An extremely handy sensor that acts as a normally-open switch until a magnet is brought within an inch or so, closing the connection. Potential uses are; low-speed rotation sensors, door switches, or any other application where a non-contact switch is useful. (keep in mind that it need not be "air" between the sensor and the magnet. Wood, plastic, etc. will be effectively "transparent" to the magnetic field!)
It is an "active-low" circuit, with a built-in pull-up, meaning that the sensor's output will read "high" until a magnet is sensed, then it will go low. The pull-up resistor is on the board, making direct connection to any microcontroller I/O very easy.
Please note that this low-cost sensor is not suitable for high-speed switching tasks, like a tachometer. High-frequency pulses will swamp the sensor, giving erroneous output. Please see the "High-speed" variant of this board for tachometer and other high-frequency uses.
The 3-pin connection follows the universal hobby-servo standard of: Signal-Vcc-GND. This makes it easy to connect to the many Cal-Eng, and other boards that use standard 3-pin headers.