has at least one pair of normally open electronic switches 11 12 which are tied operatively to the voltage value on pin 2 i e the voltage signal at pin 2 is picked up and transmitted to actuator block 6 to allow switches 11 12 to be driven an internal impedance r int is connected between pin 2 and ground and allows variations in the voltage value appearing on pin 2 to be detected switches 11 12 parallel connect the opposing ends of the internal impedance r int to non inverting input 9 of comparator 7 the comparator has an output terminal 16 for coupling to an input terminal of a circuit such as mute 24 fig2 circuit 10 is described in more detail with reference to the embodiment shown in fig4 internal resistance r int has a set of resistors r1 r2 and r3 connected in series between a signal ground and pin 2 preferably these resistors have values of 6 30 and 6 kohms respectively the first resistor r1 has one end connected to ground and another end connected to the drain terminal of a first field effect transistor m1 preferably an mos type as well as to the resistor r2 transistor m1 has a source terminal connected to non inverting input 9 of the comparator 7 and to the source terminal of a second mos transistor m2 this second transistor m2 has a drain terminal connected between the resistors r2 and r3 and a gate.