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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Experimental AM Receiver (CB17018E)
This good receiver contains 6 coils wrapped in ferrite rods. L1 has 20 turns of wire 28 and L2 /...
8-output phase shifter (CB8152E)
Found in a 1967 Electronic Design, this circuit changes the phase of a signal in 8 different ways, given...
Low battery indicator (CB21619E)
Based on an Analog Devices LTC1540, this circuit consumes only 50 uA, serving to monitor the...