Here’s a cool entry for the Instructables magnet contest, Pronteon writes – “Working with magnets and want a fast way to tell their NORTHs from their SOUTHs? Want to get an idea of their field shapes? Like making weeeeeeeooooooeeeeeeoooo noises? Build a Magno-Sniffer using a hall effect sensor, 555 oscillator circuit, piezo speaker, tongue depressor, hot glue, and a few other doo-dads.” – Link.
HOW TO – Magno-Sniffer (Magnetic sniffer probe whistler thing using a hall effect sensor)