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Ask HN: Any tools to do generic WiFi imaging?
97 points by selfsimilar 4 months ago | hide | past | favorite | 68 comments
I have an older house (1950s) and I'd really like to see behind my walls without physically excavating so I can try to run some wires without encountering surprise obstructions. There are tools which use WifI to do detect humans[1] (https://www.tomshardware.com/news/wi-fi-routers-used-to-dete...) [2](https://news.ycombinator.com/item?id=40897828) but I'm looking for a way to use Wifi for more general imaging. There's a paper from 2017 ("Holography of WiFi Radiation)[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...] and many other scholarly papers about object detection via WiFi, but I haven't been able to find any off-the-shelf products/projects that would just build a 3D environmental density map without any object detection. The resolution doesn't have to be great - not looking for millimeter scale features e.g. structural weakness. Is there anything out there that comes close? Given recent archaeological uses of drone LIDAR and satellite tomography, I figure the software for interpreting this kind of data should be pretty robust by now, just maybe it hasn't filtered down to the consumer market.



Did you look into stud finders? They’re devices specifically made to detect pipes and electrical wiring in walls.

I love that you immediately went to WiFi for this though. Gotta love us tech people over complicating things haha!


Cheap stud finders are usually not great. The good ones are radar-based using similar frequency bands to wifi, but they cost lots of $$


I bought a thermal camera as a fun toy, it is great at finding studs especially for walls that face outside. Also you can see pipes easily when hot water is running through them.

But it was not cheap, cost about $300.

If you just want to just find studs, use magnetic stud finder. Cost about $10. Very easy to use and accurate.


Cheap stud finders are only accurate for some wall construction types. They do not work on my older style plaster and lathe walls, for example.


It might be possible to rent those nicer expensive ones. I'd check with local hardware stores.


You beat me to it. I'd say "usually not great" is putting it too lightly.

They're awful and if you don't realize they're awful then you might make regretful decisions based on their slanderous output.


The key to using stud finders is never trusting them and using multiple methods as a sanity check. For example, if it says a stud is as position x I'll sanity check by checking x-16" and x+16" because there should also be studs there. If nothing, maybe I'll try x-15, x-14, etc with the key being looking for some repeating pattern. Test moving vertically as well.

Combine with some knocking to feel/listen for a stud right against the wall vs an erroneously detected pipe 1/4" away.

I'll also use a neodymium magnet in certain situations to look for drywall nails/screws in conjunction with the other methods.

I don't fully trust any of these methods, but together they'll get you as confident as you can be short of having a couple hundred dollar detector or taking the wall apart.


If it's drywall you can use use a cheap stud finder or just yours ears and hands to find studs fairly easy. Plaster might be a little harder but still doable. Also, look for outlets as those are attached to studs. Depending on the age and construction of the house studs will be 16 inches apart.


...and can be rented.


Good instinct. This way we may, one day, get a tricorder to do these things from a distance, instead of ten different tools requiring extensive setup or very manual, meticulous process to use.

That said, if you want the job actually done any time soon, stud finder sounds like a better choice for now.


Stud finders are for finding studs, not pipes and wiring.


My house's previous homeowner found a drainpipe with his stud finder. He successfully marked it and put three screws in it.

The knock test shouldn't be skipped.


i've literally never seen a stud finder that doesn't also detect wiring and pipes


There’s too much variation in size of wires and pipes for it to work well. Wires may also be tucked back farther on the stud.


Wow, I've seen stud finders unable to find studs. You must live in an alternate dimension than the one I do.


I can see the appeal of wifi/radar-like detection.

Metal detectors work well, but he'd have to go over each wall meticulously. That would probably take quite some time.


5 GHz WiFi has a wavelength around 6cm so it can’t “see” anything smaller than approximately that. The things you want to see in your walls are below the diffraction limit for WiFi.


Imaging past the diffraction limit is possible in the near field

https://en.wikipedia.org/wiki/Diffraction-limited_system#Nea...


Yes it can. Diffraction limit relates to resolution in space, not scattering.


Get an infrared thermometer camera. I have a FLIR add-on for iPhone that lets me see studs, gaps in insulation, leaks... it's amazing what shows up.

It won't show cables, unless they're hot. You can use an AC stud finder for that.

You can flip the photo between the actual and IR image, and it overlays an outline over the image.

I make a video in and out of the house on the hottest and coldest days.


get a cheap 30$ endoscope from amazon. You can go in through power outlets/lightswitches etc. (probably want to cut the breaker first though). Handy tool to have around for all kinds of tasks.


I do have a cheap one, and hate it with an absolute passion. The problem is that it’s 30ft long making it an unwieldy thing to manoeuvre, and the wifi connection means I have to hold the phone at the same time.

I’ll definitely just get a trigger style one that’s only 3-6ft max next time.


What have been your top discoveries with the FLIR device?


Used my Infriray while house hunting. Possibly saved me big time €. Great also for spotting leaks any water in/on surfaces shows up quite clearly. In the same vein I use it to spot pet accidents.

The infriray is the Chinese knockoff brand which is actually better than flir because they don’t have to follow the US weapons law. So with it I get 30(?) fps instead of the ~5 with flir. Resolution is better too. It’s very sensitive and can clearly show a hand print on a surface for a minute or so even if you only touch it for 1/2 a second.


How did it save you money when house hunting? What kind of things did you detect?


I saved money by not buying a badly insulated house. There were some walls that shocked me in how little they were insulated. Also found a roof leak in another house


2.4ghz isn't great at detecting small obstacles like wires. There's a smartphone mounted device that can do this with 60ghz: https://walabot.com/


it seems to be optimized for dry walls. not sure how it would perform on concrete or other stone walls. Also I could not find a technical description how it works.


It uses the 60ghz radar from https://vayyar.com/ . I'm not sure how or if it works, I haven't tried it myself. They also sell dev kits: https://walabot.com/products/walabot-developer-pack-new


Have you tried this product? What do you think of it?


I have, it works ok, but I still use a regular stud finder most of the time, unless something bizarre is going on.

It has been useful to find pipes and electrical with some confidence, when their location is called for.


There are slightly nicer stud finders that use magnetics to detect live wires.


I would like to ask about what cool hacky attachments can I buy for a smartphone? Such as the thermal sensor mentioned in this thread https://news.ycombinator.com/item?id=40947011.

Amazon (in India) turned up nothing when I searched for cool attachments etc.


A truly "hackish" all-in-one test/measurement gadget is;

1) ExpEYES (Experiments for Young Engineers and Scientists) - https://expeyes.in/

2) SEELab 3.0 which is a upgrade of ExpEYES-17 - https://csparkresearch.in/seelab3

I have played with the previous version of ExpEYES (ExpEYES Junior) using the Python software. I also have the Seelab3 but haven't played with it yet. You can buy them directly (don't buy from Amazon due to markup) from any of the dealers listed in https://expeyes.in/hardware.html (i bought my Seelab3 from Novatronics).

There is a Android version of the software for both and so you can use them from your Smartphone/Tablet.


Walabot sounds interesting... but here's an article about similar tools: https://www.doityourself.com/stry/4-tools-that-can-see-throu...


Walabot DIY 2 in expert mode looks great! Definitely a bit more single-purpose for walls - only works on flat surfaces and probably has trouble w corners. Great suggestion, thanks!


For anyone trying to find and map electrical wires in their walls I recommend an inductive electromagnetic listening device like the FarField on Tindie. Plug a device into an outlet and you'll be able to hear the electromagnetic field of the wire in your wall via the FarField's stereo inductors. Soma Ether is also a good one but pricey.


Do you think you could trace copper pipes with the same technique?


If there's current running through them then yes. There needs to be a EM field for the device's inductor to detect.


An educated guess, but I'd think wifi signal object detection works on recording a baseline (empty room signals) and then doing a diff to the baseline when some object moves. If that's the case, you cound not use the wifi signal to map the static room itself.


you can have a monitor wifi card that moves on an axis


The "cheap and foolproof" way to do it is to cut a hole in your wall using a drywall cutter, and then feel with your hand. When you're done, you put the drywall back and patch it.

A drywall cutter is like a cast cutter; it works by vibrating and won't cut through wires, pipes, skin, ect.

If you don't want to use a stud finder, look for electrical boxes and remove the plate. That will tell you where a studs is. The rest of the studs are typically (US) 16" apart, but vary near corners and windows. You can often make an educated guess by knocking on the wall, the sound changes depending on if you knock on the stud.


As someone who has a 1930's house... the walls are probably plaster and wood lathe. Stud finders don't work very well if at all (https://www.familyhandyman.com/article/simple-hack-to-locate...) and cutting into the wall is like chiseling morter... because that basically what you are doing.

I love my plaster walls but dealing with pipes/electrcial is a huge pain.

I ended up teaching myself to do a passable job with repairing plaster and being content with how it is.


> studs are typically (US) 16" apart,

if they are not in the typical 16" and your not near a corner, try 18" and 24". I'm in a house from an infamous builder in the early days of the cookie cutter developer cutting every possible corner and then finding new ones to cut, they are definitely not on 16". In fact, I doubt there was a tape measure used at any time during the build, and most things just seem to be placed by "that looks about right" measuring system.

Electrical boxes and light switches are the only ones to have faith in. Other types of boxes (like cable or phone) could have been added after the fact and just be attached to the dry wall if there's even a box.


Absolutely nothing about this advice is "foolproof" especially on an older home. :)


There's a youtube channel called "Thought Emporium" which earned my sub many years ago with this video series on building a wifi camera.

Part 1: https://www.youtube.com/watch?v=o6WHhqDHSQ4 Part 2: https://www.youtube.com/watch?v=VABeN4uv03s


You would likely have an exceedingly hard time getting that sort of output using WiFi. The real issue is resolution (your ability to isolate your target from nearby reflectors) which, in range, is determined by your usable bandwidth. In the best case, with the 1.2 GHz (ish) supplied by WiFI-6, your resolution would be c/(2B) = 12.5 cm (with c being the celerity of light). Then, you would have to measure from at least three different locations to get a 3D image with sufficient resolution.

An alternative approach would be to use holographic principles, but these require you to measure very accurately the position of your transmitters, whether you use a multiplicity of them placed in different locations or employ Synthetic Aperture Radar methods with a single one (by moving your transmitter).

That is why using 60 Ghz radars (with 4 GHz of bandwidth and Angle of Arrival capabilities) at short ranges would probably be the most promising direction. You can get a dev kit for one of these from Texas Instruments or some other supplier for not too much money.


You're most likely overthinking it.

For your use case, you are primarily concerned with fireblocks. They are unlikely to exist in an older home. They can be detected with a simple stud finder, which range from $30 for the simple (good enough) kind, to $1000 for the radar kind. Simply knocking on the wall is also good for this purpose.

You likely have pretty fixed locations where you want the wires to exit into the room, so you can run a borescope through those places. These are also pretty inexpensive.


Instead of imaging with RF I'm wondering if acoustic devices (like the piezo transducers they use for echocardiography or prenatal imaging?) are available for homeowners. I've never heard of any and assume they'd be stooopid expensive.


I have remodeled a dozen houses of and haven't hit anything of importance. How:

Study the building code to determine with >95% accuracy what is behind the wall based upon surrounding fixtures.


You're going to be in trouble once you get to 20 houses :)


How do you know the building code from 100+ years ago? And whether they actually followed it back then?


100 years ago there wasn't really code, or electricity or extensive plumbing and everything is pretty obvious.

As for following - after the 50's everything was, in theory, built to code with exceptions.

Worse case really is hitting a water pipe.


Wi-Fi is nowhere near the effective resolution (forget the 6cm wavelength) you need and it's subject to all sorts of electro-magnetic artefacts. Stud finders.


Wifi mapping seems to require in place training specific to the space from what I can tell so don't think that'll work for your usage case.


Not using Wifi, but radar (so it needs at least a little movement) :

Pixel 4 have SOLI 60Ghz radar chips that allows various object categorization (eventually through walls applications).

Unfortunately the radar API was never released. It's unfortunate because a smartphone already has various sensors that ideally you'd like to use to do some sensor fusion using the camera and accelerometers to determine the position of the sensor and fuse the radar information in the map, like in those 3D photogrammetry room scanning applications.

Various object categorization :

https://www.youtube.com/watch?v=B6sn2vRJXJ4

https://www.theverge.com/2019/1/4/18168083/google-project-so...

Reverse-engineered attempt have been successful (but are locked behind paywalls (since it's a 2021 paper and sci-hub doesn't download recent (>=2021) articles during its trial in india) : "Reverse Engineering the Soli Radar API for Military Applications" : https://ieeexplore.ieee.org/document/9455321/

SOLI chips are based on infineon chips https://www.infineon.com/cms/en/product/promopages/60GHz/

You can find some demo board using the chip :

https://www.digikey.fr/fr/videos/i/infineon-technologies/get...

The walabot referenced in other comments is an example. Based on a different chip from https://vayyar.com/technology/ https://cdn.sparkfun.com/assets/learn_tutorials/7/2/4/walabo...


To me, WiFi is the X in an XY problem. An endoscope can be had for <$50 and will let you actually see what is going on.

https://www.google.com/search?q=endoscope

Good luck.


I think you meant borescopes[0]. Endoscopes are for medical use.

[0] https://en.wikipedia.org/wiki/Borescope


Tell Google, Amazon, and Ali Express. I was surprised to TIL myself.


That requires putting holes int he walls, doesnt it?


It takes 2 minutes to fix a hole with filler paste. You can also find a discrete place to put it, like the opposite side of the wall, under the threshold, behind a closet, using existing holes for wiring.

Drilling gently through wood or gypsum, you will notice immediately that you got through the sheet, before you can even hit a pipe or anything else behind. Then use that hole for the endoscopy to map out the beams and sensitives behind.


As i have not done this, my concern was drilling into wiring. I have a multi story house and i figured there could be writing that drops down through the walls that are not directly aligned with any switch.

Am i being overly paranoid ?


It’s good to be cautious. Drill slowly, with the assumption that you are going to hit something.

If you assume that your walls are 12mm gypsum and you stop drilling after 12mm, nothing can happen, start slowly, use the smallest drill, tape your drill as stop reference. If you hit open air you will immediately notice that you’re through. If you don’t hit open air after 12mm, stop, think, reconsider. Maybe you have thicker walls, maybe you hit a beam, etc.

The difference between gypsum, wood and steel is so big you will feel the difference if you go slowly. With gypsum you can almost push a needle through with hand force, from there make the hole bigger. Concrete walls are a totally different story.

Don’t have the intuition for how it feels once you get out on the other end of a plank yet? Dry run using a scrap piece of wood, where you can see the back side. Pin some scrap wire on the back and see what happens when the drill hits a worst case scenario. In a real wall the wire would be assembled before the final panel so there you’d have even more spacing. Logically, the wire will be behind the sheet, not embedded inside it.


I want to let you know that this advice worked a treat. Thank you for giving me clear direction and the confidence to do it.


> Am i being overly paranoid ?

This is a good quality when drilling holes in the walls. Fortunately, technology to the rescue! Plenty of metal/stud/voltage detectors on your favorite hardware or online store.


Not really. It's not massively likely, but if you do manage to drill into one fixing it is pretty costly. And that's not taking into account the risk of electrocution

Check out the Gosforth Handyman on how to avoid pipes and cables: https://www.youtube.com/watch?v=DeTXAPYzqpY

(Note that the rules he describes on where cables should be are from UK regulations - other jurisdictions will have different rules).


you have tons of holes in your walls already you can probably use one of those. Power outlets, light switches, etc.


sure，this is X。 real way i think is to ask for Decoration worker, not these work with a computer




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