Inside a $1 radar motion sensor

Through-wall 2.4Ghz Wi-Fi CSI radar can be done with $20 ESP32 boards, https://github.com/Marsrocky/Awesome-WiFi-CSI-Sensing & https://www.cnx-software.com/2022/08/08/esp-wifi-csi-detects...

> Espressif claims it can also capture subtle movements caused by small movements such as breathing and chewing of people or animals in a static environment.. works with all ESP32 series microcontrollers including ESP32, ESP32-S2, and ESP32-C3, and does not require any changes to the hardware

2024 AI/NPU laptops with Wi-Fi 7 from Intel and Qualcomm can combine RF radar and on-device inference to identify human activity.

Related:

DIY Radio Telescope: Building a Camera That Can See WiFi (2019) https://www.youtube.com/watch?v=g3LT_b6K0Mc

Wi-Fi devices set to become object sensors by 2024 via IEEE 802.11bf standard (2021), https://news.ycombinator.com/item?id=40458766

How automotive radar measures the velocity of objects (2024) https://news.ycombinator.com/item?id=40768959

How Wi-Fi sensing of movement became usable (2024) https://www.technologyreview.com/2024/02/27/1088154/wifi-sen...

Reminds me of the HB100 teardown, but at least this one has an IC to make it more soothing that I don't really understand how it all works. https://www.allaboutcircuits.com/news/teardown-tuesday-hb100...

A little tangential, but are these things safe for humans? I have a couple of LD2410 devices and I’d like to use one of them with ESPHome in the bedroom. I did some research and they seem to be very low power and safe, but you know, before sleeping with a radar pointing at us all night long, I’m looking for as much feedback as possible.

USB C mm wave "radars" that hook to home automation are a thing.

Priced between 11 and 20 bucks they are fairly feature rich...

If you want to roll your own check out what the folks over at ESP home have going on (google esp home mm wave).

The LD2410 (B) is another option and it operates over UART. It is a bit more expensive ~$5 but has more configuration options.

MCU rp2040 from Pi Pico also costs 1$. We really are in great time for affordable hacking

I've played with these all the time! Great to know how they work!

More details https://github.com/jdesbonnet/RCWL-0516

Don't like these cheap sensors most of the time since most can't tell between a moving person and a fan rotating or curtain moving in a breeze. They are configurable to some degree but seems like not enough.

I'm sure there are more expensive options but they're more locked down/limited as well.

Wiener functions. I love 'em. An analog application too, top secret in WW II. Now also used in handset to eNodeB comms.

Since the user manual for this sensor mentions security monitoring as a possible application, I'm wondering: is there any simple way to prevent detection from such a mm wave radar? (Assuming for simplicity that we know where it's mounted, and in which direction it's pointing).

LD2410 work well too (do by the cable too, they have tiny non-standard pins)

Almost not related, but reminded me about recent RC hack I was working on.

I have an driveway alarm from mighty mule, which uses 433Mhz radio powered by two AA batteries to communicate signal from the coil sensor detection sitting next to the driveway to the base station in the house using OOK modulation. The stock PCB antenna was not very good at the distance I had it to work with, so I started experimenting with external antenna. I tried loaded antenna (i think it is what it is called - the one with the coil) and straight piece of wire.

I quickly realized that formula for 1/4 length is more of a starting point, and a lot depends on actual output components of the RC circuit (I have little to no understanding of how all of that works). I tried to cut slightly different sets of wires trying them next to HackRF/PortaPack showing me signal strength in the real time. Basically was eyeballing how strong and clear OOK bursts are, and how well or noisy they sounds through the built-in speaker... (again, I have no idea what I am doing...)

At some point I got tired of cutting wires and soldering them, so I tried to cut slightly longer wire and use thin piece of copper tubing to cover end of the antenna at various depth, hoping to simulate the antenna length changes. But at some point something weird to me happened - when just the tip of the antenna was covered by the tube, signal increased dramatically. I am talking about -55db - -50db to -36db on HackRF at the lowest usable gains settings...

I ended up with the antenna length slightly below 173mm ideal antenna length with a about 5mm-10mm "cap" made of aluminum foil tape (used for air ducts and such) at the very tip of the antenna. I also closed the other end of this wrap (in my imagination so that the signal does not escape this cap???). The cap itself is electrically disconnected from the antenna, it is just that - a cap.

I have no idea why it worked this way. I suspect by adding such a "cap" I modified something related to the capacitance or perhaps there is some resonance thing coming to play - no clue. But it became much more reliable at communicating over the distance I have it installed.

Perhaps someone who knows about such things, might give me a clue what I was dealing with.

Another thing that probably plays a role in this hack - outdoor transmitter is in the plastic box sitting vertically on a pvc pole, with batteries inside the same box. 1/4 straight antenna would not fit into it, but I also did not want to cut a hole at the top of the box to avoid water intrusion, so I pointed it down. But it also means it goes in parallel with the "USB" cable that connects to the coil-sensor next to the driveway. While system is not grounded, I suspect this USB cable is somehow became part of the antenna, since the best signal was when the line of sight between the antenna and base station, the usb cable was right behind the antenna. Distance between the antenna and usb cable running inside PVC pole is probably about 20-30mm.

You cannot try to rationalize the prices that China charges for goods. Everything is, more or less, state-owned and state-controlled.

If the Chinese government wants to undercut an American product, they will tell the manufacturer to drop the price to X, and the manufacturer will comply.

This also does not take into account Chinese currency manipulation.

Profit or loss be damned.