Hi All,
Yes, sorry Mike I realise that I didn't explain myself clearly in my original post.
I also need the microprocessor to know when either one of the supply voltages is missing. A truth table for the desired behaviour is:-
0=voltage missing, 1=voltage present
+5v +12v behaviour
0 0 unit dead (no power)
0 1 unit powers & reports "error +5v missing"
1 0 unit powers & reports "error +12v missing"
1 1 unit powers - normal operation (no error)
The device that you suggested, on1aag, is pretty much exactly what I need. However, it seems that this particular device is end of life (also non-RoHS which is a problem).
However, I'm fairly sure that it must be possible to do the same thing without having to resort to an off-the-shelf device. After all, I'm halfway there - I can easily tell if the +12v is missing (a voltage divider on the +12v input and transistor gives me a signal I can use) but my problem arises if only the +5v goes missing. Unless I put a diode in the incoming 5v rail, the +5v output of the regulator (that is being powered from the +12v) will drive the +5v rail (through the 100R resistor that Mike suggested using) and how can I then actually tell if the 5v input is missing or not?
Hope this makes sense.... thanks!
SnowyDog