Question
PIR motion sensor always on and stays on
Answer
Well, there are 101 possible reasons:
Sensor sensitivity set too high.
Sensor time delay set too low.
...
...
You forgot to read the user guide.
Suggestion
Use a bowl.
References
Introduction to HC-SR501 A complete step by step tutorial on Introduction to HC-SR501
TaoBao HC-SR501 PIR Sensor Module - ¥3.23
TaoBao DC 12V Water Proof Smart PIR Sensor Module With Switch - ¥12.80