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.
