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Hall-efect digital switch
This circuit shows how to interface a Hall-effect digital switch to a 4 amp DC load. A Hall-effect digital switch is a contactless, magnetically activated sensor. The Hall device can be a UGN3113, UGN3120-23, UGN3130, UGN3132/33, or a UGN3140. These devices incorporate a sensor, schmitt trigger and open collector output driver. This circuit will turn on a load when an activating magnet approaches the sensor. When the Hall switch is OFF (insufficient magnetic flux to operate), about 12 mA of base current flows through the 1k resistor to the 2N5812 transistor, thereby saturating it and shorting the base of the 2N3055 to ground, which keeps the load OFF. When a magnet is brought near the Hall switch, it turns ON, shorting the base of the 2N5812 to ground and turning it OFF. This allows: 12v/56=210 mA of base current to flow to the 2N3055, which is enough to saturate it for any load current of 4 amps or less.