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:-
Electromagnetic Field Detector Circuit Diagram
Notes:
I threaded a length of screened cable through an old pen tube and
soldered the ends to a radial type can inductor. I used 1mH. The
inductor fitted snugly into the pen tube. The opposite end of the cable
connects to the input of the op-amp. Any op-amp should work here,
possibly better results may be achieved with a low noise FET type such
as the LF351. The 2M2 potentiometer acts as a gain control and the
output is a pair of headphones. Stereo types can be used if they are
wired as mono. I used an 8 ohm type, but the circuit should work equally
well with higher impedance types. The probe (shown below) may be
connected via screened cable and a 3.5mm stereo plug and socket.
Detection:
The sensitivity of this circuit is good. Mains wiring buried an inch in
plaster can be detected with precision. A small load on the electric
supply is all that is needed; a 20 watt desk lamp or similar will
suffice. The hum field surrounding a transformer can be detected oat
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. Try
lifting a telephone and place the probe near the earpiece. A telephone
pickup coil can be used in place of the inductor if desired. I will make
an improved version of this circuit with a meter output later.
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