Hall-voltage slope-activated sensor

Abstract

The Hall sensor circuit includes a Hall element that is preferably followed by a Hall-voltage amplifier, and a pole end of a magnet is preferably fixed adjacent to the Hall element. The amplifier output is connected directly to one of a pair of differential inputs of a Schmitt trigger circuit and is also connected, via a single or a dual-polarity track and hold circuit, to the other of the differential Schmitt inputs. The dual-polarity track and hold circuit causes the voltage across a capacitor to track positive and negative Hall voltage slopes, and to hold the positive-going peaks and negative-going peaks of the Hall voltage presented to the fore-mentioned other Schmitt input so that when the difference voltage between the Hall voltage and the held voltage of the capacitor exceeds a positive or negative threshold of the Schmitt circuit, the Schmitt circuit output changes binary state indicating the approaching edge or the receding edge of a ferrous-gear tooth. The dual-polarity track and hold circuit is made up of two complementary plus and a minus peak detector circuits, each including a bipolar transistor having an emitter connected to the voltage-holding capacitor, an operational amplifier with an output connected to the base of the transistor, the amplifier having a negative input connected to the transistor emitter and a positive input serving as the input of the each Hall voltage tracking and holding circuit.