Radio controlled switch

[Melodolic]

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?

 

[Dorian McIntire]

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?

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

 

[Melodolic]

Have you considered the idea of using a pager or cell phone as a
trigger?

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

You might also consider using a laser, since you say the
receiver is line of sight from the transmitter.

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).

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.

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!).

 

[chuck]

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).

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

 

[chuck]

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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Sorry.

Please delete my last post.

The fingers are fastier than the brain!

Chuck

 

[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!).

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

Dorian

 

[Melodolic]

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

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).

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

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.)

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

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

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

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.

 

[Melodolic]

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

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.

 

[James Thompson]

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.

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

 

[David Harmon]

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

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.

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?

 

[Roger]

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.

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

 

[Roger]

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

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

 

[Melodolic]

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

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.

 

[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

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.

 

[Melodolic]

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

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

 

[Melodolic]

How much switching on and off,

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.

and how much precision?

Not sure what you mean - precision regarding?

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?

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...

 

[James Thompson]

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?

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.

 

[John Fields]

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.

 

[James Thompson]

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.

 

[Si Ballenger]

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.