Electromagnetic Field Detector
This circuit is sensitive to low frequency electromagnetic radiation and will detect for example hidden wiring or the field that encompasses a transformer. Pickup is by a radial type inductor, used as a probe which responds well to low frequency changing magnetic and electric fields. Ordinary headphones are used to for detection. The field that surrounds a transformer is heard as a 50 or 60Hz buzz. The circuit is below:- Any opamp should work here, possibly better results may be achieved with a low noise FET type such as the LF351. The sensitivity of this circuit is good. Mains wiring buried an inch in plaster can be d e t e c t e d w i t h precision. A small load on the electric supply is all that is needed; a 20 watt desk lamp or similar will suffice. The humfield surrounding a transformer can be detected at over 7 inches. Domestic appliances such as videos and alarm clocks all produce interference which can be heard with the probe. The electric field surrounding a loudspeaker or earpiece can also be heard.
reff : http://www.hoo-tronik.com/2011/12/electromagnetic-field-detector.html
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