With the return of the fine weather, you’ll doubtless be enjoying
lazing around of an evening on your patio or in your garden, but even
if you’re not surrounded by marshes or other shallow water it’s very
likely some intruding mosquitoes will come along to spoil this idyllic
scene. Although indoors it’s easy to get rid of them these days, indeed
even to prevent them coming into the house, the same can’t be said for
the great outdoors. We might mention the well-known Chinese coils – the
only thing Chinese about them is undoubtedly their name – which very
often drive people away as much as mosquitoes, if not more! Moreover
they are nasty things to handle.
There are also UV (ultra-violet) ‘electrocutors’ consisting of a
blue lamp surrounded by two closely-spaced grilles between which a
high voltage is applied. The mosquitoes (and fies and other flying
insects) are supposedly attracted by the color of the lamp and as they
approach, get electrocuted in contact with the two grilles. The only
thing you have to do is pull out the drawer from time to time and get
rid of the mass of dead insects. Even though the effectiveness of
these first two products remains questionable, it is less so than the
one we’re nonetheless going to describe here.
We’re talking about an ultrasonic mosquito repellent. The principle,
as described by its numerous promoters, is as follows. Only the female
mosquitoes bite (that at least is an undisputed scientific fact) and
they bite when they need to feed, and above all, to feed their eggs. In
this situation, they seek to avoid the males whose ‘job’ has already
been done, and so they fy away from the frequencies emitted by the
males when they are on heat. This is where opinions now diverge.
According to certain publications, the frequency emitted by the male
mosquitoes is said to be around 20–25 kHz, and so within the realm of
ultrasound. But according to others, it is in the region of 5–7 kHz
instead; frequencies that a human ear, even an elderly one, can still
hear very well. Rather than spending lots of money (of the order of tens
of pounds) buying such a device, which moreover generally have a fixed
frequency, we’re suggesting building one yourself so that you can carry
out your own research this summer, especially since the circuit proposed
is very simple and cheap to build.
As the figure shows, it uses just a single IC, a CMOS type 4047.
This very multi-purpose IC can be wired in very many operating modes,
including that of the multivibrator or astable used here. The operating
frequency is set by the external components C1, R1, and P1; the latter
makes it possible to slightly adjust the frequency, given the
uncertainty that exists over the most efective value…To best reproduce
the high frequencies produced by the generator, the output transducer
used is a simple tweeter, but it must be a piezo one. Such a tweeter
behaves in fact much like a capacitor, and so doesn’t overload the CMOS
IC outputs that are incapable of supplying a substantial current, as
everyone knows who’s ever worked with 400 series CMOS logic.
To obtain an output signal of sufficient amplitude while being
powered from a single 9 V battery, this tweeter is connected between the
4047’s Q and Q outputs, making it possible to apply complementary
(antiphase) signals to the tweeter so it ‘sees’ an alternating voltage
of double the supply voltage. In purely theoretical terms, this
quadruples the output power available. In practice, it’s better to
regard it as tripling it, but the beneft achieved by doing it this way
is nonetheless very real. All that remains is for you to place the
project in the middle of the patio table or beside your lounger in order
to get a taste of the calm of a summer’s evening without mosquitoes
bothering you acoustically or worse, biting. At any rate, that’s what we
wish for you…
