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A crucial failing of proximity detectors
is their unreliable and tricky nature. This is where
they are used to detect humans, not to speak of smaller
living beings. One common approach is to detect eddy
currents in a living body, which are induced in the body
through a.c. mains wiring. However, such circuits become
altogether unusable in the case of mains failure, or in
the absence of mains electricity, or even where adjacent
mains circuits are switched in and out.
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Circuits
The circuit of Fig.1 takes the guesswork
out of proximity detection by inducing eddy currents in
a living being, whether animal or human. Five turns of
enameled copper wire (say 30 s.w.g.) are wound around
the area within which detection is to take place (4m x
4m in tests), and an audio signal of about � Watt is
pulsed through this, the Tx, coil. A smaller Rx coil
(say 100 turns of 30 s.w.g. enameled copper wire wound
on a 150mm dia. former) is used as a pick-up coil. The
circuit is adjusted by means of tune and fine-tune
controls VR1 and VR2, so that it is deactivated when one
stands back from the Rx coil.
A simple clock generator (IC1a-IC1b) and power MOSFET
(TR1) are used for the transmitter, and a 7555 timer
(IC2) is wired as a sine-square convertor for the
receiver. IC2's inputs are biased through VR1, VR2 and
R4. IC2 in turn switches NAND gates IC1c and IC1d, to
drive relay RLA. Capacitor C5 switches the relay for
about two seconds, and its value may be increased or
decreased to give different timing periods. D2 is
critical to prevent back-e.m.f. from re-triggering the
circuit. Supply decoupling capacitors C1 and C4 are also
critical, and should be located close to IC1 and IC2
respectively.
When a living being - animal or human - comes within
tens of centimetres of the Rx coil, the circuit is
triggered. This coil may be placed in the threshold of a
door, under a carpet, or around a hatch, at the base of
a tree, and so on. A number of such coils may also be
wired in series.
Coils may be wound with a larger or smaller diameter,
with more or less turns, and the power of the
transmitter may be varied, as well as the sensitivity of
the receiver. Note that a.m. radio reception may be
affected at close proximity to the Tx coil. |
