Circuit checks "swamp-cooler" water level
Edited by Bill Travis
Daniel Krones, Precision Design Services, Sky Forest, CA -- EDN, October 31, 2002
A "swamp cooler" is an easy way to obtain effective air conditioning, especially in hot and dry climates, if a water source is readily available. Although most units are very reliable, the storage-reservoir control usually uses a single level-detector component. Failure of this component can lead to serious water damage. The circuit of Figure 1 provides a simple, inexpensive backup alarm signal if the water level exceeds the preset height. The circuit uses a single Schmitt-trigger IC to detect the water level, using the conductivity of the water to drop the input level of IC1A. A 1- to 10-MΩ resistor is suitable for R1. You might have to experiment to determine a suitable value, depending on the conductivity of the water supply.
The highest practical vale of R1 provides the widest range. The NAND gates IC1B and IC1D implement gated oscillators to create a pulsed tone to drive the piezoelectric-bell audible alarm. Current consumption in the off state is lower than 10 µA, thus allowing the use of a simple battery to drive the circuit. A button-cell lithium watch battery is sufficient. The small physical size and wiring simplicity of the circuit allow you to simply glue the unit to the side of the cooler. Use a short piece of twin-lead, 300Ω transmission line for the electrodes.
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