Demonic Electronic Firefly (3v Flashing Joule Thief)

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A few weeks ago I gave my girlfriend a Kinder Egg. She gave me back the (turtle!) toy container, suggesting I might have a might have a use for it. Of course I had: lucciola elettronica!

Aside for having the Best Name Ever, the Joule Thief is a remarkable little circuit:

[Pic from Wikipedia by Rowland, CC BY-SA 3.0]

It’s a switching boost converter that can run from a very flat 1.5v battery and ‘constantly’ light an LED at a very low average current by delivering a chain of very brief pulses. Demonstrates how close to magic even the apparently simplest electronics can be.

A video by Eman2000 had been recommended to me for a Blinking Joule Thief. Essentially the same circuit as above, but with a biggish capacitor added to stretch the timing. Yeah, that’s what should go in the pod.

But, part of the alchemy depends on the relative voltages around the transistor & LED. The original circuits work for a <=1.5v supply. I only had a 3v (2106) battery that would fit in the Kinder pod.

Because it’s Dark Art material, I didn’t bother analysis or trying a sim in Spice (which almost certainly wouldn’t be remotely accurate), went straight to breadboard.

Snipped a little toroid off an old burnt-out PSU, wound on 2×25 turns of thin wire (Wikipedia suggests 20t, but because I was a bit sloppy threw in an extra 5 for good measure). Went for the trial & error school of design. Starting from Eman2000‘s 1.5v blinker I had a good play.

So…first observations : the capacitor/resistor sizes bear little relation to the timing compared you might expect, seriously non-linear. At 3v the LED appears permanently lit (I think most likely blinking at a high frequency, but I didn’t bother to check).

Semi-logically, I started by tweaking the LED (the loading is significant) – variations on colour, series resistance, series diodes for voltage drop etc. Two series red LEDs seemed most promising. Scientifically changing several variables at once, I thought maybe reducing the current from the battery might help. Yes & no. It made quite a difference but only approached what I was after when backed up by a capacitor. I tried a couple of other transistors, it didn’t seem at all sensitive, probably any small-signal bipolar would work. I didn’t bother playing with the transformer, too fiddly.

This is the circuit I settled on:

It flashes a couple of times a second with either 3v or 1.5v. I don’t believe the measured current because it was a very intermittent draw (I know, should’ve got it on the scope, but I just wanted to get it done). Probably more like 3 x 25uA, which would suggest 1000 hours before the battery reaches 2v. That’s a guess. But given that it can work below 1.5v, it should last a month or two, if not a year or two.

Soldered up on a tiny scrap of stripoard:

Bit of hot glue & tape:

Then it crossed my mind, where’s she going to put this? Fridge magnet! So I hot-glued 3 tiny magnets into the Kinder pod.

Turns out there are much more hi tech approaches to similar problems, ATtiny45 EverLED and TritiLED both use microcontrollers for very long life little lights.

It is a little creepy in the dark. Leave the radio on.



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