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; high-speed tachometers, 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!)
This high-speed sensor incorporates a push-pull output and internal filtering, and is suitable for high-speed switching tasks, like a tachometer. A low-cost variant of this board is available for proximity switching and other low-frequency uses.
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 push-pull output means no pull-up resistor is required, making programming and connection to any microcontroller I/O very easy.
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.