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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).
I'm not sure if it will be legal.
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 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?
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.
Hi,Quote from: corrado33 on May 10, 2010, 07:50:39 PMOk 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/$file/8880.pdf
Hi,Quote from: macdad- on May 16, 2010, 09:46:36 AMExactly, a DTMF mod/demod chip would do the job(A M8880 DTMF Tranceiver):http://www.clare.com/home/pdfs.nsf/www/8880.pdf/$file/8880.pdfOnly if used with a different X-tal, as the frequencies needs to be changed (from standard DTMF) for better securityLM/XR567 tone decoders are a better choice, as they don't need X-tals.