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I am trying to build an arduino project with the following requirements:

  • Proximity sensors detect human presence and its distance
  • The sensors are hidden behind fabric
  • Range has to be 1-3 meters or higher
  • It is for indoor use

After a bit of trial and errors, here are my initial conclusions:

  • Available light changes, and sensors being behind the fabric, photo sensors are out of the question

  • Even though they're not supposed to be very sensitive to soft material, Ultrasonic sensors such as the HC-SR04 don't fit. Their echo bounce right off the fabric

  • Pir Motion sensors don't read through fabric either

I am starting to wonder if this is even feasible. Is there such thing as a proximity sensor that reads through soft material?

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  • \$\begingroup\$ Inductive proximity sensors have no problem detecting stuff thru material providing the material isn't conductive (too much) or ferromagnetic. Forget about the material and answer how you'd detect a human being's distance with a naked sensor (maybe that's a naked human with a sensor too LOL)? \$\endgroup\$ – Andy aka Aug 20 '14 at 19:24
  • \$\begingroup\$ Why did Stackexchange warned about low response activity on this site when I signed up? Overwhelemed by everyone's fast and clear answers. Thank you all. \$\endgroup\$ – Gregory M Aug 21 '14 at 1:54
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You could try a Doppler radar sensor such as the ~10GHz motion sensor modules which are available on the surplus market and from China.

http://www.ebay.co.uk/itm/2x-Wireless-Doppler-Radar-Microwave-Motion-Sensor-Module-/321126538770?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item4ac4a21e12

As unlicensed and likely unapproved intentional emitters, they are probably of dubious legality to use, if that bothers you. They're probably safe to use (aside from possible interference with other electronics) but caution is called for- some people get sensitive if they think their DNA might get scrambled or their 'nads nuked.

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  • \$\begingroup\$ What's the power on those? I'd gather for a short-range sensor, low power could be used, which shouldn't be remotely ionizing. \$\endgroup\$ – Jarrod Christman Aug 20 '14 at 19:41
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    \$\begingroup\$ One can buy an automatically door opener which use these inside, but, properly tested and approved. \$\endgroup\$ – EEd Aug 20 '14 at 19:41
  • \$\begingroup\$ @JarrodChristman Should just be milliwatts, and non-ionizing, so only danger would be thermal, and not much of that. There may be non-technical dangers, especially if kids are involved and you're "bombarding them with microwaves". \$\endgroup\$ – Spehro Pefhany Aug 20 '14 at 19:51
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    \$\begingroup\$ I did not use it professionally, but, these are time-matured standard mass production product. Proper one from Honeywell is security.honeywell.com/uk/products/intruder/sensors/duel-tec/… with spec. in similar range as to this one. Copying spec sheet, it output object speed Doppler signal 5mv, 31Hz per MPH (mile per hour). Sen -86dBm, ant gain 8 dBi, beam width 36 deg vert, 72 deg hori, pulse 5 micro second, 5 to 100% duty cycle, typical 20 meters working range \$\endgroup\$ – EEd Aug 20 '14 at 20:12
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    \$\begingroup\$ The Honeywell unit spec stated approval of PD6662 , TS50131-2-4 & EN50131-1 Security Grade 3 Environmental Class II. Not sure what it means, but, since major company make and sell it, it should be safe when properly installed (well, kids is another dimension and hard to say). Security alarm sensor are normally installed at ceiling looking down, so, not 3 inches away from human and it is mw and (may be, as not stated in Honey) short pulse at low duty cycle. \$\endgroup\$ – EEd Aug 20 '14 at 20:26
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I'd bet this'll do the trick:

Parallax X-Band Motion Detector ($40) http://www.parallax.com/product/32213

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  • \$\begingroup\$ Agree. Just read the Data sheet of above link, same 13 dBm EIRP power. page 6 and 8 has info on safety and disclaimer. \$\endgroup\$ – EEd Aug 20 '14 at 21:10
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Keep in mind that not all materials are opaque to near-IR light and that there is some fabric that can easily pass Near IR while blocking visible. So for us humans, we see the pattern and the device sees - nothing. Look to synthetic materials. Exposed and developed photographic film is transparent to Near Ir as an example. A loose mesh inside of a darken box will block less of a view and prevent humans from seeing inside the box.

Thermal IR (which the PIR sensor operates at) will not work as the fabric will be at a temperature that emits, it is like trying to look through gauze that is lit up.

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  • \$\begingroup\$ A web cam inside of a darken box with a loose mesh at front will do. Arduinod can hardly handle web cam. Raspberry Pi has a few examples code for web cam motion detection. I just got a RPi and tried 4 webcam, all ok. But, not tested the motion software. Webcam has IR filter, I did remove the thin filter and it see IR. Look at money note, the human image disappears and the under laying IR image revealed. I heard money validator machine use this and other techniques to check money. \$\endgroup\$ – EEd Aug 20 '14 at 21:19
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Inductive sensors spring immediately to mind, along with low-frequency (sub-ultrasonic) "click sonar" sensors. Even for ultrasonic, try speaker cloth - it's good to about 20KHz.

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  • \$\begingroup\$ Aren't inductive sensors limited to sensing metallic objects? Unfortunately, the type of fabric I am constrained to work with is rather thick. \$\endgroup\$ – Gregory M Aug 21 '14 at 1:44
  • \$\begingroup\$ Nope - I worked for years with inductive sensors in penitentiaries where they were used for escape detection. \$\endgroup\$ – TDHofstetter Aug 21 '14 at 2:55
  • \$\begingroup\$ Interesting application for escape detection in penitentiaries (which is specialize area and un-known to a lot of people). How it work? Coil size, Frequency? Coil at floor, wall or ceiling? \$\endgroup\$ – EEd Aug 22 '14 at 18:00
  • \$\begingroup\$ @EEdeveloper: can't (shouldn't) divulge too much detail, but these systems were used in perimeters to detect escapees crossing a fenceline. Very close to 100KHz. EDIT: Real PITA to tune... 8) \$\endgroup\$ – TDHofstetter Aug 22 '14 at 18:03
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Remember, if inductance will solve a problem for you, there is probably a way to solve that same problem using capacitance... Also, sound penetrates most fabrics nicely. It takes a sharp difference in acoustical impedance to get a reflection. Even infrared penetrates most fabrics nicely. You know that an infrared detector can sense humans well clothed, and animals that are nicely fured. (Or, is it furred? Or, is that even a word...?) If you asked a third grader to solve this problem, the third grader might say, "just throw a rock. If you hear the word 'ouch', then someone is there." i.e., we must think out of the box...

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