Extremely sensitive magnetic field detector

I am searching for a circuit that I can build to detect very very small magnetic field changes. I would like to be able to detect micro tesla magnetic field oscillations. Does anyone know of a circuit that can do this?

Thanks for your help...

This DIY magnetic field sensor circuit is very simple and can detect fixed magnetic fields or fields that are varying at an audio frequency. The unit is not intended to provide accurate measurement of magnetic field strength.

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.

The circuit is reasonably stable, but occasional readjustment of VR1 will be required.

Thank you Sujin for your reply. I think that this circuit will do the job nicely. One more question. Can you recommend a place where I can buy the parts for this project?

Thanks again for your help...

You can buy thee components only from Electronics shop, where semiconductors, LEDs, Transistors etc are available