ADC input from +12 to -12V - internal diodes enough protection?

I need to measure a voltage that may range from -12 to +12V, relative to ground.

I'm only interested in readings that are between -12V and 0V, though; if the voltage goes above 0 I don't care what the reading is.

From another thread I learnt you can measure negative voltages with a resistor divider, so this circuit:

... should let me read anything from -12V to +5V. That's all I need.

However, if the input voltage is higher than +5V, the voltage at the Arduino's input pin could go up to 7V. Do I need to protect the Arduino from this, or will its internal protection diodes be enough? We're talking about a maximum current of 0.1mA. Should I do this:

I'm guessing that if the verdict is yes to external diodes, I'll need to use Schottkys for their low voltage drop...

Thanks! h

With a 120k resistor you should be fine with the internal diodes.

Atmel use a 1M resistor to detect mains AC: http://www.atmel.com/images/doc2508.pdf

Up to 1mA is safe.

Spectacular. Thanks folks :slight_smile: