Water activated alarm (CB7993E)

This rain, flood or leak sensor was found in a 90s Popular Electronics. All the components used are common today. The transistor can be the BC548.

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Sample and hold (CB8873E)
This circuit is from a 70s Burr Brown documentation. The components are from the time and the...
Audio Generator 4093 (CB6887E)
The circuit, shown in the figure, consists of an audio oscillator that generates signals between...
2-input video multiplexer (CB8736E)
Found in Linear Technology documentation from the 90's, this circuit mixes two analog video...