This is a hardware watchdog, built with an ATTiny85. The aim is to retrofit an existing controller board with a watchdog. Because sometimes the watchdog built into the chips is not enough or is not reliable enough. That's what happened to me with my barrel pump project. The watchdog built into the ATTiny84 could not reliably protect the controller from freezing. I had to reset the controller by hand again and again. Of course there are also ready-made watchdogs, mostly in transistor or DIP-8 form. But they were difficult to obtain and besides, it's fun to design something like this yourself. That's why I built this hardware watchdog based on the ATTiny85, which can be plugged onto a normal 6-pin ISP programming connector. But first I designed a variant for the Digispark to test the basic functionality. The advantage of the ISP plug-in variant is that you rarely use the ISP pins for your own circuit.