This circuit was designed to capture the state of various 24VAC signals in an HVAC control system, primaril zone status which can then be monitored by a home automation system. The circuit can monitor any AC signal as long as you adjust the size of the resistor on the MID400 chip to achieve the correct input current (see MID400 datasheet for details). Be careful with higher voltage monitoring applications because the PCB track sizes are small and there were no High Voltage layout considerations. Circuit has provisions for using a Sparkfun type 4-ch logic level converter to output 3.3VDC signals for zone status. You can eliminate the logic conversion if 5V inputs are OK for your use, just jump the input to output of the level converter board and do not apply a 3.3V supply. Parts list is not very good because I am an amature PCB designer and new to EasyEDA. I also did not use large enough holes on the input connector so the screw terminals I have do not fit. You can simply solder wires for input in lieu of the screw terminal inputs. I hope someone can use this and perhaps think of new uses for the board. Be my guest...