In this setup, the voltage for the piezo buzzer is less than 2 V. If the voltage for the piezo buzzer is increased to 12 V, the alarm can be very loud, up to 108 dB when measured at 30 cm (you need to change optocoupler 4N25).
可用于吓唬野兔，耗子，鹿等。The setup could be used in a vegetable garden to scare a rabbit, a rat, a deer etc.
Parts:
a PIR sensor
a piezo buzzer
an optocoupler 4N25 (with less than 2 V for the piezo buzzer)
2 resistors
a 5 V DC power supply
https://www.youtube.com/watch?v=k0s-O24c7u0?
