Review on Stemedu Pack of 5 HC-SR501 PIR Sensor Infrared IR Body Motion Modules for Arduino Raspberry Pi by Shane Schobinger

Motion detection works quite well. I tried using this to activate a 3V relay directly and had trouble activating the relay. Using a multimeter, I was able to find out that the output from these motion sensors is only 2.8 volts, which is just below the voltage threshold for the relay. Fortunately, the project also includes a Raspberry Pi, and 2.8 volts is enough to register as a high contribution to the Pi. So now I use the sensor as an input to the Pi and the Pi controls the relay. This part is good for motion detection. Note, however, that you may have trouble detecting it on some devices.