oops

Hey mike, it's for an ecu, so there's a break out board which allows us to bridge the pins on the ECU and apply open circuit, short etc. 

We're doing OC , short to ground, short to ignition and short to perm live. 

The issue is we're using a 20cm or so loom which is literally an inline 2A fuse, so one end is connected to the pin being tested and the other is connected to ground or the ign or perm live. 

Fuse blows instantly which is expected but it's another one to throw in the bin. 

What I've done with the arduino is put the fuse in line with a potential divider which i can read off an analog input pin, this way I can trigger status LEDs which are ran from a 5V supply (from a laptop USB for example), meaning I can have one lit to say the 5V is OK, and the other to come on when the ADC readout on the analog input moves from one state to another. 

This seems a workable solution to me, but I may have either missed something or made it overcomplicated :)! 

ADC wise it's just set to pick up either a 3V signal (the potential divider gives that out), or < 3V. 

I'm still tweaking code but essentially it's gonna either flash when the 12V is applied or when it's lost. 

Lost seems more useful. 

did you actually want to emulate the fuse action with something else, and save binning a stack of fuses ? It is also possible to replace the fuse with a low value resistor and use it to fire a circuit that opens the supply more or less the manner of a fuse/ breaker. It is more involved, but I am happy to advise how in slow time.

Or you could use a poly-fuse, but they do not go completely 'off' and trickle some mA  in the hot state.

Mike