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Calling all Creatures Great and Small?

by E . A. Matthews

Among the gifts I received this year was a rat and mouse deterrent - given as a joke, I think. It was an oscillator which worked from the mains via a plug-in adapter with an output of 12V. A.C. (yes, A.C. !). For some reason the bridge rectifier was mounted with a 555 oscillator; it had an 8 ohm speaker and an LED indicator, all mounted in a purpose built box. The accompanying literature Stated that the effective frequency range for deterring rodents is between 18 and 28KHz, but when I Checked the output I found it to be marginally less than 16KHz. whether it would scare off rats and mice I can't tell, any more than I know whether any dog or other pet scarers work.

But I would like to know the recommended frequencies for a whole range of reputedly effective treatments in this and similar areas. Perhaps a member can supply a list. Has anyone found similar types of oscillator that can be used for this type of application ? What I would particularly like is information on recommended frequencies for deterring the following: dogs, cats, foxes, insects particularly mosquitoes and wasps. I would also like to know the frequencies used in ultrasonic cleaning as used for jewellery, small mechanical parts, watches - and even soiled fabrics. Information on related topics, such as fish attractors, ultra- and subsonic devices, would also be welcome.

Forty odd years ago, whilst doing training, I was introduced to a small vibrator used for silently "ringing out" some panel wiring. This gadget was housed in a cylindrical metal can about the size of a small hand torch, and worked like a very low frequency buzzer, powered by a 1.5V cell inside the can. Fitted at one end with a probe and at the other with a flexible lead terminated in a crocodile clip, it formed a novel circuit tester. It cost six old pence from a shop in Lisle Street. Its advantage was that only the user could sense continuity (by feel), as it had no sounding bell or buzzer, no light and no meter deflection. Is this idea attractive to any members ?

Editor's note: Lisle Street is a short distance from Leicester Square, and in the forties harboured several "wireless" shops which stocked a wide range of components, so it was a Mecca for the radio enthusiast.

It is generally accepted that animals smaller than man have an upper frequency limit well above the 16KHz or there abouts which is usually taken as the human upper limit, but as a zoologist I have not come across any data on the hearing ranges of different animals. It is well known that dogs respond to what we regard as supersonic frequencies, and in bygone days one could obtain a "silent" dog whistle, often referred to as a Galton's whistle. A rather superior form of this could be found in physics laboratories of fifty years ago; it had a piston for varying the frequency emitted, and its range extended to 30KHz or beyond. It was used for testing human hearing limits and for experiments on sound propagation. Although bats are not blind as is popularly supposed, they hunt their insect prey in darkness and live in dark places, so their ability to navigate by emitting and detecting ultrasonic pulses (the so-called 'bat radar') is a considerable advantage, and has been the subject of much study. Their sound emissions can be made audible to humans with a receiver using the superheterodyne principle of employing a local oscillator to beat with the incoming frequency to produce a sound within the human range of hearing. But a microphone which will detect sounds up to 80KHz or beyond is needed.

My garden is invaded by other people's cats, and as I prefer not to have my plants dug up by incontinent felines, I have tried a variety of deterrents, including lion dung (from Bristol Zoo), which was recommended on the basis that small cats fear the presence of larger cats. Our local cats hadn't heard about this, so it proved ineffective. There have been one or two designs for ultrasonic cat and dog deterrent devices in the electronic magazines, and a long time ago I decided to build one which was described in the June 1982 issue of "Elektor" (which became "Elector Electronics" in February 1984). It uses a piezoelectric tweeter (still being advertised by B.K.Electronics of Southend-on-Sea) and takes advantage of the fact that this tweeter reaches its maximum efficiency at 20KHz. An astable multivibrator, using a 40106 IC, drives two BD135 (NPN) and two BD136 (PNP) with the piezo tweeter bridged between them. This produces a square wave with an r.m.s. voltage of about 6.5V at a frequency of about 21KHz, and the application of this to the piezo tweeter produces a sound pressure of lOO dB at 1 metre, which is enough to give any cat or dog a headache. Sad to say, the project has not yet reached the finishing line, partly because the current it takes is too much for a PP9, so it has to be powered by D type (R20) cells or rechargeables. Then the whole thing has to be mounted in some sort of case with a pistol grip so that one could take it out and point it at the offending cat. But one can't spend all day waiting for the cat to turn up. So I was pleased to see in a recent issue of "Everyday Electronics" a different design, this time using a 40KHz ultrasonic transmitter as a driver for a power ET feeding a standard loudspeaker. The ultrasonic generator was pulsed by a low frequency oscillator, so that the device could be left out in the open to run continuously, ideally from a car battery. I am hoping sometime to combine the two designs, adding the pulse generator from the EE design to the Elektor sound generator. What I really need is not to have to deal with any B.A.E.C. business for about a month !