Cat and Dog Repeller

Nowadays, just about
every house has an outside lamp with a motion sensor. Such a device
eliminates the need to feel your way to the front door, and it
apparently also scares away intruders. The only problem is that
free-running dogs and cats in the neighborhood have little regard for
such lamps and continue to deposit their excrement in the garden, once
they have found a habitual location there for this purpose. This gave
rise to the idea of connecting a sort of siren in parallel with the
outside lamp to clearly advise dogs and cats that they are not welcome.

Naturally, it would be nice to avoid startling the entire
neighborhood with this alarm signal. Here we can take advantage of the
fact that dogs and cats have a significantly better sense of hearing than
people. Not only are their ears more sensitive, they can also perceive
significantly higher frequencies. With people, the upper limit is around
18 kHz, but dogs and cats can hear frequencies in excess of 20 kHz. We
can take advantage of this by building a siren that emits a frequency
just above 20 kHz. This will scare off dogs and cats, but people will
simply not hear it.

All we need for this is an oscillator with an amplifier stage and a
tweeter that can reproduce such high frequencies, such as a
piezoelectric tweeter. The schematic diagram shows how easily this can
be implemented. The power supply for the entire circuit is formed by the
components up to and including C2. The 230-V leads are connected in
parallel with the motion-sensor lamp. C1 and R1 provide capacitive
coupling to reduce the 230 V to an acceptable voltage. A DC voltage of
approximately 9.1 V is generated from this voltage using a bridge
rectifier and D1, filtered and buffered by C2. The oscillator is built
around R3, C3 and IC1a.

Cat and Dog Repeller Circuit

Cat and Dog Repeller Circuit Diagram

The frequency of this oscillator is rather dependent on the specific
characteristics of IC1, so the values shown here should be regarded as
guidelines. If the oscillator frequency is too high, it can be reduced
by increasing the value of R3 and/or C3. If the frequency is too low
(which means that the siren tone it is audible), the value of R3 and/or
C3 should be increased. The square-wave signal from the oscillator is
applied to the input of an H bridge composed of several Schmitt triggers
in combination with the final output stages (T1–T4). This approach
causes the peak-to-peak value of the square wave signal to be twice the
supply voltage.

As a result, a respectable 18 V is obtained across the piezoelectric
tweeter, which is sufficient to produce a quite loud whistle tone. When
building the circuit, you should bear in mind that it is directly
powered from 230 V and not electrically isolated from the mains network.
It is thus necessary to avoid contact with all of the components when
the circuit is in use. In practice, this means that the circuit must be
fitted into a well-insulated, waterproof box. If you want to test the
circuit, it is a good idea to first discharge C1 using a resistor, since
it can hold a dangerous charge. You must also ensure that components
F1, C1, R1 and B1 all have a mutual insulation separation of at least 6

Author: I. Fietz
Copyright: Elektor Electronics

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