power source such as a 9 volts battery applying a positive potential to the supply pin therein the operating voltage is supplied through a high resistance resistor 29 in series with the positive potential v of the power supply and is decoupled with a capacitor 30 connected to the negative potential v of the power supply as shown the high resistance resistor 29 ensures a very low power consumption by the cmos device 10 since the supply voltage is automatically adjusted to a low level because of the self biassing effect provided by the resistor 29 a transducer device 31 such as an ultrasonic transducer is connected to the input of the cascaded amplifiers so that the signal generated by the transducer 31 is repeatedly amplified by these amplifiers the low current bias of the amplifiers results in a high output impedance which provides a highly desirable low frequency attentuation various types of well known transducers such as an ultrasonic transducer optical transducer or electromagnetic wave transducer may be employed which can response to a remotely produced signal to generate a voltage signal an ultrasonic transducer is preferred in that the sound signal for actuating the transducer is not directional and can reflect from obstacles while it does not cause any adverse interference to the normal operation of any electronic circuit components in other equipment located in the same neighbourhood of the switching device the ultrasonic transducer 31 is connected directly to the input of the cascaded amplifiers the transducer 31 acts as a virtual earth because of the provision of the feedback resistor 17 which is connected from the output to the input of the first amplifier in the cascade this.