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It is well
know that pests like rats, mice etc are repelled by ultrasonic
frequency in the range of 30 kHz to 50 kHz. Human beings can’t
hear these high-frequency sounds. Unfortunately, all pests do not
react at the same ultrasonic frequency. While some pests get
repelled at 35 kHz, some others get repelled at 38 to 40 kHz. Thus
to increase the effectiveness, frequency of ultrasonic oscillator
has to be continuously varied between certain limits. By using
this circuit design, frequency of emission of ultrasonic sound is
continuously varied step-by-step automatically. Here five steps of
variation are used but the same can be extended up to 10 steps, if
desired. For each clock pulse output from op-amp IC1 CA3130 (which
is wired here as a low-frequency square wave oscillator), the
logic 1 output of IC2 CD4017 (which is a well-known decade
counter) shifts from Q0 to Q4 (or Q0 to Q9). Five presets VR2
through VR6 (one each connected at Q0 to Q4 output pins) are set
for different values and connected to pin 7 of IC3 (NE555)
electronically. VR1 is used to change clock pulse rate. IC3 is
wired as an astable multivibrator operating at a frequency of
nearly 80 kHz. Its output is not symmetrical. IC4 is CD4013, a
D-type flip-flop which delivers symmetrical 40kHz signals at its Q
and Q outputs which are amplified in push-pull mode by transistors
T1, T2, T3 and T4 to drive a low-cost, high-frequency piezo
tweeter. For frequency adjustments, you may use an oscilloscope.
It can be done by trial and error also if you do not have an
oscilloscope. This pest repeller would prove to be much more
effective than those published earlier because here ultrasonic
frequency is automatically changed to cover different pests and
the power output is also sufficiently high. If you want low-power
output in 30-50 kHz ultrasonic frequency range then the crystal
transducer may be directly connected across Q and Q outputs of IC4
(transistor amplifier is not necessary).
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