Voltage level detector (CB21504E)

This circuit was found in a 1966 EEE. The circuit can be implemented with modern general-purpose transistors.

NAB preamplifier for recording heads (CB12969E)
Many collectors of records, tapes and even audiophiles often keep recordings on tapes that...
Sensitive Metal Detector (CB4406E)
This circuit was found in a 1992 documentation and has excellent sensitivity, using components that...
AND gate with relays (CB21029E)
Found in an American brochure from the 90s, this circuit shows how we can implement a 4-input AND...