Allow us to introduce Ted. Ted is a 13-year-old black-and-white de-sexed male moggie and is the much-loved pet of a family friend.
According to his owner, Ted’s had 13 years of practice getting humans to do exactly what he wants them to do. Want to be fed? Make a first-rate nuisance of yourself until a human complies with the goodies. Want to go outside? Dig your claws into the screen door, or claw the carpet or start ripping the newspaper under the food bowl to shreds. Any one of those actions is absolutely guaranteed to attract attention and gain the necessary compliance from Ted’s owner.
Apart from that and some minor indiscretions such as the occasional fight (and a trip to the vet for repairs), Ted has been relatively trouble-free. Until recently, that is, when Ted developed a rather revolting habit.
You see, Ted has the run of a downstairs living area at night, comprising a kitchen/dining room, a rumpus room and the laundry (with his litter tray). But just recently, after 13 years of being a good pussy cat, Ted suddenly decided that he was going to jump up on the kitchen bench at night and scent-mark the glass splashback in one particular corner of the kitchen.
The result each morning was a smelly liquid pool that had to be thoroughly cleaned up and the splashback and benchtop washed down with disinfectant – not a pleasant job. And he didn’t do it just a few times. Having started the practice, it quickly became a habit, much to his owner’s disgust and annoyance.
Ted’s human has a theory as to why he suddenly started doing this. Just before the first incident, he had been shoved into a pet box and unceremoniously carted off to the vet for his annual flu injection. And while he was waiting for said injection, Ted had been forced to share the waiting room with a rather boisterous and over-friendly Labrador.
Ted was not at all impressed with this and the subsequent flu injection only added to his trauma and bad temper. Having had his jab, he was taken home and released from the confines of his pet box, whereupon he immediately made plain his considerable displeasure by attacking his scratching post.
And then that night, the indiscretions started. Perhaps it was Ted’s revenge for the vet trip or perhaps it was to re-establish territory and to let everyone know who really was the boss. But whatever the explanation, the result was . . . uggghhhh!!
Shortly after he started his shenanigans, Ted’s owner asked me if I knew of an electronic device, perhaps an alarm, that would keep him off the bench. A quick search on Google soon revealed the “Ssscat”, a battery-powered device that combines a motion detector (presumably a PIR sensor) with a can of harmless, odourless spray. The device detects the cat’s movement out to about 1m and releases a brisk spray to warn the cat off.
That got me thinking. I had a spare PIR (passive infrared) sensor, as used in burglar alarm systems, plus an old analog telephone answering machine tucked away in a drawer. Could I combine them somehow so that the PIR sensor triggered the answering machine when movement was detected?
As with most telephone answering machines, this one had a message pushbutton that you momentarily press to play back the recorded message. In theory, it would be just a matter of processing the output from the PIR sensor to simulate this button press.
A simple transistor circuit was quickly lashed up on stripboard and this proved the basic concept. This circuit detected when the NC (normally-closed) relay contacts in the PIR sensor opened (ie, when movement was detected) and produced a brief low-going pulse at its output. This output was wired across the message button in the answering machine (to simulate the button press) and it worked like a charm.
By then recording a suitably scary barking sound on the answering machine, it just might do the trick. In practice, this was more of a demented WOOF WOOF WOOF WOOF WOOF . . . Get-Ooorrrf-There-Ted . . . WOOF WOOF WOOF Grrrr WOOF WOOF sequence, which I imagined would have the desired effect.