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Ferrous metal detector
This circuit shows how to use a Hall-effect linear sensor to detect the presence of a ferrous metal. A Hall-effect linear sensor is a device that outputs a device proportional to the magnetic field sensed by the device. The north pole of a magnet should be affixed to the back side of the UGN3501. This magnetically biases the device. A 20 mV output change (decrease) is produced as a 1" steel ball rolls over the sensor. This signal is amplified and inverted by the 741 op amp circuit and drives the 2N5812 ON.