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Cat scarers becoming audible

When we moved into a house whose garden was frequently visited by defecating cats, the sellers left us a couple of ultrasonic cat repellers (they looked like PestBye's product) which seemed to be effective for a while, but then a younger guest said she could hear the sound, and a few months later we could hear the sound as well. As we weren't expecting our hearing sensitivity to be ageing in reverse, we looked for a failure mode that brings the cat-scarer's sound back down into human hearing range as the device gets older.

It turns out there is such a failure mode:

After cutting into one of our cat scarers with a small saw (which generated rather a lot of plastic dust and is not to be recommended), we were able to observe its circuit board. Apparently, it had been made 7 years earlier in 2016, had no country of origin declared, and somebody couldn't spell "sensor" but never mind. The circuit had connectors for solar input (but that wasn't fitted on this device), and legible components included a BISS0001 PIR controller chip, an HT7125 low-dropout voltage regulator, an NE556 timer chip (for keeping the alarm on after motion is detected), and D882 and 2N2222 amplifying transistors. As I'd suspected, the oscillator was not on an IC but was made of discrete components, and the biggest capacitors were 2 x 22 micro-farad and 3 x 220 micro-farad cylindrical electrolytic, with possibly some smaller ceramic chip capacitors as well (it was hard to see exactly). There were also signs of water ingress, possibly due to the rubber seal around the battery holder not being properly refitted after changing the batteries.

Thinking that mains or solar cat scarers might be less likely to be damaged in this way over time than battery-powered ones (because the event of changing the batteries is a time when things can go wrong), I replaced it with an unbranded solar-powered cat scarer bought online. This one turned out to be easier to get into with a small Philips screwdriver: it had a faux second speaker for graphic design, a D882P power transistor, a small transformer (I think), and an 8-pin "510D" chip which I'm guessing is a battery charge regulator but it's hard to make out the oscillator circuit. The seller incorrectly described it as having adjustable sensitivity (5 modes)---it does not, which meant we had no way to adjust it when it started triggering almost continuously (which is less effective) a few weeks after purchase (returns not accepted). There is an unconnected third socket on the circuit board which might have been meant for a control but it wasn't clear. The manufacturing date was given as 2022-10-15.

Any circuit may eventually be vulnerable, and having the frequency reduced to a level that at least some humans can hear seems to be a common failure mode causing many complaints about neighbours' cat scarers, so it's probably important to ensure they're still in good condition and this frequency-reduction failure hasn't happened.

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