This very simple project can detect fixed magnetic fields or fields that are varying at an audio frequency. Fixed or slowly changing field strengths are registered on a center-zero meter, which indicates the polarity in addition to the relative field strength. Audio frequency fields, such as those produced around mains and audio transformers, are detected via a crystal earphone that can be used to monitor the output signal.

The unit is not intended to provide accurate measurement of magnetic field strength, and is aimed at those who like to experiment with something a bit different. Although quite simple, the unit is reasonably sensitive. A small and not very powerful bar magnet can be detected by the prototype at about 100mm from the sensor, and drives the reading to full scale at a range of about 30mm.