit. Basically same, but the joule thief feeds signal back to secondary wiring)

pandyaketan (author)2013-11-07

Dave Kruschke (author)2013-06-10

Finally? I attached my frequency meter to this circuit and got a reading of 293 kHz...

Dave Kruschke (author)2013-06-09

Well, I've looked further into the circuit board and both sides of this board visually have connections that go nowhere. This means that the needed connections are being made somewhere between the outer surfaces.
But,... I think that I've verified that AgedP's June 8th outline is correct. My schematic of what I think AgedP said and what I've found by poking around with a meter immediately follows...

Dave Kruschke (author)2013-06-08

I have to get a better magnifying glass but I believe your description of the circuit is correct and Instructive. Thanks, AgedP...

Nevertheless, I'm still trying to get over the surprise of seeing a "Joule Thief" circuit inside the small package of a flashlight bulb!

AgedP (author)2013-06-08

Hi Dave are you sure "multilayering" is involved"? That SMT is found in Garden Lights to drive a single white LED. . If you put about a 5 micro farad capacitor across the LED, and a power diode in series with the LED you utilise the other half of the oscillator output. It will then drive a 3 color auto change LED easily. Those other components, in the bulb, look like a capacitor and a diode. So perhaps they've done that mod already?

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