LIQUID_LEVEL_MONITOR

Published:2009/7/6 9:31:00 Author:May | From:SeekIC

This monitor uses a common 741 amp conft-gured as a comparator and a low cost nontransistor as an output driver. With no liquid detected, a volt-age of about 2.92 V is present in the op amp's inverting input at pin 2. The 100-KΩ resistors establish a reference voltage of +2.5 V at the non-inverting 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-KΩ output load resistor at 0 V. When liquid reaches the probes, the 3.3-MΩ and 22-KΩ resistor circuit conductively connects to ground. When enough current, about 1.4 μA, flows through the liquid, the small 30 mV drop developed across the 22-KΩ resistor drives the op amp to deliver an output voltage of about 4.42 V. This volt-age then drives a 2N2222 transistor into saturation, which generates a voltage drop of about 3.86 V across its 1-KΩ output load resistor.



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