well to be honest I do only want to know if someone is around , the number of people is irrelevant I only need to differentiate between 0 and any. From what you say about it only detecting change though it sounds as though a still person would be counted as 0. if this is the case I think I will use an ultrasonic one
Depending on the room size, you'll need a couple/few.
you've hit the nail on the head I want it to actuate the entire time someone is in the room at a constant intensity regardless of the number of people (unless that number is 0) , sorry that took so long to get out.
retro player that would actually be the best thing ever , it would be so cool if it only started to denature if you were actually looking at it , it would bring a whole new level to the installation about observation and just be great to see people flummoxed as they realised their gaze was destroying the film.so from what your say id need a camera that picks up IR , facing in the audiences direction from the projection possibly mounted under the bottom edge of the image which will most likely be on a canvasshttp://rocketbrandstudios.com/store/pcb-s-and-boards ?and then presumably an IR bulb or IR source?
An ultrasonic sensor just tells you the distance to the nearest reflecting object that is within range, so using this to determine whether there was a person in the room would require you to perform a three-dimensional scan of the room and then compare that against the results of the same scan when the room was empty. As long as the person was close enough to the sensor to be detected, and was not hidden behind something else, they could be detected. But it would require quite a complex scanning process and more data than a typical Arduino could hold in memory, to perform it.
The sort of PIR sensors used in security lights and alarm systems are very good at detecting people moving within a room and can detect even very small movements. As long you don't need to detect the presence / absence very promptly, and a 'false negative' (reporting the room was empty when it wasn't) was tolerable, that seems the most practical approach. The idea would be that the room was assumed to be empty if no movement had been detected for some time - for example, five minutes.