Though not as simple as Proteon’s, this gauge gives a change in the pitch depending on the intensity and polarity of magnetic field.
Hall effect chips are simmetrical devices: given a small current flowing through a diagonal of the square chip, the perpendicular component of a magnetic field to the chip die will cause a voltage to appear at the other diagonal, the intensity and polarity of the voltage being proportional to the intensity and polarity of the magnetic field.
I tested the circuit on a protoboard, no solder job, no PCB. The components are easy to find and the thing can be assembled in a very short time.

The circuit

It is pretty …

By: 5Volt

More: continued here