This is a simple motion-activated buzzer. I made it to discourage my cat from jumping on the counter.

The circuit includes a HC-SR501 IR motion sensor, a 5V buzzer, an optional Arduino, and a 9V battery. The buzzer can be connected directly to the output of the sensor (middle pin). Alternatively, an Arduino can be used to drive a piezo buzzer and/or add additional logic such as timeouts.