I need to monitor 3 DC voltages that switch from + 12 volts to - 12 volts, DC. What I'm doing is monitoring 3 fuses to shut things down if one blows (ie. across the fuse suddenly measures 12 Volts ). I know a divider works to reduce to 12v to 5v, but with an Arduino I need to have always have a positive voltage for the Arduino to measure. What I really need is to just verify the presence of the voltages. Best would be if it could be isolated somehow before the Arduino sees it (optical ?). I would really like this to be in the form of a shield or brick as I'm handicapped and only have use of one hand. Does anything meet these requirements?
I think a simple diode bridge rectifer feeding an opto-isolator via a series resistor might work for you.
It would give you a digital indication if the input to the bridge rectifier was above some voltage value regardless of it's polarity. However it would not be able to give you a measurement of voltage, just its presence or not. Would that work for you needs?
To make it as simple as possible, you could risk missing out the opto-isolator and use a fairly high ratio on the potential divide, so a factor of 10 should be fairly same - although if its an inductive load, then this isn't a good idea.
You can get bridge rectifiers in a single package rather than have to make one from 4 diodes. You can use as low current bridge rectifier as you can find. So not too much soldering.
