This simple little guy takes two 5 to 24VDC inputs with input current up to 10mA and uses them to control opto-isolated solid state relay contacts. This is very handy when interfacing microcontrollers with industrial controls.
If IN1 is greater than a few volts then NO1 will be connected to COM1 and NC1 will be disconnected. If IN1 is ground then NO1 will be disconnected and NC1 will be connected to COM1. The same holds true for IN2, NO2, NC2, and COM2. Maximum contact hold current is 400mA, and maximum voltage across contacts is 50V.