The module KY-003 Hall magnetic sensor module is part of the "37 In 1 Sensor Module Board Set Kit For Arduino" module is basically a magnetic switch made by a Hall effect sensor. A Hall-Sensor-Switch (typically a 44E) of the bipolar type is mounted on the module, and an LED and a resistor are also present. The LED lights up when a magnetic field is detected. The negative pole is on the left at the "-" pin, while the positive pole (5V) corresponds to the central pin. The output is located at the pin with the initials "S". [For more information](https://www.adrirobot.it/sensori/37_in_1/KY-003-Hall_magnetic_sensor_module.htm)