Hall effect sensors are small and affordable components that detect magnetic fields. They are often used in conjunction with permanent magnets to detect the proximity or position of mechanical parts. For example, you can attach a magnet to the output shaft of a motor and use a Hall effect sensor to detect each rotation of the shaft. Count the time between moments when the magnetic field is strongest and you can calculate the motor's RPM. Researchers from Eindhoven University of Technology in the Netherlands found that NFC (Near-Field Communication) can achieve similar results. They have demonstrated their findings with an open source system called NFCSense.
NFC technology is most often used to transmit small amounts of data. When you use your smartphone to pay at a cash register, it uses NFC to send your payment information to the establishment's PoS (Point of Sale) system. But the researchers behind NFCSense noticed that NFC connections can happen very quickly — a speed of about 300 reads per second. That is fast enough to function in a manner similar to Hall effect sensors. In our previous example with the motor shaft, you can replace the Hall effect sensor with an NFC reader and the magnet with an NFC tag. By measuring the time between NFC readings, you can calculate the motor's RPM in the same manner.
NFCSense offers one very important benefit over Hall effect sensors: it can differentiate between individual NFC tags. To a Hall effect sensor, all magnets of the same size look identical. But because NFC tags contain unique identifiers, NFCSense can tell the difference between one tag and another. That introduces a plethora of potential new applications. If you line a wheel's rim with NFC tags, NFCSense can determine the rotational angle of the wheel based on which tags are within a readable distance. With a similar setup, you can measure linear motion or reciprocal motion. The downside of NFCSense, when compared to Hall effect sensors, is cost. NFC tags are inexpensive, but NFC readers are much costlier than Hall effect sensors. Hall effect sensors are also better at detecting distance, since they can measure the strength of magnetic fields.
If you're interested in trying NFCSense yourself, the team has made their API open source and it is available on their GitHub page. All you need to get started is an Arduino Uno board, an RC422 NFC/RFID reader module, and a few NFC tags.