Grumpy_Mike:
Good advice but this bit is wrong.but you do want to make sure will always be below 40 mA before feeding it into an Arduino
That current is the current limits when the arduino pins are outputs.
That's not true according to the ATmega datasheet, in section 28 under Electrical Characteristics the absolute maximum rating for "DC Current per I/O Pin" is 40 mA. There is no distinction made for whether the I/O pin is being used for input or output. If the I/O pins were capabable of withstand appreciably higher input current than output current, that would be listed because this is a potentially useful difference. Also if the limit was only for output current, than the situation illustrated here would most likely ruin only one pin, and not both when shorted together. This is because the output pin would fail before the input pin.
So you should always limit the current to 40 mA to the I/O pins regardless of how they are being used.