Found in a 1948 MIT documentation, this circuit uses a 0 to 100 uA microammeter as an indicator. The power supply is 150 V.
The integrated circuit used is not very common today. Transistors can be replaced by more modern...
This circuit was found in old documentation. The frequency depends on R1 and C1. For the...
This circuit was found in a 1990 publication. The sensors are electrodes placed in the water tank...