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