This circuit was found in a 90’s Electronic Engineering. The integrated circuits are of the time and the diodes are of general use. The generated signal is sinusoidal.
This circuit can be used in measurement bridges. We found it in the September 1995 Electronics Now magazine....
This preamp version uses BC109 transistors that are older versions of the current BC549 in plastic...
This circuit locks when the sensors are opened. The relay can be 6 or 12 V depending on the power...