PIR Sensor vs ultrasonic range finder vs?

Hey all,
I’m looking to create an ‘invisible’ trip line motion sensor. And I’m going back and forth on whether to use a PIR sensor or an ultrasonic range finder or if there’s another perhaps better idea that I’m not aware of.

I’m poking around but I do believe that people work better than search engines so if anyone has any strong opinions/ideas one way or another I’d love to hear them.


Laser + photo resistor? Much more “tripwire” like …

@bakercp that was my first thought but we are tracking street traffic and so I’m not sure how possible that would be since I don’t believe we are able to install anything on the other side of the street…

What kind of volume will you be dealing with and how precisely do you need to count people? “Counting” people with PIR and / or ultrasonic sensors is nearly impossible in crowded scenes. So, I guess the question is what will your data be controlling and how important is the individual count vs. a rough estimation of “activity” on the sidewalk?

Oh no nothing that complicated. the idea is really to just notice street traffic and use it to trigger animated 3d models to move.

Essentially, a person/car moves through this ‘trip wire’ and that gets detected and it sends a message to the app to instantiate and animate a new model basically. I’m honestly not expecting that much human traffic because this will be installed outside in minneapolis in January in a spot where there won’t be a bus stop so I’d imagine be picking up cars mostly.

The individual count is not a factor at all. I really just need a reliable way to determine if a car crossed a certain plane in front of the building and then use that signal to trigger some motion that is projected on to the building’s windows from within.

I’m a big fan of optical flow for that kind of thing.

@jefftimesten just made a nice demo for something like this:


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@bakercp I’ll check that out! though this will be installed in the evening so idk how well a regular rgb web camera will work for that…

I don’t know the regulations in MN, but in Austria, there’s heavy regulations concerning anything like projections or moving lights just beside the street, to avoid distracted drivers and accidents etc.
I’d recommend to make sure that what you want to do is legally OK (especially when you consider how lawsuit-happy your society is)

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@bilderbuchi Yea we looked into that. Essentially we will be projecting not on to the street but within our building. It will just be visible from the street. though the data will be pulled from the street via a covert motion sensor-microcontroller setup.