A switch circuit configured to receive power from either a single power source or dual power source. The circuit includes two power input terminals and two power output terminals. For a single power source, the switch circuit may receive the single power source at either of the two power input terminals. The switch circuit provides power to a load without regard to which of the power input terminals the single power source is connected. The switch circuit shorts the power output terminals for a single power input, which provides power at both power output terminals. For a dual power source system, the switch circuit may isolate the two power output terminals so each power output terminal may operate independently without shorting. In some examples, the switch circuit may be part of a thermostat or similar HVAC system controller.