I use those for presence detection in my house. 3D print a small case and for < $20 you have highly accurate presence detection that also works when sitting still. Ideal for automatic light and climate control.
My problem with similar sensors is that they don't work very well if you're sitting still around 3m away. Maybe my sensor isn't great, but when I'm sitting still at the computer, it frequently thinks I'm not there any more, which ruins any benefit.
The “Vital Sign Detection & Healthcare”[1] section is very interesting, thank you! I am looking for a way to measure the heartbeat of a single person in a completely non-invasive manner (so without contact) and I see that it can be done with Wi-Fi (2019 Paper [2]). However, I’ve noticed that there are a lot of methods (video analysis, thermal camera, etc.), but they are hard to find as ready-made products at a reasonable price. The simplest solution is under-mattress sleep trackers, but unfortunately they are not an option for me.
The gesture recognition sounds incredibly interesting. I work with vision based band tracking, and performance really isn't good enough for my application.
I wonder what kind of resolution you can reasonably achieve? Is it good enough to detect finger pose?
Aside: is there a way around IEEE paywall? I'd really love to read some of these papers
I don’t know who designed this, but they were a master of the black art of Radio Frequency waveguide engineering. I am impressed. The PCB, itself, is a major component. Not only for the patch antennas but also several RF filters, the local oscillator, and the mixer are all largely made from peculiarly-shaped PCB tracks. Apart from the PCB, there are only five “components" on the board. Five passive components to implement a Doppler radar module. C’mon. You have to be impressed by that!
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.
Non-ionizing radio waves are generally safe for humans.
The only mechanism besides ionization that could harm humans is through the transfer of lots of power into the human body (think soldiers keeping themselves warm by stepping in front of a radar emitter).
So let's try to do a ballpark estimate of how much that could matter.
I haven't found (from a quick search) any data regarding the transmission power, but the data sheet at https://fcc.report/FCC-ID/2AD56HLK-LD2410B-P/6620025.pdf says the average current consumption is 79mA at 5V, which means it uses 0.4W.
How much of that is actually transmitted? I'd guess 10%-50% (likely much less, but let's go with this more conservative estimate, from a safety perspective), so now we're in the range of 40mW to 200mW.
If you absorb 1/4th of that (again, somewhat conservative estimate; you'll likely also reflect some, and most of it is going to pass you), we're at 10mW to 50mW extra power that is absorbed by your tissue.
Again, this is a super high (and thus for our purpose, conservative) estimate. Somebody else in this thread mentioned microwatts being absorbed, which sounds much more plausible.
To put this into context, the base level of power that an adult human operates on at rest is about 100W. This is a factor of 500 to 2500 more than the power absorbed from our millimeter wave radar. Unless all the absorption happens by a very specific and sensitive part of the body (like your eyes or so), this should just be background noise.
If you want another perspective, you could try to compare it with whatever radiation (both RF and heat) that your phone emits, that you likely carry in your pocket for hours at a time.
> The only mechanism besides ionization that could harm humans [is heat]
This claim is, IMO, too strong given available evidence.
There are many chemical interactions with characteristic energies well below 1eV, or any reasonable threshold for "ionizing". Photons can couple with these interactions without ionizing anything. 4GHz range is probably fine, because the per photon energy is a small fraction of a mEv, but even then I would not rule out the possibility of multiple photons coupling to a structure without the imparted energy immediately being dispersed as heat.
Any time you have EM with low entropy/etendue, it is always theoretically possible for interactions to occur outside of the thermal regime.
Some phones have a SAR of almost 2W/kg, compared to that milliwatts of 10GHz RF are nothing. Not to mention that with standard energy dissipation formula of E~R^(-2) you're getting into microwatts at any practical distance from the antenna.
I have these all around the house, but not in the bedrooms. I use weight sensors to detect that someone is in bed and traditional PIR for motion in bedrooms.
Probably overly careful, but I didn't want to point a radar at my sleeping kids (and myself) for 12 hours per day. Similar for the WiFi access point upstairs, it's only in the hallway and not at maximum power.
> Probably overly careful, but I didn't want to point a radar at my sleeping kids (and myself) for 12 hours per day.
While I understand the sentiment. It's very very unlikely to be dangerous and there are plenty of other environmental dangers.
The biggest being the sun. But the most common man-made ones are probably auditory. Like toys and TVs being too loud or high-frequency sounds blasted from speakers in malls or under bridges to avoid "loitering."
I agree with you, especially regarding children and anyone who hasn’t explicitly made this choice, which is why I asked the question. The only thing is, I suspect we get scared by certain words, like “radar” and “microwaves”, and then we might spend all day with our heads next to a Wi-Fi router or a phone constantly downloading files on 4G.
For example: maybe the ESP32 transmitting the bed weight exposes us to more danger than the radar sensor (that can also be placed very far from the bed)? Maybe with our smartphones charging on the nightstand too.
I’m not a big fan of fear-based, illogical decisions. But, again, I understand perfectly.
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.
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).
Depends on the definition of "simple", imo. The first thing that comes to mind is research into materials with good (tens of dB), wideband absorption in the mmWave bands. It's an area of active research [1] [2] (just a couple articles from a quick google, so caveat emptor).
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.
I need to see a picture, but fyi some of components of an antenna are "electrically" disconnected, but still play a role, wave guides being one example.
Also note, if you are just receiving signals you have more freedom to experiment. Antenna tuning matters a lot more when transmitting (especially at larger powers). (Not implying that it doesn't matter with reception)
thanks for the "capacitance hat" search term. from googling it looks like this hat is electrically connected to the antenna? In my case if the "cap" touches the copper wire of the antenna - signal strength decreased. Only when I kept the cap close but not touching the antenna it performed well.
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.
Isn't the US basically the same? The US government gives massive subsidies, tax breaks or just has the military buy components for 10x the price (to offset commercial loses and R&D).
And creates regulatory capture for the rest, e.g. healthcare and big pharma ($4T a year), both of which too charge nontrivial multiples of what the rest of the world pays.
Not really. The American system of graft is more transparent. If there's a subsidy, there's a record of it. Tax breaks are public, tho there can be creative ways of couching them so they aren't obvious. Military expenditures are (with some very big exceptions) public record. And a US business under most circumstances can say "I'm not selling below cost".
None of this is really true of the Chinese system.
The government also stands to benefit greatly if they throw their weight behind smart ideas, just like VCs. It would be interesting if the government could fund itself with investment.
Through-wall 2.4Ghz Wi-Fi CSI radar can be done with $20 ESP32 boards, https://www.technologyreview.com/2024/02/27/1088154/wifi-sen...
Wifi sensing is interesting, but you can combine a $3 ESP chip with one of these for a much easier project at lower cost: https://www.dfrobot.com/product-2795.html
I use those for presence detection in my house. 3D print a small case and for < $20 you have highly accurate presence detection that also works when sitting still. Ideal for automatic light and climate control.
My problem with similar sensors is that they don't work very well if you're sitting still around 3m away. Maybe my sensor isn't great, but when I'm sitting still at the computer, it frequently thinks I'm not there any more, which ruins any benefit.
17 replies →
The “Vital Sign Detection & Healthcare”[1] section is very interesting, thank you! I am looking for a way to measure the heartbeat of a single person in a completely non-invasive manner (so without contact) and I see that it can be done with Wi-Fi (2019 Paper [2]). However, I’ve noticed that there are a lot of methods (video analysis, thermal camera, etc.), but they are hard to find as ready-made products at a reasonable price. The simplest solution is under-mattress sleep trackers, but unfortunately they are not an option for me.
1. https://github.com/Marsrocky/Awesome-WiFi-CSI-Sensing#vital-...
2. https://arxiv.org/abs/1908.05108
> measure the heartbeat of a single person in a completely non-invasive manner
NIST, "Monitoring Respiratory Motion with Wi-Fi CSI" (2022), https://www.nist.gov/news-events/news/2022/12/wi-fi-could-he... & https://doi.org/10.1109/ACCESS.2022.3230003
Google: Nest Hub sleep sensing for one person, with Fitbit Premium, https://www.techradar.com/health-fitness/how-to-track-your-s...
Interesting. I'd be interested in a breathing sensor that can work on someone working at a computer.
2 replies →
> chewing of people or animals
The ESP32 can detect the difference between a cannibal and a vegan?
That's impressive.
How can you tell someone is a vegan? Don't worry, this 1$ radar motion sensor can tell you.
13 replies →
Great links! I recall seeing somewhere that AMEX was investing in WiFi object detection…can’t remember exactly for what or how…
You can also use a single ESP and your WiFi AP as a source of packets.
These $1 boards also work through walls (I tried this).
I always go through walls.
— kool aid man
The gesture recognition sounds incredibly interesting. I work with vision based band tracking, and performance really isn't good enough for my application.
I wonder what kind of resolution you can reasonably achieve? Is it good enough to detect finger pose?
Aside: is there a way around IEEE paywall? I'd really love to read some of these papers
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...
Too fantastic not to submit: https://news.ycombinator.com/item?id=40834960
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.
Non-ionizing radio waves are generally safe for humans.
The only mechanism besides ionization that could harm humans is through the transfer of lots of power into the human body (think soldiers keeping themselves warm by stepping in front of a radar emitter).
So let's try to do a ballpark estimate of how much that could matter.
I haven't found (from a quick search) any data regarding the transmission power, but the data sheet at https://fcc.report/FCC-ID/2AD56HLK-LD2410B-P/6620025.pdf says the average current consumption is 79mA at 5V, which means it uses 0.4W.
How much of that is actually transmitted? I'd guess 10%-50% (likely much less, but let's go with this more conservative estimate, from a safety perspective), so now we're in the range of 40mW to 200mW.
If you absorb 1/4th of that (again, somewhat conservative estimate; you'll likely also reflect some, and most of it is going to pass you), we're at 10mW to 50mW extra power that is absorbed by your tissue.
Again, this is a super high (and thus for our purpose, conservative) estimate. Somebody else in this thread mentioned microwatts being absorbed, which sounds much more plausible.
To put this into context, the base level of power that an adult human operates on at rest is about 100W. This is a factor of 500 to 2500 more than the power absorbed from our millimeter wave radar. Unless all the absorption happens by a very specific and sensitive part of the body (like your eyes or so), this should just be background noise.
If you want another perspective, you could try to compare it with whatever radiation (both RF and heat) that your phone emits, that you likely carry in your pocket for hours at a time.
> The only mechanism besides ionization that could harm humans [is heat]
This claim is, IMO, too strong given available evidence.
There are many chemical interactions with characteristic energies well below 1eV, or any reasonable threshold for "ionizing". Photons can couple with these interactions without ionizing anything. 4GHz range is probably fine, because the per photon energy is a small fraction of a mEv, but even then I would not rule out the possibility of multiple photons coupling to a structure without the imparted energy immediately being dispersed as heat.
Any time you have EM with low entropy/etendue, it is always theoretically possible for interactions to occur outside of the thermal regime.
>soldiers keeping themselves warm by stepping in front of a radar emitter
Holy, did people actually do this? A quick search yielded no results. Not sure if thankful or not.
6 replies →
Some phones have a SAR of almost 2W/kg, compared to that milliwatts of 10GHz RF are nothing. Not to mention that with standard energy dissipation formula of E~R^(-2) you're getting into microwatts at any practical distance from the antenna.
I have these all around the house, but not in the bedrooms. I use weight sensors to detect that someone is in bed and traditional PIR for motion in bedrooms.
Probably overly careful, but I didn't want to point a radar at my sleeping kids (and myself) for 12 hours per day. Similar for the WiFi access point upstairs, it's only in the hallway and not at maximum power.
> Probably overly careful, but I didn't want to point a radar at my sleeping kids (and myself) for 12 hours per day.
While I understand the sentiment. It's very very unlikely to be dangerous and there are plenty of other environmental dangers.
The biggest being the sun. But the most common man-made ones are probably auditory. Like toys and TVs being too loud or high-frequency sounds blasted from speakers in malls or under bridges to avoid "loitering."
2 replies →
I agree with you, especially regarding children and anyone who hasn’t explicitly made this choice, which is why I asked the question. The only thing is, I suspect we get scared by certain words, like “radar” and “microwaves”, and then we might spend all day with our heads next to a Wi-Fi router or a phone constantly downloading files on 4G.
For example: maybe the ESP32 transmitting the bed weight exposes us to more danger than the radar sensor (that can also be placed very far from the bed)? Maybe with our smartphones charging on the nightstand too.
I’m not a big fan of fear-based, illogical decisions. But, again, I understand perfectly.
The site Microwave News keeps up on the latest health aspects of such things.
https://www.microwavenews.com
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 actual component costs about $5, if you want to roll your own with an ESP32.
The LD2410 (B) is another option and it operates over UART. It is a bit more expensive ~$5 but has more configuration options.
Get the B variant, you can configure it over Bluetooth from your phone. Otherwise it's a huge hassle.
That said, it doesn't work very well for me when sitting still 4-5m away, it thinks I've left.
These radars are designed to detect motion, not someone sitting still.
7 replies →
MCU rp2040 from Pi Pico also costs 1$. We really are in great time for affordable hacking
Now if only the GHz-range oscilloscopes came down in price ...
The competition between Siglent Rigol and Uni-T will most likely drive down the cost. From a high baseline but still.
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).
Depends on the definition of "simple", imo. The first thing that comes to mind is research into materials with good (tens of dB), wideband absorption in the mmWave bands. It's an area of active research [1] [2] (just a couple articles from a quick google, so caveat emptor).
[1] https://www.cambridge.org/core/journals/international-journa...
[2] https://pubs.rsc.org/en/content/articlelanding/2022/tc/d1tc0...
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.
I need to see a picture, but fyi some of components of an antenna are "electrically" disconnected, but still play a role, wave guides being one example.
Also note, if you are just receiving signals you have more freedom to experiment. Antenna tuning matters a lot more when transmitting (especially at larger powers). (Not implying that it doesn't matter with reception)
Just to clarify - I changed transmitters antenna.
2 replies →
A capacitance hat is a legit way to electrically lengthen an antenna.
I have had decent success with them when finetuning antennas for lower bands.
thanks for the "capacitance hat" search term. from googling it looks like this hat is electrically connected to the antenna? In my case if the "cap" touches the copper wire of the antenna - signal strength decreased. Only when I kept the cap close but not touching the antenna it performed well.
1 reply →
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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.
The BOM of this circuit really is very minimal because it’s a clever circuit.
It’s not without flaws (regulatory approval is non-existent for this) but they really did combine some dirt cheap components on to a dirt cheap PCB.
You, too, could assemble this circuit at scale for extremely low prices, no government intervention necessary.
Isn't the US basically the same? The US government gives massive subsidies, tax breaks or just has the military buy components for 10x the price (to offset commercial loses and R&D).
And creates regulatory capture for the rest, e.g. healthcare and big pharma ($4T a year), both of which too charge nontrivial multiples of what the rest of the world pays.
Not really. The American system of graft is more transparent. If there's a subsidy, there's a record of it. Tax breaks are public, tho there can be creative ways of couching them so they aren't obvious. Military expenditures are (with some very big exceptions) public record. And a US business under most circumstances can say "I'm not selling below cost".
None of this is really true of the Chinese system.
You make China sound like a VC,
Uber, AirBNB, etc ran at a loss to try to own the market.
The government also stands to benefit greatly if they throw their weight behind smart ideas, just like VCs. It would be interesting if the government could fund itself with investment.
1 reply →