A small and not very powerfull bar magnet can be detected at about 100 mm from the sensor. It has a Hall Effect sensor (UGN3503U) and IC2 (OP77GP or TL071CP), a precision opamp which is used to provide some additional amplification. Meter ME1 is connected between the output of IC2 and the potential divider and it therefore responds to the voltage difference between the two. If audio rather than DC performance is of most importance it would be advisable to use TL071CP for IC2 and then reduce the output resistor from 33k to 27k.
A 6V battery supplies power to the magnetic field sensor circuit and the current consumption is only about 9mA. Do no use a 9V battery as this will result in the maximum supply voltage rating of IC1 being exceeded.
Testing the magnetic field sensor
When the unit is first switched on it is likely that the meter will be driven fully positive or negative. With careful adjustments of Balance control VR1 it should be possible to zero the meter and placing the probe near any magnetized object should then produce a suitable response.
Applying a “North Pole” close to the surface it will produce a positive reading, and applying a “South Pole” to it generates a negative reading.
Placing the probe against the power cable of any mains powered device that is switched on should produce a 50 Hz “hum” from the earphone.
