The KY-003 is a digital Hall effect sensor module that switches its output when a magnet comes close. It carries an A3144 sensor, a resistor and a status LED. When the right pole of a magnet gets within a few millimetres of the sensor face, the S output goes LOW and returns HIGH when the magnet moves away, so it works like a contactless switch. It runs on 5 V and connects to any Arduino digital pin. It is ideal for gluing a small magnet to a spinning wheel or shaft and counting each pass: bicycle speedometers, motor RPM meters, door open/closed detection and lap counters. For fast rotation it is best read with an interrupt (attachInterrupt) so no pulses are missed.