The circuit diagram shown here is of a Hall Effect switch. Hall Effect sensor IC DN6848 from Panasonic is the heart of the circuit. The DN6848 has a built-in Hall Effect sensor, Schmitt trigger circuit, power supply regulator and temperature compensation circuits integrated to a single chip. High sensitivity, low drift and excellent temperature st
ability of the DN6848 makes it well suitable for a variety of position, rotation and speed sensing applications. When the south pole of a magnet is brought near to the front face (face with type number marking) of the IC its output pin (pin 3) goes low (zero voltage). The pin 3 is connected to the base of PNP transistor Q1 and the low stage at pin3 makes the transistor ON and makes the relay activated. In a nut shell the relay will get switched ON whenever the south pole of a magnet is brought close to the Hall Effect sensor IC DN6848. Resistor R1 will always pull the base of Q1 to a very slight positive voltage and thereby prevents accidental triggering. LED D1 glows whenever the relay is activated and resistor R2 limits the current through the D1. Diode D2 is a freewheeling diode and it protects the transistor Q1 from high voltage spikes which are induced when the relay is switched.