Wedlock movie type collar for child security...

Discussion in 'Wireless & RF Design' started by Gaurav Sharma, Aug 31, 2014.

  1. Gaurav Sharma

    Thread Starter New Member

    Aug 31, 2014
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    Dear Forum members,
    I am looking for a RF circuit which is similar to a device used in the movie Wedlock (released in 1991).This is a circuit in which there are 2 circuits (electronic neck collars) connected by RF.As soon as these 2 get separated from each other by a specific Radius of Distance...they sound Alarm.I would like to make such a project for security and safety of children who play outside homes.The base circuit can be kept at home in front of a parent and the other with a child.As soon as the child goes away outside the Specified distance,the alarm sounds at the other device which is kept with the Parent at home.

    Can someone please suggest me a circuit idea for the above device which can operate in the Radius of minimum 500 meters...

    Thanks
    Gaurav
    India
     
  2. jpanhalt

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  3. MikeML

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    500m is tough. License-free RF modules do not go that far; 50m, maybe...
     
  4. Gaurav Sharma

    Thread Starter New Member

    Aug 31, 2014
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    Thanks Mike & John for the kind reply.
    @john GPS Modules are expensive.I am making this as a prototype project for some fellow Engineering folks.The budget is limited.
    @mike I really don't see any problem with such kind of a RF project which is extending to 500 meters for an educational purpose and there is per say no voice transmission as such to cause any interference.
    Please help me if you have any circuit ideas on the same.

    Thanks
     
  5. JoeJester

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    "Carrier" interference more prevalent than a modulated signal's interference.
     
  6. jpanhalt

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    GPS modules are available on ebay for <$10. I am not recommending them, but to suggest expense as the major reason not to use that proven technology seems disingenuous.

    Since the alarm limit you set can be anywhere within a 500 m radius, a method based on static triangulation would seem to me to be impractical. Have you considered a time of flight based method, such as used in older distance measuring equipment (DME) for airplanes? http://en.wikipedia.org/wiki/Distance_measuring_equipment

    John
     
  7. MikeML

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    In the US, we have strict regulation of the use of the radio spectrum. It is covered in Part 15 of the Federal Communications Commission Rules. Projects such as yours are covered by the License-Free rules, where the effective radiated power, regardless of modulation, or lack of it, preclude reliably transmitting radio signals of distances much over a few 10s of meters through walls, buildings, trees, persons, etc., especially with small antennas.

    If I were contemplating a system like yours, I would periodically transmit a coded message from a transmitter attached to the wandering kid. If the receiving site (parent) fails to receive the coded message after three or so scheduled transmissions, that would alert the parent to go looking for the kid. Reception of any given transmission is a statistical probability problem. As the kid gets further away, the probability of missing a transmission increases...

    The idea of transmitting the kid's GPS position is a good one. If the kid gets out of range, at least the parent would know where to begin looking...

    I use a licensed radio link to telemeter the GPS-derived position of my airplanes. This system is a bit too big to stick on a kid, but it shows what can be done by marrying a GPS to a radio transmitter. In this case the receiving sites are on various mountain tops (placed there at their own expense by Ham radio enthusiasts), and the radio link operates in the 2m radio amateur band (properly licensed).

    Here is a recent flight...
    (zoom out the Google Map to see the entire flight path). Click on the red dots in the track history to see the path that the radio signal took to get to the internet...

    btw- the aircraft depicted on this flight is one that is my avatar.
     
    Last edited: Sep 1, 2014
    BR-549 likes this.
  8. skeptic

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    Mar 7, 2010
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    In the U.S. both 27 MHz CB and 462 MHz FRS are unlicensed and will go that far.
     
  9. alfacliff

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    neither 27 mhz cb or frs are allowe3d to build your own circuits, they must be type accepted. also, neither allow data type transmissions, voice only.
     
  10. Alec_t

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    Hey guys, the OP is in India. Perhaps India has no regulatory system for radio transmitters?
     
  11. BR-549

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    Sep 22, 2013
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    I have installed many of these 900 MHZ hoppers for data links. I have cheated sometimes with these....using beam antennas out west for long reach.....~30 miles. (and maybe a little increase in voltage). It never caused any problems that I heard of. I have installed many of the MURS radios also. These are great. A lot of power(2 watt) and beams. Voice and data. Did a lot of farm and ranch, industry, and environmental monitoring. With short high quality feed line and some elevation......Great range. Great un-licensed private radio systems any where in the US.

    http://www.stevenswater.com/articles/murs.aspx
     
  12. alfacliff

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    a beam antenna might not be any help mounted on a child.
     
  13. skeptic

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    I believe, in the U.S. there are FRS radios with integral GPS units for which the FCC specifically amended the rules to allow them to transmit the GPS coordinates digitally.
     
  14. BR-549

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    I have also heard the FRS radios have circuits that alert you when you get out of range of companion radios. With all these features, FRS would be the way to go with kids.
     
  15. alfacliff

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    one brand I lnow of has this feature, Yeasu.
     
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