Abstract: A highly reliable annunciator circuit which is immune from spurious signals generated in an elevator environment is provided. The circuit employs an input condition verification network which performs in conjunction with low true logic two-bit inputs by carrying out switching only when those inputs fall below a preset control value, highly enhanced performance is achieved. Volume control over the piezoelectric output developing a chime or gong sound is achieved through a variable impedance control which is adjustable on site.

Abstract: Method, system and apparatus for carrying out the cathodic protection of structures wherein evaluation of true current and true voltages at the counter electrode is carried out by an integration procedure which is timed in conjunction with the cross-over times of the originating a.c. input to the power source. The system also may employ an IR drop free potential measurement of a variety employing a reference electrode and a control which periodically turns off the fluctuating current drive to the working electrode for purposes of carrying out potential measurement from the reference electrode. To improve performance of the system, as well as the true current measuring features, difference networks are used to develop the difference between the signals of one channel, i.e. ground, and the parallel channel, i.e. positive, leading to the working and reference electrodes. With the arrangement, spurious noise phenomena are cancelled.

Abstract: A frequency selective ringer circuit which utilizes a constant current source in conjunction with an array of resistors and switches for developing a frequency selective window with which to detect an incoming frequency at the tip and ring inputs of a telephone system. The incoming frequency is doubled and converted to a d.c. level which is utilized as the converted level which is evaluated by the circuit. Where the converted voltage level falls within the established window, a drive circuit is energized for actuating a transducer such as a piezoelectric crystal or loudspeaker. Frequency selection is made by the simple expedient of closing one of a plurality of SPST switches. The circuit also may include a circuit for automatically deactivating the mechanical bell of an existing telephone set without the removal thereof.