for example a motor and brake therefor but it is to be understood that the invention disclosed herein is not limited thereto operation of sensing circuit 3 will now be described a current i in external circuit 5 flows through primary winding 11 of transformer 10 inducing a voltage in secondary 12 diodes 15 and 16 limit the voltage drop across secondary 12 and reflect back to primary winding 11 to reduce the load imposed by the sensing circuit 3 and to keep the voltage drop small in external circuit 5 the diodes serving to clamp the voltage between predetermined limits resistor 17 and capacitor 18 serve to filter out noise present in the induced voltage voltage divider 20 provides a voltage reference point at junction 25 and threshold voltages at junctions 23 and 27 junction 25 serves as the voltage reference point for the voltage induced in transformer secondary 12 comparator amplifiers 30 and 35 conjointly compare the induced voltage to a particular threshold voltage amplifier 30 comparing it to the positive threshold at junction 23 and amplifier 35 comparing it to the negative threshold at junction 27 the threshold voltages are designated positive and negative with respect to the reference voltage at junction 25 amplifier 30 is provided for induced voltages positive with respect to the reference voltage and amplifier 35 is provided for negative induced voltages thus assuming a non constant current i is flowing in external circuit 5 as the induced voltage increases to a level above the positive threshold voltage at junction 23 the output.