Continuity test (CB9328E)

This audible continuity test was obtained in a book from the 90's describing simple projects. The feeding is done with a single battery.

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Residential Alarm (CB17228E)
This simple alarm circuit was found in a 1998 documentation. When the sensors are activated, the...
Microphone balanced amplifier (CB9388E)
This circuit was found in an old American documentation. The integrated circuit used is uncommon...
Increasing the voltage of a fixed regulator (CB8476E)
Found in a 90s Electronic Design, this suggestion to have a higher voltage in a fixed regulator...