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Location devices

Discussion in 'General Electronics Discussion' started by Dhakr, Jun 9, 2016.

  1. Dhakr

    Dhakr

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    Jun 9, 2016
    I am posting to ask for help with creating two devices that will alert/alarm when they are separated by a preset distance. If you can imagine two small devices that are linked by some signal, other than wired, that will alert or alarm if they become separated by a distance that has been previously set in them. Any information on how or where to get started on this would be greatly appreciated. Thank you.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    This is a very tricky problem.

    And it is also one that comes up a lot.
     
  3. BobK

    BobK

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    If the distance is in 10s of meters or more, you can use GPS, otherwise there is no really good solution in general.

    Give us more specifics about the distances involved, and the environment in which they operate.

    Bob
     
  4. KMoffett

    KMoffett

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    You didn't say how far you wanted before alarming. Google: separation alarm toddler suitcase

    Ken
     
  5. Dhakr

    Dhakr

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    Jun 9, 2016
    I wanted the alarm to sound at approximately 10 feet. It would be used in a relatively low electronic interference environment. Thanks again.
     
  6. Bluejets

    Bluejets

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    Ultrasonic as in car garage parking sensor perhaps?
     
  7. davenn

    davenn Moderator

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    the problem is ... that Steve alluded to ... is that ultrasonics, like a radio link will not cut off at a specific distance

    transmit and receive variations along with obstacles in between will all contribute to wildly varying operational distances

    there is no real easy solution to this problem


    Dave
     
  8. Bluejets

    Bluejets

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    If one item is stationary then perhaps the ceiling mounted "laser" parking sensor then....?

    I think Op needs to be a bit more specific though.
     
  9. Herschel Peeler

    Herschel Peeler

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    One or both ends battery powered? How long for the batteries to last? A low power RF link sounds appropriate for this. Do you imagine this as a product for sale or just one personal item? Did you have a cost in mind? How about size?
     
  10. Bluejets

    Bluejets

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    hevans1944 likes this.
  11. Amar Dhore

    Amar Dhore

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  12. AnalogKid

    AnalogKid

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    It would take some calibration, but what about this -

    Without getting into the phase and Doppler tricks of real ranging devices because of size, weight, complexity, and cost, one fizix principle left is the Inverse Square Law. Thinking of an ebay $4 wireless doorbell, the transmitter is a CMOS inverter banging on a tank circuit. 4 of these sewn into a belt that is strapped around a toddler would give a reasonably consistent 360 degree radiation pattern. 4 receivers sewn into a belt that is strapped around mom would give a reasonable consistent receiving pattern. Calibrate transmitter output against a known receiver sensitivity, and calibrate receiver sensitivity against a known standard transmitter output power. The transmitters chirp once per second, the receiver needs to miss 5-10 chirps in a row to sound the alarm. A similar trick could be cone with Bluetooth to enable identification coding.

    ak
     
    Amar Dhore likes this.
  13. hevans1944

    hevans1944 Hop - AC8NS

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    You need to be more specific about the separation distance and the accuracy and resolution required to sound an alert or an alarm. For example: devices must be battery operated, 3 V coin cell preferred; range must be programmable from 10 feet to 30 feet with a resolution of one inch and an accuracy of plus or minus one half inch. This is for illustrative purposes only. We have no idea how you are going to use the two devices. The answers to minimum and maximum range, range resolution, and range accuracy will determine what technology is appropriate.

    A very simple technology is co-operative ranging. One device has a transceiver that transmits a "ping" pulse pulse periodically and then listens for a return "ping" from the other device. The device receiving the ping immediately re-transmits it with a measured and fixed delay. The first device measures the time interval between transmitting and receiving the "ping" and subtraxts the fixed delay to calculate distance. This is the basic principle used with radar as well as ultrasonic ranging.

    Passive ranging is less complicated than co-operative ranging, but it is also more limited by the reflective and scattering properties of the object whose range is being determined. Co-operative ranging is more difficult because the delay in re-transmitting the "ping" must be precisely known and constant because it must be subtracted from the two-way "time of flight" of the signal. Since light travels about a nanosecond per foot, the delay must be known very accurately and must be very constant. Using ultrasonic ranging is only slightly simpler, because of the slower speed of sound in air, but that would be my first choice for a prototype.
     
    (*steve*) likes this.
  14. BobK

    BobK

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    The trouble with what Hevans is suggesting is that it would not work if an object intervened. This is why we need to know more about the environment in which it will be used.

    Bob
     
    davenn and hevans1944 like this.
  15. hevans1944

    hevans1944 Hop - AC8NS

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    Well, as you noted, it doesn't work with ultrasonics if there is anything in between. Problem doing it with RF is timing constraints. So, yeah, let's hear from the OP about what he is trying to DO before providing anymore "advice" on how-to-do-it.
     
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