This circuit from the 80's makes use of two transconductance operationals of the time. Modern equivalents can be used. The circuit does not need to be powered by a symmetrical source.
This simple rain, leak or moisture detector circuit appears in other places on this site with...
This circuit was obtained in a 90s EDN. The components used are from that time. The voltage...
This circuit is from a 1967 General Electric SCR manual. The capacitors are polyester and the...