Cheap Walkie-Talkies for General Instrumentation & Contr

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harrogate22
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Cheap Walkie-Talkies for General Instrumentation & Contr

Post by harrogate22 » Thu Jun 29, 2006 3:50 am

Any ideas on this please:

With two-way PMR446 radios being so cheap these days – less than GBP30 ($50) a pair – and with a range of 3Km (2-miles), it occurred to me that they could be very useful for remote control. The control of models, for example, with a greater range than the usual R/C systems.

In fact two pairs of these could be configured to provide a full-duplex instrumentation & control link. This could be with modems handling a reasonable data rate in the full speech bandwidth, or even more crudely, using DTMF signaling.

Has anyone here had experience of two-way radios in this context? I would be very interested to know if the practical implementation matches the theory.

Ken

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Post by JPKNHTP » Thu Jun 29, 2006 10:07 am

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Chris Smith
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Post by Chris Smith » Thu Jun 29, 2006 10:10 am

DTMF

Just add in a DTMF encoder and sender, and your walkie talkies can do anything the tone will let them.

Its old news, common phones did this trick for many years, and if you need to go wireless, DTMF.

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haklesup
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Post by haklesup » Thu Jun 29, 2006 4:41 pm

I bet you could hack an old 300/1200/2400 baud modem (external RS232 interface) to modulate onto the radio channel instead of the phone line it was intended for. A newer modem with better error checking would be even better but I am unsure of the max rate you could get away with. As long as you had a good SNR, it should run fairly fast.

Best part, you would have instant support in windows for addrressing the data. You could use a modem console and type real time or you could use DOS commands to copy a TXT file to the COM1 port and send the whole lot at once.

The hack would consist of level shifting and isolating the modem output so you could drive the radio microphone. Might even be able to do it using the Modem's built in speaker if it has one.

This is not much different than NASA talking to the Rover

Most of the FRS radios that have sub channels and some with only the base 14 channels broadcast a digital signal that is encoded and decoded by the radios (some even have security scrambling codes built in) so I don't think there is any prohibition against using an analog FRS radio to transmit your own digital signal as long as you do it from an unmodified transmitter.

These bands are used by many products and users so any application can't be critical since interference is likely at any time.

DTMF is fine but you only have 16 codes. Playing them is sequence gives lots more Op codes if you go that way but decoding is more complicated. two codes (beeps) yeilds 8 bits of data. One beep is equivelent to 4 bits.

As for these having greater range than RC, that depends on a lot of things beyond just output watts. What may be acceptable for voice may not be good for data or vice versa. I wouldn't guess if this method has greater range than traditional RC. It may be that voice could be transmitted further than the effective range using a typical RC.

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Chris Smith
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Post by Chris Smith » Thu Jun 29, 2006 8:16 pm

From memory, DTMF will give you 12 distinct codes.

If operating machinery is your goal, 12 should suffice quite well, is already operative, and everything is already off the self with no need for any modifications? Simple switches.

Speakers will accept a signal as is, receivers will accept signals already, and not much modification is necessary? Just flip and flop.

Your only limit is imagination and code quantity.

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Post by Robert Reed » Thu Jun 29, 2006 9:00 pm

If these radios have any quality to them, the IF passband will be on the order of +/- 8 Khz.You can judge you data rates accordingly, They may also have upper end limits of voice frequency (at about 3Khz ) Built into the mic amplifiers.As far as the FCC goes, your not under their jurasdiction, your in the UK. However the international committee on communications may follow the same guidelines.

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Post by JPKNHTP » Thu Jun 29, 2006 10:00 pm

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harrogate22
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Post by harrogate22 » Fri Jun 30, 2006 3:31 am

Thanks very much for your thoughts - Haklesup's look promising, with the potential (data-wise) to mange control of a reasonably sophisticated remote device.

I imagine the use of hacked modems would reqire the use of two pairs of radios in order to gain full-duplex. I'll have to check on whether the transmitter can be left switched to TX without damaging the output stage.

Thanks aagain.

Ken

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Post by Dean Huster » Sat Jul 01, 2006 5:06 am

DTMF provides 16 codes. The "extra" keys aren't found on telephones but are common on the DTMF keypads available from "surplus" electronics outfits.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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Post by Will » Sat Jul 01, 2006 10:30 am

Hi Harrogate,
Your idea seems fairly good but, if you were going to use this for process control you would presumably be doing it with some sort of relatively sophisticated computing device (A PC ?) in which case you can buy (Here, but perhaps not in the wilds of Wannee in Bradford - I'm a Geordie !) Wi-Fi (USB Plug-in) devices for $20 or so which could accomplish all you need without messing around with modems etc. this way you could achieve relatively high speeds and build your own security codes etc ? Have fun !
BB

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Post by JPKNHTP » Sat Jul 01, 2006 11:13 am

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