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.


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 mm!