Monitor voltage supplied to the power jack?

Using Arduino Project Guidance
1

Hi,

I am trying to figure out how I can monitor the voltage of a battery supplying power to the Arduino. I've found a couple forum posts about connecting a battery to Vin and using a voltage divider to do so. I would rather have my battery hooked up to the power jack, though, because a) I already have a 2.1mm center-positive plug with a 9V connector on the other end, and b) I feel like the power jack is a more sturdy connection than Vin, and I am using the board in an application where it will be subject to bumbs and vibrations.

Is this possible to do? I supposed I could cut the wire and put a voltage divider in there anyway, but I'm not 100 percent sure how to do that, and I thought that the board may already be measuring it anyway and I just need to know how to get the value.

Basically I just want to be able to have an alert sound when I'm running out of battery. I don't want the board to die and I don't know about it.

Thanks

2

With the power going into the jack, take the voltage at VIN trough a voltage divider to an analog input.
Voltage on input is to be less than 5 volts.
The voltage difference from the jack to VIN is about .7 volts.