Tachometer Square Wave output Signal

A high sensitivity Hall Effect sensor is usually within an IC on the printed circuit board (PCB), whilst the magnet assembly rotates with the shaft. When the North Pole passes past the sensor, it generates a positive peak, whilst the South Pole generates a negative trough.

Since the Hall sensor detects magnetic fields, its output is not a square waveform, and therefore not recognised reliably by digital circuits. The pre-driver circuitry is responsible for shaping the output to a square wave. The diagram above shows the relationship between the dynamic magnetic field and the open collector transistor switching.

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