A Hall effect latch works like a switch. It will latch on when the magnet is near the sensor (south pole). It will turn off if the opposite magnet pole (north pole) is applied.

It can be used for counting rpm of rotating shaft or fan by putting 2 magnets with opposite poles after each others.

Features

• 3.5V to 24V DC operation voltage
• Low current consumption
• Temperature compensation 
• Wide operating voltage range
• Open-Collector pre-driver
• 50mA maximum sinking output current
• Reverse polarity protection

A3144 hall effect sensor datasheet.

A3144 hall effect sensor datailed tutorial to meaure fan rotaion RPM.