Walkie talkie as a wireless (robot) communication device?

[corrado33]

Nowadays, walkie-talkie's can work from miles and miles away.  (I've seen some that can work from 10-20 miles away.)  I read up on them a bit, and it seem they work on a half duplex channel, meaning you can either talk OR listen.  You cannot do both simultaneously.  I was throwing the idea around in my head of building myself a car alarm (as 400 bucks for a couple of sensors is a rip-off.)  Don't worry, hooking it up to the car would be the easy part.  The first thing I did to my car when I bought it when I was 15 was to take the entire interior apart, and fix all of the wiring that the previous owner had messed up/fried. 

Would it be possible to transmit data using actual walkie talkies?  Say, instead of using a the microphone, hook something else up to it?  Then on the other end you'd just have to decode what is being transmitted?  I mean, it's just a EM wave, just like radio stations, RC-remote controls, etc.  I don't see why it WOULDN'T work. 

The only reason I was thinking this is that it'd be a great, inexpensive way to transmit a signal from a car being broken into to a receiver that a person has, no need for it to be two way.  Then again I don't know of any wireless technologies besides bluetooth that I can manipulate.  Maybe we can simply by the transmitters that walkie talkies use, I don't know.    Anyway, let me know what you think.  I know this isn't technically robot related, but let's just imagine that I'd put a camera mount in the car and I'd be able to control it with my remote.  There, now it involves a robot.

[Soeren]

Hi,

Reaching 10 to 20 miles takes a good quality roof mounted antenna with a good amount of gain (but if you're 10 miles away, you cannot do much about someone breaking into your car either).
In reality, you'll probably have to settle for perhaps 2 to 5 miles, dependant on the environment (even less in a city with tall buildings screening it).

If you just want to have a signal when someone breaks into your car, you can simply transmit a tone signal, a sequence of tones or more than one tone at once, like DTMF, but if you go for that, transpose the tones up or down a bit, or you may get false alarms from people using DTMF to open repeaters on the 2m band.

On the receiving end, tone decoders could trigger an alarm circuit, so that you don't have to listen to traffic on the channel.

I assume your Walkie-Talkies are NBFM (Narrow Band FM) and even if AM, it will be "narrow band", so you are limited to frequencies between roughly 300Hz and 3kHz (and don't go to the limits, they're the first to change if the band is crowded).

The regular traffic on the channel you'd use is a risk factor, as it may stop the alarm signal from reaching you - a consequence of using an "uncontrolled/non moderated" frequency.

It could be set to send out bleeps to track and should then be made to continue when the ignition is off, that would give you a slim chance of using it as a very primitive Lo-Jack.

I'm not sure if it will be legal. It isn't in Denmark, but if anyone breaks into a garage I own, about 1 mile from my home, my priorities are clear - the fine would be negligible compared to what I have in my garage and the chance of anybody being bothered by a few bytes of transmission on a noisy Citizen Band is nonexistent and nobody can find a transmitter that has stopped transmitting anyway.
But you have to decide what risks you wanna take for yourself.

[corrado33]

Hi,

Reaching 10 to 20 miles takes a good quality roof mounted antenna with a good amount of gain (but if you're 10 miles away, you cannot do much about someone breaking into your car either).
In reality, you'll probably have to settle for perhaps 2 to 5 miles, dependant on the environment (even less in a city with tall buildings screening it).

Ok maybe I exaggerate a liiiiiiitttttle bit.   

If you just want to have a signal when someone breaks into your car, you can simply transmit a tone signal, a sequence of tones or more than one tone at once, like DTMF, but if you go for that, transpose the tones up or down a bit, or you may get false alarms from people using DTMF to open repeaters on the 2m band.

On the receiving end, tone decoders could trigger an alarm circuit, so that you don't have to listen to traffic on the channel.

Ok so if let's say something would set off the alarm, I'd transmit a set of tones (maybe corresponding to a random number, like how garage door openers work).  Then, on the receiving end... a tone decoder?  How would that work?  I was just thinking that instead of wires running to a speaker, they'd run to an ADC.  (Properly regulated of course) Is there something I could use that could limit what ranges get received?  Kinda like a high pass filter, then I could just keep my transmission above or below a certain threshold (hopefully away from human voice ranges) to lessen false alarms?  (I might be thinking about that wrong)

I assume your Walkie-Talkies are NBFM (Narrow Band FM) and even if AM, it will be "narrow band", so you are limited to frequencies between roughly 300Hz and 3kHz (and don't go to the limits, they're the first to change if the band is crowded).

I have no idea, I was just pondering, not trying to use anything that I had.  But, theoretically, if I had narrow band FM ones, could I use the antenna in my car to transmit the signal?

I'm not sure if it will be legal.

Really?  Hmmm... Where can I find laws regarding that? 

Last question.  How would someone go about transmitting things without the use of a walkie talkie?  Just with an antenna?  How would you even output the correct signal?  I guess you could hook PWM up to something, set the frequency to something in the FM range, tune a radio to it, then output tones depending on the duty cycle of the PWM?  Would that work?  Again, I'm just pondering here.

[Soeren]

Hi,

Ok so if let's say something would set off the alarm, I'd transmit a set of tones (maybe corresponding to a random number, like how garage door openers work).

You're thinking of Rolling Codes (which is absolutely not random, or it wouldn't work) and that needs a set consisting of coder and decoder.

  Then, on the receiving end... a tone decoder?  How would that work?

If you generate eg. two tones concurrently and send them, two tone decoders (or one double) will each detect one of them and if they both are detecting their respective tones, you have an alarm condition, so they could set off a buzzer or a pager vibrator.

  I was just thinking that instead of wires running to a speaker, they'd run to an ADC.  (Properly regulated of course) Is there something I could use that could limit what ranges get received?

I don't see where an A/D-C enters the picture?

  Kinda like a high pass filter, then I could just keep my transmission above or below a certain threshold (hopefully away from human voice ranges) to lessen false alarms?  (I might be thinking about that wrong)

As I wrote, you can only use the range 300 Hz to 3 kHz on narrow band comms radios, as the channels are too close for wide band use, so if you could get it through the W.T. (which you cannot) you'd take up several channels.
But this is not needed, just send two tones of 1073 Hz and 1378 Hz respectively (that is 14% above the frequencies used for the asterisk sign in DTMF. I'm basing it in DTMF as those frequencies are calculated to avoid issues with their sum and difference frequencies and raising them 14% to avoid  the chance of someone using DTMF on the frequency and giving off a false alarm (14% is close to midway between the two frequencies).

I have no idea, I was just pondering, not trying to use anything that I had.  But, theoretically, if I had narrow band FM ones, could I use the antenna in my car to transmit the signal?

FM is just a modulation scheme and has nothing to do with the frequency.
The FM broadcast band is 88 MHz to 108 MHz.
W.T. may be 27 MHz, the PMR radios is UHF (around 485 MHz) but this is just in DK, you have to find out what's used where you live.

Really?  Hmmm... Where can I find laws regarding that? 

I don't know where you live, so I really can't say, but Google around a bit and you should find something.

Last question.  How would someone go about transmitting things without the use of a walkie talkie?  Just with an antenna?

Well, if you are allowed to add a piece of cloth and you have two antennas, you could allways learn semaphore comms 

Seriously, the antenna is a very important part of a transmitter, but it can't stand alone.

  How would you even output the correct signal?  I guess you could hook PWM up to something, set the frequency to something in the FM range, tune a radio to it, then output tones depending on the duty cycle of the PWM?  Would that work?  Again, I'm just pondering here.
 

You are over complicating it.

From what you write, I can only advice you to mod a W.T. if you are gonna go with it, as I sense that HF isn't your strong point and to build such a system from the ground up takes a large amount of knowledge in the field - a transmitter is fairly easy, but the receiver takes a sound knowledge of how components and PCB behave at radio frequencies.

Besides the W.T.s you'd need a circuit generating the tones and sending them (appropriately dampened) to where the mic used to be while keying the transmitter. A small controller could do that with very little external components.
On the receiving end (your pocket), the speaker signal (dampened) would go to two LM567/XR567 or one XR2567. Their outputs AND'ed with diodes should go to the alarm circuit proper (sound/vibe/flash) which could contain a monoflop or a bistable circuit needing you to reset it to stop the alarm.

[macdad-]

Hi,

Ok so if let's say something would set off the alarm, I'd transmit a set of tones (maybe corresponding to a random number, like how garage door openers work).

You're thinking of Rolling Codes (which is absolutely not random, or it wouldn't work) and that needs a set consisting of coder and decoder.
...

Exactly, a DTMF mod/demod chip would do the job(A M8880 DTMF Tranceiver):
http://www.clare.com/home/pdfs.nsf/www/8880.pdf/$file/8880.pdf

[Soeren]

Hi,

Exactly, a DTMF mod/demod chip would do the job(A M8880 DTMF Tranceiver):
http://www.clare.com/home/pdfs.nsf/www/8880.pdf/$file/8880.pdf

Only if used with a different X-tal, as the frequencies needs to be changed (from standard DTMF) for better security
LM/XR567 tone decoders are a better choice, as they don't need X-tals.

[macdad-]

Hi,

Exactly, a DTMF mod/demod chip would do the job(A M8880 DTMF Tranceiver):
http://www.clare.com/home/pdfs.nsf/www/8880.pdf/$file/8880.pdf

Only if used with a different X-tal, as the frequencies needs to be changed (from standard DTMF) for better security
LM/XR567 tone decoders are a better choice, as they don't need X-tals.

Slipped my mind