Prevent external input voltages "back-powering" circuit through Arduino when powered-down

[jmulder88]

Hi guys,

Just wondering what you think is the best way to prevent an external +5V signal (connected to an Arduino input pin) powering my circuit when my Arduino is powered down? I guess I could use an NPN transistor, with the Arduino pin connected to C (with internall pull-up active), Ground connected to E, and the +5V signal connected to B through a 100k resistor or something. Could I also just use a basic hex buffer (I have 3 of these +5V signals connected to separate inputs, so this would mean fewer components)? I'm not sure what happens to the output of a buffer when the IC's +5V line is "off" but the inputs are at +5V... Will the outputs stay in High-Z during power-down regardless of the states of the inputs?

Thanks for any help.

EDIT - just in case I wasn't clear, I mean I want to prevent an input powering the Arduino through the diodes present on the IO pins.

[flashko]

Try to connect in series between the pins a high resistance resistor, I guess 100k or bigger. However, it depends on what this connection is used for. It's generally not a good idea to have a piece of electronics that has power and is connected to other components that don't have power.

[gilshultz]

Your buffers will work but the results will be interesting. You will forward bias the input protection diodes, that will in turn power the outputs at a lower voltage, that in turn will forward the protection diodes in the Arduino. You could use a tristate buffer that will enable when the Arduino is powered.