# How do I keep the “detected” state ON in a PIR detector until a human has left the detection area

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?

• I developed a system that is used to shut off a 40A 240VAC kitchen stove/oven system if the kitchen isn't occupied continually. It ensures that there is someone present in the room with the stove/oven and only allows very short periods of departure (settable.) The system involves the use of multiple ultrasonic sensors and a radar unit to achieve it. PIR wasn't that useful so it's not part of the system. – jonk Aug 2 '18 at 20:43
• I made an edit. Please revert, if you thought something else or confirm, if the edit is OK. – user287001 Aug 2 '18 at 20:55
• @jonk Did you use a doppler radar or an FMCW/chirp radar? – Henry Crun Aug 2 '18 at 23:20
• @HenryCrun No chirp, though if I revisit this I will probably look at that possibility (depends on cost, size, distances, etc.) I used doppler radar. If you need reference papers on using these for room occupancy, I can provide a few. – jonk Aug 2 '18 at 23:27

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.

• +1. The OP might require an "active" infra-red system that can spot something in the human body temperature range and block out background, low temperature and very high temperatures such as lamps. The problem will be that other devices such as TV / electronics will give off signatures close to that of body temperature and of similar size. – Transistor Aug 2 '18 at 21:29
• specifcally they sense angular motion across the plane of the sensor. They are insensitive to radial movement. Microwave doppler is the opposite, sensing radial, and insensitive to angular. – Henry Crun Aug 2 '18 at 23:19

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.

• But how will this reset when the person leaves? You are really only providing a latching motion sensor whereas the OP is looking for a presence sensor. – Transistor Aug 2 '18 at 22:28
• I believe the OP was looking for a simple solution - without the need for more advanced technologies. I tried to answer the question providing a simple and low cost one, considering certain requirements present on the short text provided by OP (I made that very clear in my answer). Regarding the reset: Someone could to open (personally or remotely) the "normally closed contact" (Reset switch or transistor on figure) after being advised of the intrusion, for example. – Dirceu Rodrigues Jr Aug 2 '18 at 23:46
• I believe he has a practical reason. An intruder has come in, triggered a sensor, but the sensor is not under monitoring because it's still not the time to close the doors. The questioner wants that a person who is in the detection area at the closure time would be detected, no matter if he moves or not. The relay system you presented is included to normal alarm central logik. – user287001 Aug 3 '18 at 1:12
• @user287001: OK. You may be right. So, I would like the OP provide his opinion on this. – Dirceu Rodrigues Jr Aug 3 '18 at 2:05

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.

Other possiblities:

• 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
• Your 'other possibilities' are so creative, but if I may, maybe not very practical? The last one made me laugh though, thanks! I wonder though (seriously): Wouldn't a PIR sensor moved back and forth by a motor go off when someone turns on something hot, like an oven or a lightbulb? – MartinF Aug 2 '18 at 22:52
• @MartinF I assumed there's no remarkable temperature differences in the detection area. Moving the sensor of course is useless, if the assumption was false. – user287001 Aug 2 '18 at 22:59
• @user287001 "persuade the human to want to move" A friend made security systems for warehouses, and they had a real problem with being too sensitive or not sensitive enough. Thieves had learnt to move slowly, as the sensors had to be turned down to prevent falsing. His solution was to have two thresholds. At the very sensitive (falsing) one, it gave a short blast of the siren and strobe lights. If it was a sneaky human, they would jump or run, and then trigger the higher threshold, sending an alarm call. – Henry Crun Aug 2 '18 at 23:31