Known Issues - leesm5/Arduino-PIR-Motion-Detector GitHub Wiki
Known Issues
Sensitivity/Accuracy
The main issue is the sensitivity of the HC-SR501 sensor. It is cheap and not accurate enough to be relied on for real security. It is perfectly acceptable for a fun project or annoying your housemates with a buzzer when they take too long of a shower.
Timer
Implementing a timer with the arduino based on a high output of the motion detector was more troublesome than anticipated. I got around this by counting up in a loop and basing the threshold times for each state around that count. For my system, 210 counts was equivalent to 1 second. If this timer was implemented, the sensor could be mounted to see when someone enters/exits the bathroom instead of being mounted near the shower to detect continuous motion.
Buzzer Volume
The piezo buzzer is cheap and found in many kits, but it is not very loud. To improve this project a louder speaker could be used to make the project more useful. A link to an example of this is below.
Startup Time
The HC-SR501 takes about a minute to fully boot up. This should be taken into account in given applications.