adrian_h:
Chagrin:
The AVR chip itself can monitor its own battery voltage. You have to set the analogReference to INTERNAL (which is 1.1V) or use an external voltage reference such as a TL341 and then measure the battery voltage against that. Once your chip recognizes that the battery is low you can set it into a sleep mode, waking it periodically to blink your LED, etc. and that will be much simpler and more efficient than any external monitoring circuit.Isn't 1.1 V too low a voltage? Doesn't that mean that if the voltage goes above 1.1V that the value returned will just be maxed out at 1023?
Yes, that's correct; you would need to use a voltage divider to keep it under whatever your reference voltage is. Well, point being that you have to compare the battery voltage against some known reference. Leaving it as analogReference(DEFAULT) would be comparing it to itself.