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Thread: How to detect / find plastic objects outside

  1. #11
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    Sep 2010
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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...
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  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?

  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).
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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: - I Like this one but its out of stock :-( - way too big - 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":

    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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    Jun 2015
    Northern Virginia
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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 09:19 AM.

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