Let current flow when signal is low?

Other Hardware General Electronics
6

So just to clarify:

If the Arduino is active, it will read an existing PWM signal and some other signals, and construct a new PWM which goes to the fan
If the Arduino is inactive, the existing PWM will go direct to the fan, and the other signals are ignored

Is that correct?

SnipeYa:
Correct!

Then it's not just a simple matter of a relay is it?- needs some kind of logic to switch the existing PWM to the fan if and only if the Arduino's out of play. (You need another Arduino to see if the first Arduino's there or not :wink: )

So I'm not really sure what the relay of which you speak is going to do: I need a diagram.