An input circuit includes an input line for providing input regarding state of a load. A first impedance is connected to the input line at a first node for connecting a voltage source to the input line. A second impedance is connected to the input line at a second node for connecting the input line to ground. A Zener diode is connected in the input line in series between the first and second nodes to impede current flowing through the Zener diode below the Zener voltage thereof in a direction from the first node to the second node, and to allow current above the Zener voltage thereof in the direction from the first node to the second node.