I cant find any data on the Pro Mini but I did find the Arduino Nano 33 BLE. Looks like it can handle 21V and will do the job. Not sure what the ST7805 regulator will do but the next pat of this project will be how to integrate it into a real automotive circuit. I have been reading papers on filtering the automotive circuits and I came up a paper that describes what needs to be done and different ways to do it. See attached. While the 0.1uf cap got it to work, it may really be a problem of voltage spikes that really need to be properly filtered out. I'm really suspecting some voltage spike, probably when I turn the ignition switch off is causing it to not reset, and the 0.1uf cap is just a band aid.
When I get to that phase I will probably start another thread on just that topic, Or I'm guessing there are probably threads already discussing the situation. It would be really nice if I an off the shelf voltage regulator with all that stuff built in already exists in a small package. Might that be in the ST7805 regulator?
Transient Circuit Protection.pdf (353.2 KB)