Design a printed circuit board that illuminates an LED when the voltages from four sources are all good.
There are three power supplies. The first and second voltage sources come from one power supply, +15 and -15 VDC with a shared common. The third source is 24VDC from one power supply. The fourth is 28VDC from one power supply. The commons returns from these three power supplies shall not be joined together. The tolerance of "good" is +/- 5%. The PCB shall be approximately 2"x3".
How much are you willing to pay for this fairly complex design.
Your specs are unclear. The two voltages that have independent grounds, measure relative or absolute, and the tolerance need to be clarified, or your design will be too complicated, or not meet customer needs.
Not so. Use properly biased and adjustable Opto Isolators on the independent supplies and use a simple summer into a single isolator for the +-15 with common return.
Unfortunately, he did fail to indicate what source would be used to power the circuit. Under the conditions, only battery operation would work or diode-blocked shared source from any common supply, but not tying grounds together complicates things.
Easier than that...use two PV422 mosfet ssr's (8-dip), and the proper limiting resistors for the opto led sides. The on-resistance of this part is so low you could simply tie the outputs in series to the indicator led. It might take 10 minutes to lay out the board.
Digi-key has them.
Disclaimer: I am not affiliated with International Rectifier but am one of their fans.
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