Cat scarers becoming audible

It turns out there is such a failure mode:

• Most electronic cat deterrents are based on a piezoelectric buzzer driven by an oscillating voltage, which usually means there'll be a resistor-capacitor (RC) oscillator somewhere in the circuit.
• To picture how an RC oscillator works, imagine taking the output of an amplifier, delaying it so there's a phase shift, and feeding the inverse of this back to the input---so after the output has gone up for a while, the input starts to be pulled down, and then after it's gone down for a while, it starts to be pulled back up, and so on. (In a real physics paper you'll want a more mathematical approach than that, but that's basically what's happening---except usually we don't bother to separately invert the feedback signal but just use an inverting amplifier.)
• In these circuits, the capacitor takes time to charge and discharge through a resistor (usually a variable resistor so you have some control over the frequency). I previously said here that capacitive reactance (inversely proportional to capacitance) will come into play, making loss of capacitance cause to frequency drops (and looked up some preprints trying to quantify the capacitance loss), but I later realised capacitive reactance does not affect the frequency in this type of circuit because we don't already have an AC signal passing through. Instead it's the increased resistance as the circuit board degrades which will drop the frequency. The capacitor takes longer to charge and discharge through more total resistance.

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 dual-timer chip (probably one half for keeping the alarm on after motion is detected and the other half for being the oscillator---this chip still requires an external resistor and capacitor to set its switching frequency), and D882 and 2N2222 amplifying transistors. 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.

(We stopped maintaining cat scarers after our local cats were rehoused: it turned out their previous human had been unable to provide them with litter, and fixing that solved the problem.)