Connecting grounds/negatives of different supplies

Forum 2005-2010 (read only) Hardware Interfacing
1

Since I require an alternative 12V>5V power regulator to power a certain sensor requiring quite some current which the arduino power supply cannot cope with, I was wondering whether I could safely connect the ground of the Arduino to the negative pole of the other power regulator without blowing up stuff.

As you may have noticed my electriotechnical knowledge is zero to nothing. :-[ I need to do this since I need to measure the 0V-5V signal comeing from the sensor while the sensor will be supplied with current from the other power regulator.

I figured, since it is a car system, in the end all negative poles are connected to the batttery's negative pole. Am I correct? :-?

Is there some kind of basic rule like always connect all grounds/negative poles with eachother?

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In essence yes that's what you do. The rule is don't connect two rails together. For example you can connect the +ve of your 12V to the earth or -ve of your Arduino and then the -ve of the 12v will be a -12V supply. You can mix and match as you like with more than on supply as long as they are isolated supplies, that is there is no hidden connection through say the mains earth.

On what you want to do it is always best to try an isolate or at least buffer a signal if it is coming from a higher voltage supply to make sure that the higher voltage does not get onto the input of the Arduino otherwise you will kill it. A series resistor and a catcher diode to +5 are usually sensible precautions in these cases.