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cell phone jammer - cheap and easy

make a cellfone jammer out of old cellfone
  (+12, -5)
(+12, -5)
  [vote for,

Lets ignore the legal and ethical questions for a moment, and acknowledge that sometimes it would be nice to have a device to silence cellphones and their users in our vicinity.

You can buy ready-made units to do this on the internet, but they are very expensive. There are also vague plans out there to build your own (ie wave bubble) but they are way overcomplicated, and here's why.

The plans essentially have you construct the RF filter, amplifier and signal generator yourself. This requires obtaining obscure expensive components, performing surgically difficult SMD soldering, and using advanced test equipment to test and align the RF stages.

Why bother with this when most people already have one or more complete RF amp, antenna, and signal generator packages lying in their junk drawer: an old obsolete cell fone!

Simply program a small microntroller to generate a random-noise base signal and possibly the serial control strings for the PLL chip and wire it into the cellfone's circuit. Ok, that part might be a little tricky, but a little collaboration with the online hacker community could easily produce diagrams for a handful of the more popular phones showing which wires to solder into to inject your rogue signal and activate the RF amp section etc... But all the hard work of creating the RF signal generator and amp and wave shaping circuits and antenna would already be done for you!

So worst case you would have an old cellfone with a small circuit board sandwiched to it externally which would have a PIC or other microntroller on it.

Or even better, invest some time into deciphering how to download a new firmware program into the phone's main CPU. If you can do that, it would be simple enough to write a small program to do nothing more than send random noise, at full power, on the correct frequency.

An effective jammer does not necessarily need to be outputting a significantly higher power level than the devices it wishes to jam. most Cellfones use CDMA technology which means they are all essentially broadcasting on the same frequency, but they are all synchronized with the base station and transmit different "codes" within that frequency to allow the base station to decipher them individually. However, a single rogue phone on the CDMA frequency that is constantly blasting out garbage noise instead of carefully synched CDMA codes will drown out its immediate neighbors...

ServoMan314, Jun 19 2008

GSM Worldwide Percentage http://www.microsof...b01/02-19gsmpr.mspx
"Its focus is GSM because [snip] that supply over 70 percent of the world's customers for wireless services." [Klaatu, Jun 19 2008]


       //most Cellfones use CDMA technology//   

       30% of the world's market is not "most". <link> And, that doesn't even take into account the few that still use TDMA.   

       But [+] for the idea.
Klaatu, Jun 19 2008

       This won't work on the latest 3G phones; the code spreading used in W-CDMA (UMTS) is deliberately designed to get round this.
8th of 7, Jun 19 2008

       //Ok, that part might be a little tricky//
With multi-layer boards and BGA packages, this may be nigh-on impossible.
coprocephalous, Jun 19 2008

       Sorry [ServoMan314], one phone 'blasting' isn't going to do squat.   

       Let's assume you have a 'rogue' phone - if it's CDMA then it isn't going to raise interference over the whole band enough to jam the system. As [8th] says W-CDMA is more robust than the old 2G.   

       Now let's assume you are trying to jam the GSM system, which is TDMA based (everyone is given on of 8 timeslots). You pump out at full power and you have to do it over all time slots. Your battery is going to disappear like noone's business and it's going to all be in vain as the carrier-to-interference ratio will still be sufficient for the base station to recieve the signal.   

       All in all, it ain't happnin'
Jinbish, Jun 19 2008

       [+] any post which starts with the phrase "Lets ignore the legal and ethical questions for a moment"
FlyingToaster, Jun 19 2008

       ...except for the missing apostrophe in "Lets".
hippo, Jun 19 2008

       Jammer? Or....hammer? Hmm? One always works, the other, sometimes.
Noexit, Jun 19 2008

       I see some good points here, my idea isn't as feasible as I thought at first. Some of the limitations could be ovcercome by using three or four phones attached together. One or two would be boradcasting on CDMA channels, one on GSM channels, one would be on analog mode, changing to different analog frequencies every 250 mS. Wire up all the phones to a big fat lead acid battery in your backpack to provide plenty of power.... at that point it starts to be more trouble than its worth though...   

       I think i will focus instead on a personal-size EMP device to permanently silence those phones.
ServoMan314, Jun 19 2008

       //I think i will focus instead on a personal-size EMP device to permanently silence those phones//
Try explosive collapse of a TWT.
AbsintheWithoutLeave, Jun 19 2008

       [+] Maybe I am a total pig for giving this bun but sometimes people just need to hang up the phone. Especially those guys who use the earpiece and attempt to talk to you while they are on the phone and you have no idea who they are talking to.
Jscotty, Jun 20 2008

       I liked this, and then I loved it after ServoMans anno salvaging his idea with extraphones, a backpack mounted battery, and four more epicycles.   

       This project reminds me of the stuff in Make magazine. I also fondly recall when my Dad and I were fooling around with our Heathkit and we made a transmitter that turned every channel on the TV to white static when we pressed the key.   

       I am sad to say, however, that I found no rouge phones in my junk drawer - only 1 gray, one gray and black, and one a worn aqua. So I will not be able to help with development.
bungston, Jun 20 2008

       Well... with the car battery, you probably would need a car carrier. Depending on how people react to having their call cut off, this might make driving safer. I suppose use in the office might also be suitable.   

       Have you considered targeting the bluetooth wavelengths? Surely they are designed to use lower power to begin with, and at least then you could check for people holding a phone in their hand as proof that they are talking to someone other than themselves.
ye_river_xiv, Feb 03 2009


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