magnetic field sensor

Magnetic Field Sensor Circuit

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.

Sent by Viktor Kapra, Ukrain.

DIY Magnetic Field Sensor Circuit Diagram

magnetic field sensor


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  • Sleyvin

    Hello, I work at small restaurant in an interior located factory. We cater for factory staffs..around 350 to 400 staffs. For the past 1 year, we have to constantly buy dishes, bowls and cups (ceremicware) as it keeps getting missing… i have an idea to attach a small neodymium magnet under these utensils and a device similar to yours at the door so that if any1tries to take the utensils out the door, the sensor would activate the alarm… any help on how to modify your circuit to cater for my purpose.. the door way is 1meter wide and the magnet has to be detected through mild concrete plaster which will be used to attach the magnet and an overall suit as thats how its smuggled. Thank you

  • deljo

    is this a woking project

  • B S Murty

    A very very useful circuit indeed.
    We shall revert back after testing in our lab.
    Thank you very much.

    B S Murty.

  • Ed

    Mr. Kapra, Sir, thank you for this circuit, I will build this to tell me of the poles on my magnet’s when playing with “Bedini SSG’s” but I have a question as to if could be used to sense the position of a wheel with a installed magnet some where on the wheel. Like a crankshaft sensor? and if so could I use thee adjustment to delay or advance fireing a “MOSFET” to signal a spark? “EFI’s” on auto’s have no adjustment for this and are control spark by the on board computor,giving difficulty when useing alternative fuels or boosters to increase fuel milage,,this sound like something close to the advancement of this type alternative fuel enhancer technology,,close but you would of course know more then I. Thank you,,,,,Ed

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