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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Solar intercom (CB20503E)
This circuit can be used in demonstrations or science fairs. The circuit uses headphones and can...
Long-Range Wiretapping (CB19321E)
We found this wiretapping circuit in a publication from the 90s. The circuit allows the cable...
Tone activated Triac (CB7234E)
This circuit uses a resonant reed switch at the trigger frequency. The circuit is from General...