intercom using cell phones

[ujjvalshah]

is it possible to use the phone without a sim card or change the frequency at which the mobile communicates.. actually what i want to do is use a couple of cell phones or more to communicate between each other over a short range without using any service provider, i am ready to buy/make a kit which can work as a kinda service provider between those two or more mobiles in a certain region.. say of radius 300-500m......

on more larger scale.. i want to set up a kind of inter-com system using cell phones which people already have.. as of now.. the cell phone model nos are not important.. i just want to tweak them for my purpose


any suggestions, links or books will be very very helpful......

thanks

-Ujjval

 

[Sceadwian]

You can purchase two way radios for very low cost that have greater range than this. Modifying a cell phone to work as a walkie talkie would be illegal and extremely complex. They simply were not meant to work that way.

 

[ujjvalshah]

i dont want to use them commercially. all i want to do is experiment with cell phones.... dont worry about legal matters.. in my place, where i wil be experimenting, no one wil bother about some signals going here and there..
moreover i would really like to know how complex is it. If not practically,.. i would definately like to do it theoritically atleast.

 

[Hero999]

It makes no difference whether it's for commercial purposes or it's for fun, it's still illegal.

You would need to modify the firmware which isn't possible on most phones, some might use flash memory but I wouldn't even know where to start as far as re-flashing it with your own code is concerned.

How much experance do you have?

 

[ujjvalshah]

How much experance do you have?

how much do you think it requires?... and experience in wat?...

as i said earlier, i don thave any specifications for a cell model.. if there is an open source mobile out there which can serve my purpose, i am ready to buy it.

to introduce myself i would say that i am third year student doing bachelors in electronics and communication engineering....i have experience whcih makes me think that its possible to do it and how to do it.... but i dunno how to do 'how to do it'... i mean i can think of the programming.. but cant think of how to interface the mobile and tweak it with the pc so that i can program it.. as for now what i really need is a way to program the cell !!!

 

[Hero999]

Well you probably know more about it than me!

I'm sure you know how a mobile phone works and that it's the network operator who is licenced to use that perticular frequency band. Therefore hijacking it and using it for yourself is probably illegal.

I've heard about some phones that run on Linux which is licensed under the GPL which means if they have to release all the source code if they use it on their phone. Perhaps a Linux phone is your only hope as (unlike a proprietary operating system) it should be easy to hack.

 

[ujjvalshah]

well i have made some initial research on open source mobiles.. i have also come across some of the open source mobiles (eg. greenphone)whihc are completely linux based.. but.. BUT they all are open source with respect to the software applications.. i couldnt find anything that could change the 'internal' settings..

adn as far as the illegeality goes by hijacking the frequency band.. the place where i will be working is located on the outskirts and has no network connections... if at all, i will be working in an educational institute so i think they wil certainly help me in getting an amateur band for a short range

 

[HarveyH42]

Try this link: http://www.spletomat.com/tech/a1018

I looked through it once, just out of curiousity. Personally, I think its foolish to spend so much time with a mini-microwave oven pressed up against your head...

All cellphones are designed to be used through a provider, pretty sure they made sure it wouldn't be cheap or simple to bypass or steal the service. If I remember, cell towers are only about a mile apart. Doubt the phones have the circuits or antenna work without the towers large antennas and amplifiers. By theirselves, I'd imagine a range of only a few feet (shouting distance).

The guys on that hack site have been working to figure out how to make use of the microcontroller that's built in, don't think they are concerned about comunications. They do have it pretty well mapped out, and some software tools for reprogramming. If nothing else, it should give you some idea at just how much time you can expect to put into this project. It's a good thing, that you want to recycle electronic trash, but think your goal is unrealistic.

 

[mahmoud1919]

i know some information about the antenna but not in mobile phone and u cant use the antenna of the mobile as u think(because the mobile= microstrip or monopole antenna + ampilfiers) and it will be illegal because u will work in the band of the mobile and the company of the mobile buy this band this is the illegal part but if u want it u can make it by take band to ur self and make two devices if u want more information in the antenna part i guess i can help u i hope goodluck for u

 

[ujjvalshah]

for time being lets keep the issue of legal battles aside and concentrate on what engineers and geeks are suppose to do...

All cellphones are designed to be used through a provider, pretty sure they made sure it wouldn't be cheap or simple to bypass or steal the service.

firstly..
i am not at all saying that i would be using 'their' services. what i am proposing to do is send some audio signals from one cell and receive it on other cell... it wil be within a very short and specified region.

its true that the cell wil not work unless it has a sim card. but sim card has nothing to do with the sending data and receiving it. all it does is send some info to the cell and service provider....... now its also possible to get a blank sim and write the required data to it. so when a cell with this sim sends the info it will be received by my own small 'central hub' which will again send confirmation back thus bringing my cell to life.. a kind of my own small service

(i know all this might sound stupid and may be not be feasible.. but just out of curiosity .. i want to know how far can i go)

If I remember, cell towers are only about a mile apart. Doubt the phones have the circuits or antenna work without the towers large antennas and amplifiers. By theirselves, I'd imagine a range of only a few feet (shouting distance).

secondly...
as it will be only within small area, i think the antennas of the cell will be able to transmit the signals.. moreover i think the main purpose of the big towers is to handle traffic and contact the other towers.. . more towers are used to enable reusing of the same frequency..
though i am not sure but i guess mobile's antenna is powerful enough to send signal upto few hundred meters

 

[ujjvalshah]

i know some information about the antenna but not in mobile phone and u cant use the antenna of the mobile as u think(because the mobile= microstrip or monopole antenna + ampilfiers) and it will be illegal because u will work in the band of the mobile and the company of the mobile buy this band this is the illegal part but if u want it u can make it by take band to ur self and make two devices if u want more information in the antenna part i guess i can help u i hope goodluck for u

see as far as antennas are concerned then i think they send the signals on the frequency decided by the service provider which is ultimately provided by the sim.. so after duplicating a sim i think i might be able to program the cell to communicate at a certain frequecny.

can you pls tell me how far can the mobile send its signals?

thanks

 

[Sceadwian]

Simply setting two phones to use the same frequency will do nothing, there is a communication protocol between the individual phones and the cell phone towers, two phones can not be set to talk to each other directly. If you're going to be buying something anyways why not just buy walkie talkies? The range on consumer models is upwards of 5 miles. You can get two hansets for less than 60 dollars.

 

[ujjvalshah]

can you please give me some more info regarding this comunication protocol?

thanks

 

[CheapSlider]

All the previous posters are correct.

A cell does more than act as a relay. Putting aside the legalities, a mobile phone cannot communicate directly with another mobile phone. You would have to build your own private cell and switch.



If you still want to persue the idea of using cell phones:
The most common worldwide standard is 'GSM'. This an open standard and specifications are available from their **broken link removed**.

Have you considered using DECT phones ? The range is only 100m but they can have an intercom facility between handsets.

 

[RadioRon]

Wow, this is quite a lot of discussion for a question with such a simple answer! To the question from the original post "is it possible", the answer is simply, no.

 

[picbits]

You would be better off using old Nokia phones which have an LCD that many people have interfaced with.

Then get yourself a couple of low-medium powered transceivers. Use a PIC or similar to interface between the keypad, LCD and tranceiver and use the PIC to compress the audio / duplex it.

Medium powered tranceivers will give you a range of a couple of KM.

As far as using the actual existing RF stuff on the phone - I'd be with all the answers above and forget it.

You should be able to build a complete system for two phones for around $150-$200 including the transceivers but personally I'd buy a pair of PMR radios for a tenth of that .......

 

[parabyogi]

it is possible to send me intercom using cell phone circuit diagrame and it softwear details .

 

[HarveyH42]

it is possible to send me intercom using cell phone circuit diagrame and it softwear details .

As far as anyone can determine, this does not exist, most likely illegal, and probably considerably more expensive than a set of walkie-talkies, or just to pay for cellphone service in the first place. Sorry to break the sad news...

 

[ujjvalshah]

eol

ok guys.......
i accept..... its not posssible.....
well my aim was not to design something expensive and sensless.
i just had an idea which i shared....

thanks everyone...

 

[Super_voip]

if you have new mobiles with wifi and voip capabilities builtin, or PDA type phones it may be possible.