Low-Cost Anemometer Fights Dust

This application note describes a low-cost circuit that can detect when a chassis fan filter has become clogged with dust. It does this by using a temperature sensor to determine the rate of cooling of a transistor in the air path of the cooling fan, thereby implementing a simple thermal anemometer.

source: http://pdfserv.maxim-ic.com/en/an/AN844.pdf

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