Fan-Failure Detector Circuit
Original Publication Date: 1983-Dec-01
Included in the Prior Art Database: 2005-Feb-08
In this fan-failure detector circuit, a light-emitting diode illuminates a spot in the path of a fan blade and a light-detecting diode detects light reflected from a reflective spot on one blade. As the blade rotates, the light-detecting diode produces a sequence of up-level pulses that are spaced apart by the period of fan rotation. A single-shot circuit is connected to be set by the pulse from the light-detecting diode. The period of the single-shot is just longer than the normal period of rotation of the fan, and while the fan runs at its normal speed the single-shot remains reset. If the fan speed fails, the single-shot completes its timing sequence and produces an up logic level at its output.