Syonyk’s Project Blog writes about tearing down one of those rodent repellers – what do they do?

One of the problems I have on my property out here is burrowing critters.  We have voles, rock chucks, and gophers – at a minimum.  I’ve attempted to deter them from areas we care about (like the garden) with some little solar “burrowing critter” emitters.  In theory, they make enough annoying noises under the ground that the critters just go somewhere else.

Maybe you’ve seen them – a solar panel on a stick that you wedge in the ground.  A 4-pack on eBay runs about $25, maybe a bit less.

But what’s in it?  What makes it tick?  What sort of interesting Chinese-designed electronics are in this gizmo?

Inside the point, there’s a little black plastic box.  This is the buzzer, which creates both noise and vibration to annoy burrowing critters nearby – in theory.

The removed guts are a buzzer of some sort.  It faintly reminds me of the old style flexible metal strip buzzers from the 50s and 60s, but it’s not – there’s no actual contact involved in the pulsing of the electromagnet.  It detects the magnet position via feedback, through some fairly simple circuit (I haven’t worked out details), but it happily buzzes the strip back and forth when voltage is applied.  The electronics consist of a small transistor and diode (at least, I assume the L5 gizmo is a transistor – it’s got three legs).  It starts buzzing around 1.1V and 20mA, and pulls about 40mA by 3V – it’s not using much power at all.  I didn’t push the voltage past 3V, as it normally operates around 2.7V and I didn’t want to burn it out.

What does this racket look like if you stare into the frequency chart?  A useful little spectrum analyzer on my phone answers that question: It looks like the FFT of a square wave, give or take.  There’s energy throughout the spectrum, heading up into the ultrasonic (I’d wager the drop after 20kHz is more my phone’s mic and sampling system quitting), and a surprising amount of it.  As a human, this is a genuinely irritating noise.  Were I a gopher, I’d probably also find this very annoying!

This is the sort of engineering I enjoy finding.  It’s radically different from what I’d typically do (toss a small microcontroller in to run everything), but it works.  Chinese engineering for cheap consumer products tends to look really hard at the datasheets and say, “What’s the simplest thing that will mostly work for the common use cases?”  Charging a NiMH or NiCd battery with a diode and a small solar panel isn’t the most amazing option out there, but it works.  Using analog timing circuits is dead simple in terms of component cost, and it works.  Is it consistent over time as the components age?  Nope.  Does it matter?  Also, nope.