Notes:
This tester is designed to locate stray electromagnetic (EM) fields. It will
easily detect both audio and RF signals up to frequencies of around 100kHz. Note, however that this circuit is NOT a metal
detector, but will detect metal wiring if it conducting ac current. Frequency
response is from 50Hz to about 100kHz gain being
rolled off by the 150p capacitor, the gain of the op-amp and input capacitance
of the probe cable. Stereo headphones may be used to monitor audio frequencies
at the socket, SK1.
Probe Construction:
I used a radial type inductor with 50cm of screened cable threaded through a
pen tube. The cable may be used with a plug and socket if desired. My probe is
shown below:
A layer of insulating
tape or glue is used to secure the pen body to the inductor.
Meter Circuit:
The output signal from the op-amp is an ac voltage at the frequency of the
electro-magnetic field. This voltage is further amplified by the BC109C
transistor, before being full wave rectified and fed to the meter circuit. The
meter is a small dc panel meter with a FSD of 250uA. Rectification takes place
via the diodes, meter and capacitor.
Testing:
If you have access to an audio signal generator you can apply an audio signal
to the windings of a small transformer. This will set up an electromagnetic
field which will be easily detected by the probe. Without a signal generator,
just place the probe near a power supply, mains wiring or other electrical
device. There will be a deflection on the meter and sound in the headphones if
the frequency is below 15KHz.
In Use:
Switch on, plug in headphones (optional) and move the probe around. Any
electrical equipment should produce a hum and indicate on the meter.I remember once building a high gain preamp (for
audio use). I made a power supply in the same enclosure. The preamp worked, but
suffered from an awful mains hum. This was not directly from ripple on the power
supply as it was regulated and well smoothed.What I
had done was built the audio circuit on a small piece of veroboard,
and placed it within a distance that was less than the diameter of the
transformer. The transformers own electromagnetic field was responsible for the
induced noise and hum. I should however note, that
this was when I was new to electronics with very little practical experience.
You can now buy toroidal transformers which have a
much reduced hum field.
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