AVRs are still pretty popular, and I think that predates the Arduino frenzy (starting about 2005). If you disassemble a bunch of devices in your house, you might find a few of these.<p>E.g. when I went to replace a faulty switch on my Logitech Perfomance MX mouse, I found an ATMEGA324PA. Debug pins seem to be exposed as well: <a href="https://steelcityelectronics.files.wordpress.com/2015/05/img_3821-e1431606516130.jpg" rel="nofollow">https://steelcityelectronics.files.wordpress.com/2015/05/img...</a> (not my site).
But since I still use that mouse, I'd hesitate to flash the Arduino firmware on it ;-)<p>Another ICs for shenanigans of this kind: In smaller, bluetooth-enabled devices I usually expect to find a nrf5x these days (and FCC docs often allows pre-purchase clarification). I think both nrf51 and nrf52 are supported by Arduino and ZephyrOS. Bonus points, though I not yet tried that: Their firmware dumping protection seems to be faulty (though the nrf52 requires power fault injection), so reverting/modding could also be an option.
Here's a page with a picture of the soap dispenser this project works with: <a href="https://www.gojo.com/en-INT/Product/1388-04" rel="nofollow">https://www.gojo.com/en-INT/Product/1388-04</a>