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12 v power checker9/26/2023 You really need a Schottky diode for this, a common P-N diode has a 0.7 V drop instead of the Schottky's 0.2 V, and then the 5.7 V maximum input voltage may be too high. Then any input voltage higher than about 5.2 V will make the Schottky diode conduct, and the input voltage will be limited to the 5.2 V. A better solution would be to have a Schottky diode between the Arduino's input and the 5 V supply. But a zener voltage varies with the current, and at the low input current the resistors give you it will cut off at lower voltages. (Actually nominal is more like 12.9 V, at 2.15 V per cell.) You can place a 5 V zener diode in parallel with R2, and this should cut off any voltage higher than the zener's 5 V. ![]() The car's 12 V is never quite 12 V, but most of the time higher, with peaks several volts above the nominal 12 V. You mention an automotive environment, and then you do need some extra protection. Will the voltage still be seen as high? I would stick with the 15 kΩ. Even 18 kΩ will still give you a sufficiently high 4.3 V, but then you have to start thinking about the 12 V a bit too low. You also have a bit of headroom in case the 12 V should be a bit too high. ![]() Your input voltage will be 4.8 V instead of 5 V, but the Arduino will see that still as a high level. Now 14 kΩ is not a standard value, but 15 kΩ is. If your R2 is 10 kΩ then R1 should be 14 kΩ. ![]() Resistor values in the range of 10 kΩ are a good choice. A simple resistor divider will bring the 12 V down to the 5 V an Arduino can digest.
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