A PIR detector stops alarming after a while, if a human in the detection area doesn't move. How do I keep the alarming state ON as long as there's a human in the detection area even if he doesn't move?
PIR sensors do not actually sense the presence of a person, they sense the motion of some large thing that has a significantly different temperature than the surroundings. If a person stops moving then a PIR sensor will not detect the presence of the person; that's just a fundamental characteristic of the sensor.
A possible solution considering that the human has already been detected by the PIR sensor and your notes: "... stops alarming after a while ... How do I keep the alarming state ON ...?": A relay (K1) with two contacts (K1_1 and K1_2) can work such as a flip-flop (interlocking in true) storing the ON state. The additional switch shown (or transistor) could to RESET the system.
Assuming there's no large temperature differences in the room you can try to rotate slowly the detector forth and back few degrees with a motor mechanism. I bet the intruder cannot follow it accurately enough.
- after a detection start to monitor possible ways off. As long as nobody hasn't gone assume there's still someone (not good if there's enough room for 2 or more and they know the logic)
- use another detection principle - a static one which doesn't need motion. There are many of them. For ex. weight on the floor, changed normal camera image, changed IR camera image, changed ultrasonic standing wave pattern, incrased losses in a big induction coil, reduced capacitance between the walls, detected breathing sound, increased CO2 or other gases in the air etc...
- persuade the human to want to move. Have for example a massive tesla coil discharge to increase the restlessness