It’s something [mircemk] demonstrates, with an Arduino-powered magnetic field strength meter that uses a UGN 3503U Hall effect device. The circuit is extremely simple, comprising the sensor ...

By placing a unit equipped with an ADXL345, an accelerometer for detecting motion, or a Piezo sensor on a door, it can notify you when someone knocks or tries to kick in the door. Linking the Arduino ...

which is why [mircemk] decided to build a hand-held version using the Arduino Nano and a BMP180 pressure sensor. Since you don’t want to be staring at a little screen in mid-air, the device ...