Found in old documentation, this circuit has only the most difficult component to obtain the PUT. The sensors are electrodes placed in the reservoir. The circuit directly controls an AC power load.
This circuit was found in a 1992 publication, using an operational amplifier not very common...
This is a amplifier for XTAL high impedance earphones or piezoelectric types. Current drain is...
This circuit is from the LM111 manual of the 1980s. The component no longer exists, but...