Radio controlled switch

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?

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Melodolic Spielberg
Reply to
Melodolic
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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

Reply to
Dorian McIntire

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!).

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Melodolic Spielberg
Reply to
Melodolic

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

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Reply to
chuck

Sorry.

Please delete my last post.

The fingers are fastier than the brain!

Chuck

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Reply to
chuck

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

Dorian

Reply to
Dorian McIntire

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.

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Melodolic Spielberg
Reply to
Melodolic

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.
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Melodolic Spielberg
Reply to
Melodolic

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

Reply to
James Thompson

On Sat, 17 Jun 2006 14:40:41 GMT in sci.electronics.basics, "Melodolic" wrote,

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?

Reply to
David Harmon

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 :-)

Reply to
Roger

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 :-)

Reply to
Roger

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.

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Melodolic Spielberg
Reply to
Melodolic

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. :-)

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Melodolic Spielberg
Reply to
Melodolic

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

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Melodolic Spielberg
Reply to
Melodolic

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...

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Melodolic Spielberg
Reply to
Melodolic

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.

Reply to
James Thompson

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The OP asked for "nearly 20km."  Your "2 miles" falls a little
short.
Reply to
John Fields

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.

Reply to
James Thompson

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.

Reply to
Si Ballenger

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