How can I block a PIR sensor(passive infrared) to stop the alarm going off?
The sensor views the infrared image of things in its field of view. It there is a moving point of temperature difference with the background, it actuates the alarm. All you have to do is block the sensor field of view with something opaque to long wave infra red. Metals will certainly do that, but so will some plastics and many other materials. Sometimes you can just redirect the sensor away from the moving heat source (over the top?). Incidentally hotter or colder than the background works, and there is no real difference between animals and humans, we are the same temperature within the limits of size, clothing , fur etc.
