I can easily hook the voltage sensor to a 9V or 1.5 V battery and it gives me a perfect reading on the Arduino. However, I tried wiring to the 12V source on the vehicle (which is also the Arduino's power source!!) and my readings are very off. It gives me about 2.2 V.
So after searching these forums I figured it would be easier to read from the Arduino's VIN pin. It is also strange, giving me about 7.2 V reading. My electronics skills are limited, and I'm not sure what's going on here in either case. In the latter, I'm thinking its measuring the voltage left AFTER the Arduino takes its own portion?
So after searching these forums I figured it would be easier to read from the Arduino's VIN pin
VIN isn't an input. VIN can be used to supply >7V to power the arduino through the on-board regulator or to power external hardware from the supply provided at the barrel jack. I'm not entirely sure what you were doing, but it is clear you were not using it correctly.
I'm thinking its measuring the voltage left AFTER the Arduino takes its own portion?
Sorry, but that doesn't make any sense.
Going back to the differential sensor you have, how have you connected it to the vehicle's battery? Do you have a common GROUND connection for all your hardware?
Most likely, your vehicle has positive ground. Direct voltage measurements in this case become complicated, you will need special sensor with galvanic insulation.
James, for the VIN pin I was going by what was said in this thread:
"1. Vin pin is the power supplied from the Arduino external power connector and is in the range of 7.5 to 12vdc depending on what you are using. This Vin voltage has no effect on the analog reference accuracy of the analog input pins, however you may or may not have a desire to measure the Vin voltage, that is not clear to me." http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1263440087