Mice - at least European mice - can go through holes where a pencil can go through.
The fur makes them appear much bigger.
As already stated in another answer, ultrasonic sound could be a solution.
An approved recycling- solution needs this equipment:
- An old smart phone with charger
- An old stereo set with boxes where at least the high frequency chassis of one channel/box must be working
- A cable to connect the phone plug with the amplifier
- A free signal generator app, f.e. Keuwlsoft Dual Channel Function Generator
The hardware can be found in the "obsolete electronic box", in the basement, attic (of friends/relatives) or in the next recycling yard.
After installing the SW and connecting everything the sweep function can be set to an interval between 15kHz and 40kHz. If any tenant is disturbed, enlarge the lower interval setting, f.e. to 17kHz.
The waveform should be triangle in order to produce many high frequencies, sinus waveforms are less suited.
The volume should be set to a level where the amplifier is not heating up too much - and where everything can work for a long time without starting fire hazards.
The level depends also on the hearing capability of all tenants and neighbours - cats and dogs could be annoyed by that signal, and also bats.
The box(es) can be directed to the wall with the mice population.
This set up is much more powerful then plug- and- play solutions. The sound is much more annoying - comparable to a neighbour, who is a professional musician in an orchester who plays the diatonic scale or the same short piece of music 24/6.
And it is more flexible, and if it can be all made out of recycled devices, it is free.
Edit: Taking a screenshot disclosed the fact, that this App can only sweep up to 22kHz, which should work fine, since the triangle produces higher frequencies. Both channels have the same parameters except for the sweeptime in order to get more annoyance = 2 musicians playing the scale with variable offset at the same time. Only deaf mice or mice with Cr-Vd- hardened steel nerves will stay.
