Abstract: The apparatus is a detector for ferromagnetic objects using a small passive coil to sense disturbances in the earth's magnetic field when an object moves near the coil. The circuit uses a multiplier in the form of an electronic switch on the output of the coil to impose a much higher frequency on the output of the coil. This separates the frequency of amplification from the frequency of the coil signal and makes it possible to sharply filter the signal, prevent false alarms from electrical noise, identify the direction in which the object is moving, and reduce the power consumed through the use of a sampling technique. The frequency conversion technique also allows for multiplexing of multiple coils by one signal processing circuit.