One project I've started recently is a simple device to monitor a voltage level and warn when it drops below a certain level. The specific application this is meant for is to monitor the 12V power in an airplane, and issue a warning if the alternator stops working. It should work equally well in an automobile.
I've been asked by someone to design this device, with the intention of manufacturing and selling a small quantity. These would obviously be for experimental aircraft only. Since my electronics experience is at best "amateur hobbyist" level, whatever design I come up with will be examined by an EE before selling anything.
My little bit of electronics experience is in working with microcontrollers, and mostly on the programming side. This project seems a bit too simple to justify using a µC, so I went in search of appropriate components to use.
A bit of googling revealed that I probably wanted to use a "voltage comparator." I also found a number of schematics demonstrating how to use them. These are the two pages that I found most useful for my purpose:
Based on those two schematics, I created my own circuit. I don't have a copy of the original circuit, but if you just ignore the PTC and diode in this one it's pretty much the same: