Abstract: A duty cycle controller for an air conditioning system. Solid state switches are connected in series with the heating and cooling switches of a thermostat in circuits for energizing the cooling and heating subsystems, respectively, of an air conditioner. A switch control circuit, when enabled, causes the solid state switches to be conductive when set and nonconductive when reset. Timing signals from a counter applied to the switch control circuit determine the state of the switch control circuit and thus the duty cycles for the heating and cooling subsystem. The controller is provided with a safety circuit which disables the switch control circuit to render the solid state switches nonconductive for a predetermined period of time after a power interruption or after a cooling or heating switch of the thermostat opens. The controller assures that each heating and cooling duty cycle begins with a full energization period. The length of the heating and cooling duty cycles can be readily varied.