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Radio controlled switch

Discussion in 'Electronic Basics' started by Melodolic, Jun 17, 2006.

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  1. Melodolic

    Melodolic Guest

    I want to switch a light (or a circuit controlling a light) on and off from
    a distance of nearly 20km. There is line of sight between the two locations,
    across a valley. The set up is to be used for a one-off experiment in the
    UK, and is expected to last about an hour. I don't have a ham licence (but
    did do the non-morse course years ago), so suggestions for licence-exempt
    frequencies are welcome.

    Both items need to be carried - by human - to locations in countryside and
    operate on batteries, so I need a sensible size (no car batteries!) for
    each. The receiver's location is several miles from the nearest road. It
    would be good if the transmitter was a small pocketable hand-held device - I
    want to be able to operate a camera between switching the light on and off.

    My thoughts thus far are some sort of CW transmitter, with the actual keying
    being done using a PIC (I have some experience coding PICs, and making a
    working cct shouldn't be an issue). The receiver would listen for the series
    of pulses (say, morse for ON and OFF), and respond accordingly.

    Doing metalwork for making antennae is no problem - provided I have some
    idea of the shape(s) I need. On the assumption that the licence exempt
    frequencies are low power, would I be right in thinking that a directional
    transmitter antenna would be a good idea?
     
  2. Have you considered the idea of using a pager or cell phone as a trigger?
    You might also consider using a laser, since you say the receiver is line of
    sight from the transmitter.



    I'm not trying to lead you completely away from the idea of using RF
    equipment but an un-licensed receiver/transmitter pair with the range
    necessary is neither a trivial project nor recommended.



    Dorian
     
  3. Melodolic

    Melodolic Guest

    I haven't - how would I hook into a cell phone? Do something with the audio
    ring signal on the speaker?

    How could I be sure a laser was aligned correctly, though? The purpose of
    the experiment is to be able to pinpoint the receiver's location, as viewed
    from the transmitter - I have a reasonable idea of what way to look, but it
    isn't very precise (maybe within 2-4 degrees).

    Would 10mW at 865MHz be too feeble, then? I'm not above repeating the novice
    course and exam (and hoping an admin screw-up doesn't get in the way this
    time!).
     
  4. chuck

    chuck Guest

    That's a different objective entirely.

    What do you mean by "pinpoint" relative
    to 2-4 degrees?

    You want a radio direction finding
    receiver/antenna that will let you
    determine azimuth to better than one degree?

    Even if your antenna gives you that
    accuracy, how will you actually measure
    azimuth?

    Do you only want to sight the
    transmitter through 'scope crosshairs?

    Please tell us more.

    Chuck
     
  5. chuck

    chuck Guest

    Sorry.

    Please delete my last post.

    The fingers are fastier than the brain!

    Chuck
     
  6. I origionally read that you wanted to control something at a distance. Are
    you talking about an RF foxhunt?

    Dorian
     
  7. Melodolic

    Melodolic Guest

    Pinpoint visually - I want to view the exact location of a rock in relation
    to a particular moonrise. The rock is about 3m long and lies close to the
    horizon as viewed from the transmitter (on a hilltop on the other side of a
    valley). On rare occasions, the moon rises over this part of the horizon,
    and I want to find out how close the rock's azimuth is to that of the moon
    as it rises (likely that the moon's lower limb just clears the horizon at
    the azimuth in question).

    No, I want something to switch on a light that can be detected by the MkI
    Eyeball. Accuracy to a handful of minutes is preferred - I'm looking for a
    little dot just below the horizon, and I want to see where the moon is in
    relation to this as it rises. (I appreciate that atmospheric effects may
    impact accuracy.)

    It's not about absolute measurement, but comparison with the azimuth of the
    moon.

    Don't want to view the transmitter at all. :) I'll likely use binoculars
    to look for the light. I'll be shooting video of the moonrise, and it would
    be nice if I could capture a few frames showing the light.
     
  8. Melodolic

    Melodolic Guest

    No - in a nutshell, I want to view the location of a rock which is nearly
    20km away, during a particular moonrise, for the purpose of observing the
    difference (or not) in azimuth between the rock and the moon. See post to
    Chuck for a bit more.
     
  9. You could do the whole thing with light. Make your receiver sense a
    modulated laser. The light can be timed to run for a short time then go
    back off, after being triggered by the laser. You would only have to scan
    the area with the laser to get it to hit your receivers phototransistor.
    Can you see from the rock to your viewpoint that your camera is located? If
    so, then you just point the receivers optic to there and you are set.
    Much more simple to do that radio, less battery power and no license
    required. JTT
     
  10. David Harmon

    David Harmon Guest

    On Sat, 17 Jun 2006 14:40:41 GMT in sci.electronics.basics,
    How much switching on and off, and how much precision? Perhaps the
    easiest thing might be to talk up the idea at your local ham club
    and get a couple of hams to help out with your experiment for an
    hour -- one at each end?
     
  11. Roger

    Roger Guest

    Borrow a GPS reciever and get exact coordinates of both locations.

    Then post to an astronomers ng, I think you could make them quite an
    interesting question :)
     
  12. Roger

    Roger Guest

    Simplest interface I have ever seen is a photosensor over the display,
    just give the phone a quick ring and the LCD backlight fires the
    sensor. Just give a ring and hangup, dosn't even cost for the call :)
     
  13. Melodolic

    Melodolic Guest

    I have exact coords of both locations. It all becomes a bit sticky when
    refraction is taken into account - the actual rise point of the moon isn't
    neccesarily on the calculated azimuth. At a certain point, empirical
    observation is the way to do it. :)
     
  14. Melodolic

    Melodolic Guest

    As mentioned elsewhere, I'm not sure where the rock is within the landscape
    as viewed from the transmitter, so I'm not clear on how I can orient the
    laser correctly before the fact (considering that the purpose of the test is
    to determine the location of said rock within the landscape).

    Ease of hitting the target could be said to be a function of how much the
    laser disperses. The actual distance in question is 18.4km. If the laser
    alignment was out by 1 degree, it would need a dispertion half angle of 1
    degree to be in with a chance - but 1 degree is at 18.4km is 320m. Wouldn't
    the light be desperately attenuated if it had dispersed that much, over that
    distance, such that a sensor might have trouble detecting it? I don't know
    how much a cheap'n'cheerful laser disperses, and I have no idea how far, for
    a given power, the beam can travel and still be detectable by a suitable
    sensor. My gut feel is that it doesn't sound easy.
     
  15. Melodolic

    Melodolic Guest

    And doesn't involve butchery of the phone...
     
  16. Melodolic

    Melodolic Guest

    I'm thinking of something like 3 short flashes started by the transmitter -
    something that lets me get a quick visual check of the direction to look in.
    I don't want something that stays on until switched off. It should, however,
    be possible to capture the flashes on a video camera. So, hit the button,
    three half-second flashes, switch off and wait for next trigger, kinda
    thing. It would maybe be activated about 10 times or so.

    Not sure what you mean - precision regarding?

    A possibility - just contacted a local ham to find out about courses. I'll
    have to pick a fit-looking one for the receiver job, though...
     
  17. Just another thought: If radio is what you need, then look into the little
    FRS style of 2 way. They have a range of about 2 miles and they are not
    very expensive, plus you wont need the license to operate it.
    You could have your light sound activated from the radio speaker.
     
  18. John Fields

    John Fields Guest

     
  19. Ok, your right. Im off about 10 miles there :( I didnt think to convert it
    as im used to miles. Thanks for looking over my shoulder there. Looks like
    the ham radio friends is the best answer.
     
  20. Si Ballenger

    Si Ballenger Guest

    Line of site they have been known to make contact at 50 or more
    miles under ideal test conditions. Using dtmf tones might be a
    way to control several switches.
     
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