Electromechanical Oscillator


One of the problems with basic electronics courses is that before you start learning about active components (transistors, diodes etc) there isn’t much fun you can have. Passive components are… well passive and we’ve grown accustomed to using active components whenever we need to generate non-DC signals.

So I was sitting in the bath, wondering if it was possible to make a passive component oscillator. A little googling, found some early oscillator designs that used delay lines. But I didn’t have the required components. But I did have some relays…

Now, I’m not sure if I’d classify a relay as a passive component but they are at least simple electromechanical devices, which can be easily understood. But they are suprisingly powerful, in fact some of the earliest computer designs were based solely on relays.

I thought that using a DC supply, I could charge a capacitor until it builds up enough voltage to switch a relay, which cuts the supply, allowing the capacitor to discharge, flipping the relay back to the on position, which would start the process all over again. Hopefully the following schematic makes things clear:


I used a 9V supply, a 5V relay, 1000uF capacitor and a 200Ohm resistor. I also put a 1K resistor in parallel with the 200Ohm so I could play with the frequency. It circuit works reasonably well, though only up to around 15Hz! The output produces reasonable looking sawtooth patterns:


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