Electronic temperature control system for thermostatic control

Claim 1

A temperature control system for thermostatic control of a standard two-wire voltage relay control mechanism without the presence of amplifiers and circuitry requiring a power source other than the low voltage relay control mechanism and the calibration of which is unaffected by relay voltage variations comprising a circuit having two main terminals, one terminal being adapted for connection to terminal of an alternating current operated relay of the low voltage heating and/or cooling system relay control mechanism, the circuit comprising a temperature sensing circuit including a temperature responsive thermistor and potentiometer adjustable in response to a desired temperature setting, a programmable unijunction transistor (PUT) the gate of which is connected to the thermistor and the anode of which is connected to the potentiometer wiper, a diode bridge connected across the system circuit main terminals and to the temperature sensing circuit and a SCR thyristor connected across the output of the diode bridge and to the cathode of the programmable unijunction transistor to conduct current during alternate half cycles of the alternating current through opposite legs of the bridge.