The interface circuit (IC) has a single input terminal (i) for to receiving an analogue or digital (PWM) control signal (Vin) indicative of the required speed of rotation of the motor (M). The type of input signal (Vin) is recognised by means of comparator circuits (C1, C2). The input signal (Vin) is processed differently according to its nature in order, in any case, to output an analogue signal (Vo) acceptable by the regulation and control circuit (Z) which pilots the motor (M).