If you're having problems with mice at home, then you can attempt to catch them with (animal friendly) mouse traps. But you can also repel them with the aid of an ultrasonic mouse repeller, devices that are sold these days by garden centres and hardware stores. How do these repellers work, and can you build something like that yourself? Of course you can, and it is even quite simple, as we will demonstrate here.
Last autumn there suddenly appeared to be mice in our attic. The most annoying thing was that they ran around at night exactly above our bedroom. The first measure I took was placing a few mouse traps. But this was not successful, after a week I still hadn't caught anything. Cheese, bacon, or peanut butter, it all was apparently not tasty enough!
As an alternative, I bought a so-called ultrasonic mouse repeller, a device that produces a high-frequency sound that mice are supposed to be not that fond of. This appeared to be effective after a few days, and just to be sure, I bought another one from a different manufacturer that I put at the other end of the attic. Once the mice were finally gone, and as an electronics engineer, I naturally wanted to know what these devices did exactly. Time for a screwdriver and an oscilloscope!
The cheapest version came from China and I suspect that this one is available in various types of enclosures with practically the same contents. The internals consist of a circuit board with an electronic circuit based around a 555 timer IC and a little piezo speaker. The power supply is obtained straight from the mains via a capacitor and resistor. The thing also has a separately switched light using blue LEDs, a nice gimmick.
For measuring the output of the piezo element I built a circuit on a breadboard using an ADMP504 MEMS microphone that I had around, followed by an amplifier stage using an OP27. The microphone is specified to 20 kHz in the datasheet, but because of its diminutive dimensions I hoped that it would continue to work well past that frequency.
And indeed, even around 40 kHz there was plenty of signal so it wasn't even necessary to set the oscilloscope to its most sensitive range. As the oscilloscope image shows, the mouse repeller generates a sinusoidal signal of about 38 kHz. The frequency remains nearly constant, so only a single oscillator is built around the 555.