Circuit for check PSU positive and negative rail

[firstcolle]

hi

I'm designing a PSU with 3 rails, +15 -15V for the analog section and +5v for the digital one. the AC comes from 2 secondary of the same transformer. the 2 GND are separated and will be connected only at one point near the A/D IC.

I want to implement a check on the analog rails.. something like 5v if the rails are upper certain voltage (+14 / -14 for example).
the signal could be going to the microcontroller or to a relay to enable or not a certain part of the circuit.

is there an easy way to do that??
I thought to use BJT's in on/off config but at this point analog GND and digital GND are connected together and I don't want that.
could 2 optocouplers help? obviously, the one on the negative rail must have the GND on the cathode and -15v on the anode of the driving led. could it work?

thanks

[David Hess]

The brute force way is to use window comparators.

The three supplies could be combined with 3 resistors to produce a zero volt "balance" output.  Then if any of the three are wrong there is an indication of a problem.