Proximity alarm (CB21300E)

This circuit was found in a Popular Electronics from the 90's. The components used are common and instead of the buzzer we can use an LED or a relay.

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Motion sensor (CB8762E)
This sensitive motion sensor with inductive sensor was found in a 90s Elektor. The circuit uses only...
Hearing Aid Amplifier (CB6286E)
Suggested by Amperex in a 1968 documentation, this circuit is based on TAA370, an integrated circuit...
Band Pass Filter I (CB19684E)
This filter, found in an old manual, has characteristics of reduced overvoltage given by the graph...