https://10maurycy10.github.io/projects/motion_sensor_hacking/
I recently got some cheap RCWL-0516 microwave motion sensors, mostly because I was wondering how China managed to make a radar for under a dollar:
Getting one working was quite easy, I just connected the VIN pin to 5 volts, GND to ground, and added a 1 uF decoupling capacitor on the 3V3 pin. When someone moves within ~5 meters, the OUT pin goes up to 3 volts for 3 seconds.
So it works, but how?
Generally, motion and speed sensing (doppler) radars work by sending out a continuous carrier and mixing the received signal with the transmitted carrier to create a low frequency IF signal. If reflections are coming from a moving object, the received signal will slowly drift in and out of phase with the transmitted signal, creating a beat frequency at just a few hertz. Because a motion sensor doesn't care about the exact speed, all the chip has to do is look for millivolt-level changes: all the hard work is already done.
(Score: 5, Informative) by anubi on Thursday July 04 2024, @11:30AM (6 children)
I leave a search string here for juicy tech info.
https://duckduckgo.com/?q=rcwl-0516+ghz [duckduckgo.com]
I've played around with these....they make a dandy "people detector". They are not really long range but can "see" right through plastic enclosures.
For example...detect people visiting your front door, whether or not they ring the bell. Package Delivery?
It will detect right through the door.
Or detecting anyone getting within several feet of my van. Mine had some directional sensitivity, and that could be enhanced uslng empty food cans
Yes, it detects cats too.
The same antenna ( PCB etch ) is both transmitter and receiver, and senses oscillator current draw changing as RF reflective environment changes from return wave aiding or damping the oscillator. It's looking for a low frequency current ripple as the Doppler return from a moving object interacts with the radiated emissions.
It uses very little power...
Mine had a tendency to phaselock to each other if several were on simultaneously within several feet of each other, and became unstable.
Shopkeepers would probably like to have one of these to let him know if anyone gets too close to his till.
Or remind him to check the bathroom for stragglers come closing time.
I got mine via AliExpress, several years ago, but I imagine they are all over the place now. These are a fun thing to play around with, and you might come up with things it can watch for you.
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
(Score: 2) by janrinok on Thursday July 04 2024, @01:06PM
I've just ordered 5 of them from Amazon - delivery tomorrow.
Something to play with when I have an hour or two spare.
[nostyle RIP 06 May 2025]
(Score: 3, Interesting) by epitaxial on Thursday July 04 2024, @03:42PM (4 children)
It would be great to tie these into my security cameras. The cameras record all the time but having a log of detected motion would be cool.
(Score: 3, Informative) by RS3 on Thursday July 04 2024, @04:59PM (2 children)
Yours might be somewhat older? Most newer security camera systems can be configured to only record when there's motion in the video feed.
I have one, only set up one camera on my front door area, and it works well. Only annoying thing is whenever the wind blows plants, leaves, branches, etc., the system records. But the playback is pretty easy and lets you skip to the next recorded event.
If your system can not be configured to only record motion events, then yes, you might be able to tie some of these motion detectors in.
However, you might want to check the link I provided above. In that article the author does some good analysis, including figuring out that these little motion detectors work well indoors where there's much RF reflection and activity, but they don't work well, if at all, outdoors. However, he figured out that by using two or more of them they can work outdoors. I think he modded one or both, but check it out- it's an easy read / skim.
(Score: 3, Interesting) by bussdriver on Friday July 05 2024, @09:49PM (1 child)
Camera motion detection takes electricity. it would be nice to combine these so the camera is off until actual interesting motion happens at which point an AI tracker could ignore cats etc.
(Score: 2) by RS3 on Saturday July 06 2024, @12:16AM
Reasonable point. I'm not sure how much power the camera system consumes. It may be significant. Ones I've seen, including the one I have, maybe use 20 watts or so. Mine doesn't say on it, but I'm pretty savvy with electrical things. The biggest power consumer in mine is a 1 TB spinning rust HD. It's very quiet, but I'm pretty sure it spins constantly. They could optimize power consumption by spinning it down when not needed. I'm sure better ones do.
Again, to be clear, I agree with your point.
I would make the point that there will be some latency if you power up the camera system. You might wish to capture some video before the motion event.
I'll also check- it may be that some (most?) camera systems are in sleep mode until needing to record.
(Score: 2) by RS3 on Thursday July 04 2024, @05:01PM
Correction:
(Score: 3, Informative) by RS3 on Thursday July 04 2024, @02:07PM
Here's some more info on the module, including more depth / detail:
https://lastminuteengineers.com/rcwl0516-microwave-radar-motion-sensor-arduino-tutorial/ [lastminuteengineers.com]
(Score: 3, Insightful) by VLM on Thursday July 04 2024, @02:58PM
The author would likely greatly enjoy getting a ham radio license and messing around with microwave stuff "for real".
One of the strangest counter-productive regulatory effects I've seen over the decades is wifi using ISM bands means consumer-ish product mfgrs no longer use ISM bands to avoid interfering with "important" signals like wifi. So mfgrs no longer see FCC certification nor do they use legal ISM bands. So, I'm not entirely surprised the author claims this runs at 3.1 GHz or whatever nonsense instead of somewhere legal.
(Score: 1) by rst on Thursday July 04 2024, @09:05PM
I wonder how different it really is from something like this: https://www.dsc.com/?n=products&o=view&id=1015 [dsc.com]
(Commercial motion sensor with PIR and microwave combined.)
The approvals documents say that this runs at over 10 GHz and of course this has some extra circuitry for the PIR and communication with the panel, and I'm sure having that fancy plastic case with the silk screened logo adds a lot to the price, but if you ignore those things for the moment, I wonder just how different it really is.