A thermostat for controlling an HVAC system includes wiring terminals adapted and configured to make an electrical connection with an HVAC system wires such as common, heating and cooling control and return wires. The making of the connection with a common wire actuates switching open a loop of an electrical circuit used for power harvesting. The wiring terminal includes actuation of a moveable part of the terminal so as to accommodate the common wire that in turn actuates the switching open the power harvesting loop. More than one other loop can be switched. The wiring terminal can be used to automatically connect and/or disconnect Rc and Rh circuits when one or both Rc and Rh wires are present. The wiring terminal can be used for electronically sensing the presence of the HVAC system wire.