Posted on Jun 11, 2012

Hall-effect switches have several advantages over mechanical and optically coupled switches, They`re insensitive to environmental light and dirt, they don`t bind, and they don`t sustain mechanical wear. Their major drawback is that they require three wires per device. The circuit shown, however, reduces this wire count toN + 1 wires for N devices.

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Amplifier IClA is configured as a current-to-voltage converter. It senses the sensor assembly"s output current. When the Hall-effect switch is actuated, the sensor"s output current increases to twice its quiescent value. Amplifier IC1B, configured as a comparator, detects this increase. The comparator"s output decreases when the Hall-effect switch turns on. The circuit also contains a fault-detection function. If any sensor output wire is open, its corresponding LED will tum on. If the power-supply line opens, several LEDs will tum on. A short circuit will also tum an LED on. Every time an LED turns on, Ql turns on and the alarm relay is actuated.

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