In brief, in accordance with the above and other objects of the invention, there is provided a programmable electronic digital thermostat for controlling a temperature conditioning system to maintain a scheduled set ambient temperature. The thermostat includes at least one controlled semiconductor switch for selectively activating or deactivating an external relay connected in series with an external power source of the temperature conditioning system. The fan coil unit thermostat includes a power supply circuit utilizing the external alternating current power source. The power supply circuit includes a bridge rectifier connected between the controlled semiconductor switch and the external relay for rectifying the alternating current to produce a direct current voltage. A voltage regulator is coupled at its input to the bridge rectifier for providing a predetermined supply voltage. A capacitor is connected to the voltage regulator input for charging to a first predetermined potential. Control circuitry is connected between the capacitor and the controlled semiconductor switch for sensing the voltage potential of the capacitor and for deactivating the semiconductor switch in response to a sensed voltage potential below a predetermined value, thereby to enable the capacitor to recharge to the first predetermined potential without deactivating the external relay.

An electronic digital thermostat is disclosed which is capable of use in a pneumatically controlled temperature control system of the type which has a pneumatic supply line which extends to various components of the control system and wherein the control elements of the system are controlled by varying the control pressure that is communicated to such elements. The thermostat is capable of operating in a LAN system environment, and can be retrofitted into unit ventilators and the like as a substitute for a conventional pneumatic thermostat. The thermostat has a processing means that enables sophisticated control to be achieved.