MAGNETIC FIELD CHANGE DETECTION CIRCUITRY

摘要

Apparatus for detecting a tooth of a body. The tooth has an edge terminating in a notch. The tooth and notch are disposed along an edge of the body. The apparatus includes a pair of Hall effect cells disposed adjacent to, and laterally disposed along, the edge of the body. A magnet is positioned to provide a magnetic field through the edge of the body and the Hall effect cells. The magnitude of the field passing through the Hall effect cells is related to the relative position between the tooth and the Hall effect cells. The Hall effect cells produce an output voltage related to the magnitude of the magnetic field passing through the cell. A differencing circuit is fed by the pair of Hall effect cell produced voltages for producing a difference signal having a peak when the edge of the tooth is positioned between the pair of Hall effect cells. A peak detector detects the peak produced by the difference signal. A threshold circuit is fed by the differencing signal for producing a gating signal when the magnitude of the difference signal exceeds a threshold level. The gating signal enables the peak detector produced pulse to pass to an output of the apparatus and inhibits such detected peak from passing to the output of the apparatus in the absence of such gating signal. A threshold level adjusting circuit is responsive to the difference signal for tracking maxima in the difference signal and for establishing the threshold level of the threshold circuit in accordance with the tracked difference signal maxima.