Instead of a servo you should try using a solenoid to pull the trigger. They are noisier but much faster. By the time the rat hears the noise it will be too late.
I would but that means having a 12v battery and a solenoid not to mention a relay and a switch along with additional wiring, yes the solenoid would be faster but I don't have the money to buy all the parts, I've got gobbs of servos so I might be able to find a faster servo and quieter one, I have some Digital high speed ones I could use, and they are metal geared so they are very quiet compared to the plastic geared, much finer teeth on the gears......
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