Abstract: A power source circuit for a discharge type sensing circuit, comprising a blocking oscillator (1 ) and a feedback circuit (2) for changing the period of oscillation of the blocking oscillator (1). The sensing circuit as a load is connected to a secondary winding (L.sub.2) of a transformer (T) of the blocking oscillator (1). The base bias of a transistor (Tr.sub.1) of the blocking oscillator is controlled by the feedback circuit (2) to which is applied a negative voltage produced in the primary winding (L.sub.1) in an off state of the transistor (Tr.sub.1). As a result, when the sensing circuit is in the sensing mode of operation, the period of oscillation becomes shorter for rapid supply of electric power and, when in the waiting mode of operation, the period of oscillation becomes longer for reducing supply of power, so that the power consumption is reduced as a whole.