- is it dangerous for arduino the below circuit because GND pin is common with -12V battery pole?
- is it dangerous for lots of 5v circuit to be grounded on -12v battery pole?
- in case i don't connect GND arduino pin with -12v , does arduino works?
- in case that 1,2 question is dangerous for arduino,does it safe if a connect a diode at P1 mark?
Voltages, being a potential difference are always measured relative to another voltage. Normally that reference voltage is referred to as ground, though may not always be.
The + and - on the battery refers to the polarity of the battery not the polarity of the voltage.
For instance, relative to the - terminal the positive terminal is 12 volts higher, or +12V. Relative to the + terminal the negative terminal is 12V lower, so -12V.
When one of your battery terminals is connected to a point in your circuit nominated as ground (the - terminal of your battery in this case) then your battery voltage at the + terminal is measured relative to ground, which we say is 0V. So the + terminal is 12V higher than the - terminal, and the - terminal is 0V, so therefore the + terminal must be 12V with respect to ground. The - terminal, since it is connected to ground is at the same voltage as ground, so it is 0V with respect to ground.
If you were to connect the + terminal of a battery to ground instead, then the - terminal would be 12V lower than ground, so would be -12V with respect to ground. The + terminal would be 0V with respect to ground.