The PiWatcher is a very small board that helps you fully shut down or reboot a Raspberry Pi, in case of an incident or simply for power saving purposes. You can even program your PiWatcher to shut down and automatically reboot your Pi a few minutes or hours later. In essence, the PiWatcher is a programmable watchdog circuit for the Raspberry Pi, usable with the humble Pi Zero up to the power-hungry Pi 3. It not recommended for the Raspberry Pi 4.

https://www.omzlo.com/articles/the-piwatcher