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Author Topic: Mod an old 900mhz cordless phone?  (Read 3321 times)
KF5EGM
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Posts: 52




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« on: June 19, 2010, 09:22:38 PM »

ok so has anyone thought of using the tx an rx in an old 900 mhz cordless phone and base station? you get 2 transmitters and 2 receivers on 2 different frequencies that are well within the frequencies of our hobby, and the things are dirt cheap at garage sales or on ebay. I have been curious about this for some time now. The idea is to mod it and make either a fixed frequency or possibly tunable radio out of it. The value here is more in "geek value" or "street cred" or what not, rather than actual use. If anyone has any thoughts on this I would appreciate it. They are FM modulated, correct?
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AE5NE
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Posts: 91




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« Reply #1 on: June 21, 2010, 02:54:10 PM »

It is probably do-able.

Standard FM devices, but I notice they are usually pretty wide deviation.  Frequencies are likely synthesized inside of a controller chip that may be difficult to modify, but then again the things are already in the band - but on (for obvious reasons) noisy channels.


Many people have handheld FM voice radios. I think modifications turning the phone handset into a full-duplex data transceiver would be well received. perhaps a PIC with G3RUH or FM-FSK modem and TNC firmware?
« Last Edit: June 22, 2010, 08:28:43 AM by Joe K » Logged
AE5NE
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Posts: 91




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« Reply #2 on: June 22, 2010, 09:55:51 AM »

I found some PDFs with phone schematics here
http://www.electronica.ro/service_manuals/panasonic/cordless_phone/


it looks like some of those panasonic phones use a commodity RF module. Extracting this RF module would yield you a FM transceiver with a digital PLL interface.

such a module: http://projects.uniprecision.com/sc_upload/images/B150_DATASHEET_RAW.pdf?PHPSESSID=8a9afe06d503ee44da49f86d234fbb8b


« Last Edit: June 22, 2010, 10:05:26 AM by Joe K » Logged
WM9V
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Posts: 106




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« Reply #3 on: August 19, 2010, 12:58:56 AM »

ya we though about using old vtech phones
problem with the cordless products is that a lot of them were spread spectrum
only the early ones were fm
that and the firmware stuck them on preassigned channels
not to mention the low output power
better off at the batlabs site
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KE3WD
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Posts: 5689




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« Reply #4 on: August 19, 2010, 07:31:09 AM »

Quite a few of those things were channel hoppers and not really true spread spectrum.  

When the channel selected has a signal drop that goes below a set threshold, the thing simply switches to another assigned channel in an attempt to get a better signal.  There is a little data dance between the two units involved, of course.  But once on the channel, it stays there unless and until it detects another signal drop.  This works for signal interference more than anything else, changing channels is not likely to yield a better signal one for the other if the freq area is free from interferences and noise. 

Shouldn't be too hard to defeat that channel hop feature.  Look for the single data bit connection(s). 


« Last Edit: August 19, 2010, 07:34:38 AM by Clark McDonald » Logged
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