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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Display brightness control (CB21614E)
Based on Allegro's A6275, this circuit controls the brightness of various displays. The circuit is valid...
AM Transmitter (CB513)
This small transmitter can act as a wireless microphone operating in the medium wave range,...
SCR dimmer (CB21652E)
This is the circuit of an RCA Dimmer sold in 1967. With it it is possible to control a load up to...