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.
This simple circuit uses a 200 µA galvanometer to measure voltages in three ranges. The circuit...
Today we can call this circuit a baby monitor. It is a microphone amplifier found in a publication by...
This circuit is from Motorola's Linear Interface ICs from 1993. The MC1723 is the equivalent of...