Long story short, I'm trying to build some form of device that you can wear and it will detect when something moves behind you. I wanted to use PIR sensors for the motion detection, but I have no idea if they can detect objects if you're moving. Any ideas?

3 Answers 3


OP didn't really specify the actual requirements other than detecting movement and wanting to use PIR while in motion.

PIR (Passive Infrared) implies looking for warm bodies. In general, PIR motion sensors look for the presence of thermal radiation that moves between the individual parts of the sensor. On TV it is shown that you can fool PIR detectors by carrying a large cold panel between you and the detector. ;-)

They also detect reflected thermal radiation that is moving. It does not need to be a direct view. So moving the sensor around outside would pick up a lot of reflected IR sunlight changes. And certainly incandescent lights would do the same since about 80% of their emitted energy is IR.

The way they are designed, (with lenses like a bee's eye) they are more likely to detect your own arms swinging back as you walk and generate false alarms that way. So you would need some kind of mounting arrangement that only allowed it to "see" directly behind. And even then only in a dark room.

PIR sounds like a lot of trouble for an application like this. (As in, probably not suitable)

Here is a decent instructable that explains how they work: http://www.instructables.com/id/PIR-Motion-Sensor-Tutorial/

If PIR is not a be-all requirement, here is an alternative to be considered, even though it would require some work to get it right:


Here is a nice, inexpensive radar unit that will arrive eventually on the slow boat, and the price is right. It could work well in motion if you program it to detect changes that occur at speeds faster than your own motion. And you would want to program a pulse gate it so it only senses objects at more than a certain distance behind you.

Anyway, in the spirit of hacking, I offer this idea:

LFS-DC04 2.7GHz Microwave Radar Module DC 5V


It is small enough to wear. But it is slow, so I wouldn't recommend using it to detect intruders with violent intent. I believe it would be good mounted on a bicycle-pulled trailer or something like that, to make sure you know if something is coming up on you too fast.

Product Introduction:

4.Input Voltage:DC 5V
5.Output Type:level signal output 
6.Connection Method:VCC,GND connect with positive negative input, OUT,GND signal connect with output 
7.Installation Height:1-3.5m
8.Sensing Ditance:5-8m
9.Delay Time:30s(quick testing mode is 4s)
10.Sensing Angle:360 degree

It is low enough power that it shouldn't fry your kidneys. Probably very similar to sitting next to a high-power WiFi router all the time, or like having a cellphone in your pocket with WiFi and 4G turned on all the time.

  • New information: I hooked up a PIR sensor and tried moving with it hanging over my shoulder. It would not stop triggering constantly. I couldn't hold still enough to make it stop. So I believe that isn't going to work for you.
    – SDsolar
    Nov 12, 2016 at 2:30

PIR sensors detect the movement of a warm body relative to the location of the PIR sensor. Therefore if the PIR sensor moves then the location of a static object relative to that PIR sensor will change and motion will be detected.

So no, PIR is not really suitable for this.

TBH I can't, off the top of my head, think of a simple way of achieving what you want short of image processing with OpenCV on something considerably more powerful than an Arduino.


A PIR is entirely passive. The sensor has an enclosed vertical and horizontal IR receptor which reacts to the change of IR coming through a faceted lens, to accentuate the effects of movement, striking the surfaces through a small window on top. Having a passive system attached to a moving body would result in mostly false positives. It could be worth looking at active systems such as video, microwave, or even ultrasound

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