Piezoelectric alarm (CB8970E)

This circuit is from the 80s. The integrated circuit used is uncommon as is the transducer. Equivalent components can be used.

Water level meter (CB6959E)
This interesting capacitive water level sensor was found in Popular Electronics. The circuit can be...
LTC7841 PolyPhase® Synchronized Boost Controllers
LTC7841 PolyPhase® Synchronized Boost Controllers Mouser Announces Analog Devices LTC7841...
Variable Reluctance Pickup Preamplifier (CB941E)
This circuit was fond in an old publication of the years 1950. The transistors are germanium types and...