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Proximity detector/switch

Discussion in 'General Electronics Discussion' started by Marsh, Dec 29, 2014.

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  1. Marsh

    Marsh

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    Dec 29, 2014
    Hello--I am a new member of this forum, in fact I joined to ask this question.
    I am trying to determine what type of device I would need to act as a switch or trigger by moving it to within 6 inches of another component. I don't know whether both would have to be powered, or if one could be of a non-powered nature, such as a magnet. Both components need to be rather small in size, say the size of a wristwatch or smaller. I know very little about electronics, and any help would be greatly appreciated. Marsh
     
    73cat likes this.
  2. Gryd3

    Gryd3

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    875
    Jun 25, 2014
    Only thing I can think of is IR.
    Same LED's that are in your TV Remote.
    Game Boy Color devices used (... attempted to use ...) IR as a near communication device.
    Older laptops had this too. Range was not too good, but 6 inches should not be a problem. Sunlight, and other IR sources will create interference however...

    Other than that... Ultrasonic would be too large, and radio detection of range that small is beyond my skill-set.
     
  3. Marsh

    Marsh

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    Dec 29, 2014
    Thanks so much for that input--the size is key here. Also the cost would have to be under a couple of hundred bucks to be commercially feasible.
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Funny you should ask this. I have an application in mind, inspired by all the personally-worn electronics (watches. fitness monitors, etc.) appearing on the market lately. There are several possible solutions that I am considering. One of the simplest is a bracelet that uses near-field communication to activate a resonant tuned LC circuit and detect the energy absorbed. The ubiquitous anti-theft tags found on high-value merchandise (such as compact discs) work on this principal. I am somewhat given pause by the realization that the "screening" devices used at retail store exits are huge! OTOH, RF IDs can easily fit in a key fob, or a badge, or on one of my credit cards, so I know that the technology exists to do what I want.

    A wrist-band transmitting in a GHz ISM band would probably work but I am concerned about battery life. Better would be a two-way near-field communication between a powered target device and a mostly inactive wristband. Bringing the wristband close to the "antenna" of the powered device would "wake up" the electronics in the wristband and start an exchange of validation information. I think these devices already exist, for example in the form of electronic badge entry-control systems. The badges may even "sip" power from the near-field emissions to enable their RF response.

    The Dayton Amateur Radio Association (DARA), of which I am a member, issues key fobs for electronic locks controlling entry to clubhouse facilities. You bring the fob to within an inch or so of the transmitter/receiver mounted near the door to activate the lock. No power necessary (as nearly as I can tell without destroying the fob) in the fob itself.

    73 de AC8NS
    Hop
     
  5. Marsh

    Marsh

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    Dec 29, 2014
    Interesting possible solutions, all. I am thinking another one might be a small metal detection application--they can be fairly small, such as devices used to find nails in walls, or hand wands used in law enforcement. Garrett makes one called the Pro-Pointer that I
    am going to test in the task at hand.
     
  6. 73cat

    73cat

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    Dec 30, 2014
    Great answer, I like it.
     
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