We find this circuit in an old documentation, but it can be made using modern transistors. The circuit trips, producing a continuous sound when the sensor is wet. The 1 mOhm resistor is actually 1 Mohm.
Found in a documentation from the 90s, this circuit has a typical accuracy of 2% with 20 V signals at...
This circuit, from a 1988 documentation, consists of an automatic temperature control using an NTC...
This circuit of a Popular Electronics from the 80's selects tones around 60 Hz, acting as a filter...