This monitor uses a common 7 41 amp configured as a comparator and a low cost nontransistor as an output driver. With no liquid detected, a voltage of about 2. 92 V is present in the op amp`s inverting input at pin 2. The 100-KO resistors establish a reference voltage of +2. 5 V at the noninverting input at pin 3 of the op amp. Under those conditions, the op amp`s output is -3.56 V, which keeps the 2N2222 transistor turned off and the voltage across its 1-KO output load resistor at 0 V.
Liquid-level-monitor - schematic

When liquid reaches the probes, the 3.3-MO and 22-KO resistor circuit conductively connects to ground. When enough current, about 1.4 p.A, flows through the liquid, the small 30 m V drop developed across the 22-KO resistor drives the op amp to deliver an output voltage of about 4.42 V. This voltage then drives a 2N2222 transistor into saturation, which generates a voltage drop of about 3.86 V across its 1-KO output load resistor.

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