Hi all. I have an Arduino Uno in a car, running off a USB lighter charger. All good there. However, I want it to be able to sense 12v on a pin. This will be tied into the headlight dimming wire, which goes hot when the headlights are on. How do I safely get that input into the Arduino?
GOOGLE : Read 12V with Arduino. 
Thanks I was trying a lot of "common ground" searches
Or, Google 'voltage divider'
for signal only, not power.
Use an opto-isolator.
I think I’d go for the opto isolator too.
As an alternative I have seen people use 5v voltage regulators - car supply in; 5v out .
Single transistor is the output of an optocoupler.
If one has a lot of signals then cd74HC4049 (inverting) or cd74HC4050(non-inverting) can convert 12+V down to 5V.
Optos are great, but you'll loose isolation if Arduino GND is connected to Auto GND (unless the Arduino is powered with a separate battery).
dlloyd:
Headlights voltage goes to 13.8V sometimes 14.2V on very cold days (battery temperature compensation).
14.2 / 2 = 7.1V... not good.
@Paul__B circuit would be the best for potential divider method.
Opto Coupler method isolates any voltage spikes that may appear on the vehicle positive circuits.
Tom....
Meh, even without the pulldown, the voltage could reach 52V before it would inject (52-5)/47K = 1mA into the MCU's input protection diode. Without the pulldown, (14.2-5) /47K = 195µA injected through the pin.
Note that here, Atmel has up to 240VAC mains (rated for 1000V pk) connected through a 1M series resistor to an input. In this case, with 240VAC there would be ±310µA injected through the pin.
The 47K/47K isn't intended as a divider ... when the headlight switch is off (open-circuit), the 47K pulldown resistor ensures the input goes to 0V.
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