I'm currently building a sonar running off an arduino. My radar includes an ultrasonic sensor that will detect the distance an object is sitting at, and a servo motor that the sensor will sit on top of. I was wondering if it was at all practical to use a PIR sensor with this setup so that the radar will be able to detect people instead of just inanimate objects. I'm worried that while rotating, the sensor might just constantly stay high if it sees any moving person.

I have no prior PIR sensor experience and that is why I was hoping someone with some experience could let me know if this is worth pursuing or if will be too much of a problem.

The range of this sonar isn't too important. Although I would like it if it could scan several meters, under 1 meter would still be fine.

  • $\begingroup$ sonar!=radar... you can't rotate an ultrasonic distance sensor and measure with it at the same time; it needs to be still and have enough time for the sound reflections to arrive. the PIR should be independent. $\endgroup$ – dandavis Mar 25 '17 at 23:57
  • $\begingroup$ Yes I mean sonar not radar, I will edit the post. And in the practical sense, ya I cant do that, but this thing only has a range of a couple of meters at most so as long as it is rotating slow enough, that shouldn't be an issue for the ultrasonic distance sensor. $\endgroup$ – Sheriff Mar 26 '17 at 0:37
  • $\begingroup$ no, you have to stop stop until the bounce returns or accuracy will be way off or not working at all. you have a stepper, so that's not a problem. a pir or two might help to avoid the constant noisy pulse rotation. $\endgroup$ – dandavis Mar 26 '17 at 11:44
  • $\begingroup$ @dandavis - yes you can. That's how a grown up radar works. It's just that you have to be careful regarding the angular speed and effective beam divergence angle. $\endgroup$ – Paul Uszak Apr 15 '17 at 20:37
  • $\begingroup$ @PaulUszak yes well light moves pretty fast by comparasion. For OP: Instead of a PIR sensor you might consider a infrared distance sensor. Its much cheaper than a PIR sensor (you can get ones for 50 cents easily though better ones cost more) and is analogious to the ultrasonic sensor... only its more sensitive to surface reflectivity than the ultrasound. Or you can just get a lidar... $\endgroup$ – joojaa May 16 '17 at 16:02


A hobbyist level PIR sensor is actually a digital output device. It sees parts of 2D space split into various patterns and detects thermal transitions /movement between them. If the thermal emitter moves from zone a to zone b within a certain time, a positive detection event is output. Just like you see with your PIR burglar alarm. If the person doesn't move, there is no transition with a static sensor. But your sensor is rotating. However, this might even happen with no one around as the thermal background radiation will be constantly changing wrt the sensor. Again these rules govern the siting of PIR burglar alarm detectors. One thing. You might have to screen the sensor as they have very wide fields of view. That might increase angular accuracy. They only cost a few quid. Try it.

I mentioned that the detector is 2D which seems counter intuitive, but it has no ability to judge distance as far as you're concerned. All it sees is a map of zones spread out over the x and y axes. There is no depth perception that you can use. It's not analogue like the ultrasonics.

Note. If you have access to a scientific thermal sensor or camera you could judge distance as well so get 3D scanning as with ultrasonics, but I'm assuming that this is out of scope here.


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