Outcome

The device works successfully. After completing my fully-working device, I learned that IR sensor is better in terms of sensitivity than PIR sensor. I also learned thinking very basic and simple is better when writing a code. Drawing a flow chart of the tasks I want to accomplish with my code has helped me a lot to modify my code. Finally, I learned being patient and keeping myself motivated during coding! Public sources for codes has helped me a lot to build my code. 

Next steps:

1) Try with a IR sensor.

2) Try alternative ways to make the sensor less reactive to external factors, such as running the sensor through a tube (cardboard or PVC) to reduce the sensor to a narrowor beam of detection.

3) Add buzzer to put the house mate invading your personal space in panic.