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Thread: How to detect / find plastic objects outside

  1. #11
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    Re: How to detect / find plastic objects outside

    jwatte was describing multilateration of an active transmitter using multiple microphones and signal processing (measuring time difference between arrival of wavefront at each microphone; more difficult than it sounds if not using an easily distinguished sound/frequency and/or many sources of noise, distortion, echoes, etc.) to detect its direction and distance from the microphones/receiver. i.e. the tracking collar using sound instead of RF. Could be as simple as a lithium coin cell and a tiny PCB with a cheap microcontroller and piezo speaker to make a chirp at a fixed rate after some length of time without its snooze button being pressed.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  2. #12

    Re: How to detect / find plastic objects outside

    Yes, I was assuming an active transmitter on the disc, and using a microphone array to triangulate phase. A chirp with a known frequency at a known interval is necessary to tell signal from surround.

    There are small and cheap acceleration sensors. PCBs can be made very thin -- even flexible, depending on specifics (I'd recommend rigid.) There are small and cheap piezo chirpers, lithium batteries (or supercaps!) and 8-pin microcontrollers. You can get this into a few grams and half a square inch or less. Make the thing only turn on and chirp after it's seen acceleration and then stopped -- so it's laying on the ground -- and time out after a few minutes to save battery/charge.

    Is $100 enough? It'll probably cost you more than that just to make the first set of PCBs, and the solder mask for soldering the surface-mount parts to make it small and light. And you'll likely need to re-do it a few times to get it right.
    Also, microphones, and multi-channel sound input, and a CPU + direction indicator output, will likely be another $100+ (I'd go for a Raspberry Pi 2 and a USB stereo sound input for simplicity)

  3. Re: How to detect / find plastic objects outside

    If I have to go the active transmitter route, I think i will need a lot help... I write software for a living but I am very much a n00b when it comes to designing/building electronics. Thanks to my high school electronics class 15 years ago i know how to read basic schematics and hold a soldering iron with out burning myself lol.. I'm rusty but ambitious :-)

    Would anyone be willing to partner up and help me design something like this? I doubt I can make something that will be small enough...

    I'm still very interested in finding a passive way of detection. I might start looking into active RFID as well.

  4. Re: How to detect / find plastic objects outside

    I spent too much time looking at uhf rfid stuff again... i found a few projects doing things like what i want but most have faded out and never got completed. I think to make this portable i need to accept a smaller read range and raise my budget. here are a few things i found, thoughts?

    http://www.edaboard.com/thread200506.html
    http://ams.com/eng/Products/UHF-RFID
    http://www.ebay.com/itm/Cheap-Mini-S...-/111631527104

  5. Re: How to detect / find plastic objects outside

    Anyone? Thoughts?

  6. #16

    Re: How to detect / find plastic objects outside

    Let's re-think this problem. You're looking for a disc in some field/woods/thickets.
    Discs, alone, tend to be kind-of hard to spot, even if you get them in neon-ish colors.

    However, the eye is good at spotting glimmering lights. And the environment of the disc ought to reflect/amplify some light source on the disc.

    Thus, the simplest possible solution to "how do I find my disc" is to get a coin cell battery, a small trickle-flashing circuit, and a high-intensity LED. And perhaps a very lightweight switch, if you don't want to use "insert battery" to turn on the flash. Mount this to the center of the disc. The disc will now make a flashing light, perhaps once every second, which should make it much easier to find. With the right brief flashing period, and flashing interval, the coin cell battery will last a long time (although you want to turn it off when not playing.)

  7. Re: How to detect / find plastic objects outside

    Do that, and add a chirping buzzer so that you can get a general idea of the location even if it's hidden in a bush or under leaves. Also helps in area with a lot of contrast (lots of sun and some dark shadows from trees).
    ---
    Personal blog: http://xevel.org
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  8. Re: How to detect / find plastic objects outside

    I like your idea and I have been doing a ton of research. I am a complete n00b and have never designed anything on my own. I've spent hours on digikey trying to figure out what I need, but I'm lost.. so I looked up a few kits to try to figure this out on my own:

    http://www.radioshack.com/radioshack...0/2770230.html - I Like this one but its out of stock :-(
    http://www.radioshack.com/velleman-d...t/2770101.html - way too big
    http://www.radioshack.com/velleman-m...t/2770231.html - also too big

    Can someone give me a hand? kick start me with a parts list or something? I want to learn and build something but I'm in over my head.

    On a side note, I came across this "Simple Low Cost UHF RFID Reader": http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

    They built a basic reader but the read range isn't that great. Any thoughts on how it could improve the read range to at lest a few feet?

  9. #19
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    Re: How to detect / find plastic objects outside

    I think it -is-possible passively. The key is (as others have pointed out) a matter of selectivity--you need something that 1. Presents a strong signal when interrogated, even if only a small portion has LOS. 2. Is different signal than anything else (as much as possible)

    i would suggest a NIR illuminator (IR LED array) with a narrow band, and then a cheap little board cam with a matched IR bandpass filter (cheap, filters can be had for $10-20). Then put retroreflectove tape on the disk. If you want even better SNRS, cover the retro tape with NIR transparent (but blocks visible) film--that way only your interrogating light gets retroreflective.

    that should work at really good range, even a small area can be readily detected (retroreflective >>> just reflective, as its 1/(2r)^2 intensity instead of 1/r^4), and the sensor / illuminator can be really cheap.

    it doesn't address situations where there isn't a LOS, but then the periodic acoustic beep could work, as Xevel said,
    Quote Originally Posted by Xevel View Post
    Do that, and add a chirping buzzer so that you can get a general idea of the location even if it's hidden in a bush or under leaves. Also helps in area with a lot of contrast (lots of sun and some dark shadows from trees).
    but I wouldn't bother with auto-detection of when to beep (accelerometer, etc) I would just make it active when you use it...a short (<50ms) beep every 10s coukd use an extremely small amount of power (microW average), can be built using low cost parts (small piezo disk buzzer small SMT discretes, home-made flex, and a really small LiPo or coin cell). For start/stop make a latching circuit that latches on/off when two contacts show lower R (with a finger?) or a very easy flex-circuit based switch (have a folding tab with a bit of tape that you can tape closed or open circuit depending which way you fold it).

    for RX, two microphones (be wary, most mics dramatically lose sensitivity above 15-20kHz, unless they are really expensive) record signal, either analog-filter or digital filter a bandpass around a high resonant freq of the piezo buzzer, then centroid each signal's envelope, and do a TDOA calc to determine azimuth. Moving a short baseline (hence the 10second interval) and repeat should give okay range estimate.
    Last edited by birdman; 06-30-2015 at 10:19 AM.

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