Stealing a Joule Thief From a Cheap Solar Garden Light




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Introduction: Stealing a Joule Thief From a Cheap Solar Garden Light

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To run a LED from single 1.5 V, 1.2V or in particular largely depleted batteries, the famous Joule Thief circuit using a coiled toroid is great.

However, when salvaging the solar cell and battery from a cheap solar garden light to make a charging circuit, I realised the electronics are in fact a simple LED driver. The only part involved in the charging is a diode. So I took a new diode for the charging project and used the complete little circuit board as a Joule Thief for a cyborg zombie.

I used solar garden light bought in a package of 4 for 10 euros at [ Conrad] under nr. 572759. If you happen to have another use for the solar cell, on-off switch and battery, as I do, this comes very cheap for LED + driver circuit. You can simply cut the wires from the solar cell and just leave them like that. Replace the battery with the one of your choice (or keep the one from the solar garden light) and you're ready.

The circuit from the solar garden light is bound to be at least somewhat more efficient than the famous three-parts-joule-thief. The latter is great with almost dead batteries, but so is the solar garden light circuitry. Tests show it works till the battery voltage drops to about 0.6V. While toroid three-parts-joule-thieves are reported to go to 0.3 V, the ones I managed to build mine also need about 0.6 V.

For the cyborg zombie I cut the wires to the battery and fitted them to a connecting block. As the wire connections on this kind of circuit boards is quite fragile, I fixed the connecting block on the back with hot melt glue. You can leave on the switch and its wiring and glue it to the assembly in the on-position. This way no soldering is required. Of course you soldering a little bridge on the printed circuit board, replacing the switch.

To make a cyborg eye, I desoldered the led, ran the legs through a button and resoldered it on the printed circuit. Because this way the cyborg zombie misses the cool toroid-thingy, I first left the complete circuit on the outside of the zombies face, which is pretty cool to. However as the sharp edges are not very child-friendly, I implanted the board, leaving just the LED + button outside.

The cyborg zombie is an entry in Halloween Contest, so please check it out, and if you like it, do not forget to vote Nomember 9th and 15th  2009. Thanks!



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The 5251F is in fact 2 devices in 1. As you look at the device with the writing and the slopes facing towards you, the left hand leg is the solar cell cell input which is connected to pin 2, which is the battery positive, pin 3 is the battery connection negative and pin 4 is the output. The little device that looks like a resistor, is in fact an inductor and the key to the whole system.

The whole set up is known as a buck charge inverter, The device is in fact a tiny inverter or oscillator feeding the small inductor, its the back emf, or the collapse in the magnetic field within the inductor that lights the LED, and NOT the direct DC from the battery.

The circuit will still light an LED as long as it can oscillate, which would be down as low as 0.6 volts, and still feed the same amount of power at 6 volts, the amount of power available is determined by the size of the inductor. I have 5 high power LED's running quite happily at full brightness on 0.8 volts

Sourcing the 5251F is difficult unless you want 10,000 of them for your next project. I have details of a similar set up that uses only 3 components, transistor, resistor and a simple hand wound coil, very easy, and another circuit that can drive 30 leds with about 10 components.

12 replies

I´ll be very pleased if you post the circuit for 30 leds

Can you please post the circuit used to drive 30 LEDs.

Also the IC adds an auto off in light and solar charging capabilities that the "torch" circuit lacks.

I've been trying to find an efficient auto off in light option out there with NO luck. They take too much voltage. I just wanted to upgrade this solar light to lithium, which doesn't need the boost, but I still needs the auto off. (Solar direct to protected cell.) Finding a circuit that doesn't eat the voltage has been impossible.

You were correct. It was difficult to find. At least at a good price. Finally found a supplier for small quantities. Ended up paying about $0.25.
The convenience of the small all in one package makes it super easy to add to several projects since all you need is two components plus the LED.

Thanks for the info.

I use the "joule thief" circuit similar to the one shown in my other "cyborg zombies":

I forgot to add, that these circuits are NOT mine, they come from an extremely good and informative site called run by Colin Mitchel

The site will not let me add a pic for some reason. It's already here.

Here is the English PDF.

BTW the company is called QX micro devices co. You can search google for that and qx5251f.

You are right. It's even economical now to build your own. I don't have the doc's handy. English ones where difficult to find. I can give you an email of a vendor if you are interested in that IC. Actually most IC's. msg me if you'd like that email addy.

I apologize for not updating this sooner. You are correct. It is custom, but not just for one company. There are actually several versions. I bought them for 0.25 plus shipping. With battery, the LED and the inductor, it's still less than a buck.

Of course on ebay you could by the solar lights for about $1.5 and it saves you the trouble of finding the components. They come with batteries that are half of what they are rated, ie 600mA is really 300mA if you are lucky.

1000 picofarads would not work with this circuit. 330 is the most. That is dim, but last days and days. 35pf is very bright and last about half the night. (dies around 1am)

i noticed that in the circuit the green thing resembling a resistor appears to be an inductor, am I wrong? If it is, then its the same as the three parts joule thief here is pic of qs's torch with a similar inductor

3 replies

It could very well be an inductor. Many LED driver circuits make use of an inductor. However the active component is not an ordinary transistor (it has 4 legs, in other examples there was another IC with 5 legs), but probably some kind of dedicated LED driver IC. So it is probably not exactly the same, but does make use of the same principle of storing energy in an inductor.

My solar lights are even simpler. It has this mystery IC, an inductor, 1 led. That's it. It drains the battery down to nothing. The component is labeled (line 1) 52521F (line 2) 0909A. It has 4 legs and looks like a transistor. The only reference I could find was to a Chinese website for an IC. If the IC was cheap, this would be the easiest/ most efficient circuit I've seen around, but I can't find that component anywhere. Can anyone id this component ???

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Hey, look, theres a picture of my nightmares! D: lol

Genius!!! Absolute Genius!!! It's like... hugging the battery!!! Now i have a use for all those used batteries left over from my railgun experiment!!!

can i use a speaker magnet 2 replace a toroid?