WaterWaiver, 2 years ago

You could probably increase the 82K and 10K resistors to be much bigger (by a factor of 10x or maybe even 100x). Lookup the input impedance for the ADC of your model of ATmega, as long as it’s >10x the size of your resistors then your circuit will probably be accurate enough.

A couple more things to keep in mind:

• a fresh alkaline 9V battery is actually 9.6V or more, not 9V.
• 9V battery voltages droop noticeably when under load because of their high internal resistance. Make sure to measure under the same conditions.