Does anybody know how to make one? I was thinking about adapting the baybus i made to have one built in, i just cant workout how its supposed detect when there is no current flow through the fan.

you would probably use the rpm monitor lead from the fan, when the current stops, it triggers the alarm.

in that case i could just use the power right? cos one of my fans doesnt have a rpm lead. but that brings me back to my problem, i dont know how to tell if current stops in a wire.

I would use the RPM lead because there can be other problems with fans other than the power. You could make it so it triggers an alarm when it gets to a slow speed.

There is this prog that came with my MSI KT3Ultra that does that its called PC doctor 3.0 or something I don't know if it can be used on all boards but just search on google.

if all you're looking for is is a software notification of fan failure you can use mother board monitor. you can even use it to shut the computer down if the fan fail. once again you need fan with rpm monitoring. also like general said, fan may stop even with power flowing to it.

if you want fan failure to trigger an alarm, you would probably need some sort of transister, or IC. i really wouldn't know how, specfically.

if you want to buy something. the digidoc 5 has rpm and thermal monitors that can be set to trigger alarms.