Abstract: An automatic temperature setback system for controlling a heating unit includes the use of two devices. One device includes a oscillator circuit which is actuated by a timer control. The oscillator circuit can be an astable or free-running multivibrator. The output from such device is coupled across the low voltage secondary transformer winding of a typical heating system control unit so that, at specific timed intervals, the low voltage (i.e., 24 volts RMS) can have high frequency modulations superimposed upon either the positive cycles of such voltage or alternatively the negative cycles or alternatively both cycles. Such one device could be termed, in general, means for half-wave modulating the low alternating voltage carrier from a typical heating system control unit. The low voltage wires from the typical heating system control unit, in a conventional household, are coupled through the house to a low voltage thermostat located in one of the rooms of the house.