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Heya. So I found people building perpetual self-exciting pendulums (pendula? pendulae?) and showing them off on youtube. Nice 🙂 So far the most elegant that I know of seems to be THIS ONE by youtube channel [TinselKoala] … which is in turn a “cover version” of some original circuit by David Williams, published in the magazine(?) “Nuts & Volts” in August 2012. The circuit is shown at the very end of TK’s video.

The basic idea is: You have a magnet on a string – the pendulum – which swings by a coil that acts both as a sensor (detecting the magnet is above) and an electromagnetic motor (giving the magnet a repulsive kick). Obviously the kicking has to occur at exactly the right time, i.e. when the magnet just barely swooshed past the center of the coil.

Let’s be honest: The Williams/TinselKoala circuit is very close to perfection: Only two transistors and barely a handful of passive components. Hats off! There is just two problems (arising from my ignorance):

BC556B and BC337-25 … errm I have never heard of these transistor types, I can probably buy them easily, but the thing is: I don’t have them at home.
I did not understand the circuit after staring at it for three minutes.
But I wanted to build such a pendulum the same evening. And in the end I did 🙂 (as you see below). I decided to rely on an Arduino to control the behaviour. You might argue that an Arduino is overkill here, since we already know it can be done with two transistors only. But since a chinese Arduino leonardo clone (pro micro) costs me only 4€ and in return I get fine-tuning of the trigger threshold and FET switching timing and all that in software, I’d say that it’s a fair deal.”

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