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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Micro FM Stereo Integrated (CB25238E)
Using a BA1404 integrated circuit this small FM stereo transmitter was found in a Radio...
Tremolo Amplifier (CB19609E)
The operational amplifier is not very common today, but in its place an equivalent, even of...
Semi-isolated Collpits oscillator with parallel crystal (CB25555E)
This circuit was found in the February 1993 Radio Electronics magazine. The frequency depends on...