# Cell Phone Ringer Detector

#### walkonfire

Joined Apr 5, 2006
13
I am interested in what kind of circuit would reliably detect when a cell phone gets a call. The same concept as is used for those vibrating belt clips that were popular before phones had built in vibrators.

Can anybody point me in the right direction? I know a little about circuits, but not an EE or tech. Thanks.

#### Papabravo

Joined Feb 24, 2006
13,689
Originally posted by walkonfire@Apr 5 2006, 06:46 PM
I am interested in what kind of circuit would reliably detect when a cell phone gets a call. The same concept as is used for those vibrating belt clips that were popular before phones had built in vibrators.

Can anybody point me in the right direction? I know a little about circuits, but not an EE or tech. Thanks.
[post=15869]Quoted post[/post]​
It is not really clear what you are asking for. The cellphone itself will detect a call. Most of them will indicate it in a variety of ways. Do you want something connected to the phone? Do you want something which communicates with the phone wirelessly? Can you be more specific?

#### walkonfire

Joined Apr 5, 2006
13
I want to be able to switch on a DC battery operated device when the phone rings.

#### Papabravo

Joined Feb 24, 2006
13,689
Originally posted by walkonfire@Apr 5 2006, 07:14 PM
I want to be able to switch on a DC battery operated device when the phone rings.
[post=15874]Quoted post[/post]​
Let me try one more time. We need an idea of what you think can be detected. We need to know if a "wire" connection to the phone is possible. We need to know if you think a wireless solution is what you need. A cell phone is normally a closed system. The people who make it don't want you messing around with it. It probably does not have a little accessory port on it that you can connect to, so you can ask it things or it can tell you things. If you are willing to take it apart and hack the internals then have at it. If you want something to detect what the phone is doing without a wire connection, then life gets more difficult.

Maybe we should start with the phone. Can you get any technical information it?

#### walkonfire

Joined Apr 5, 2006
13
Well, this would be a wireless connection, that would either detect a change in magnetic field, RF or whatever else could be detected in the proximity of the phone by virtue of the fact that it 'woke up' to receive the phone call.

#### walkonfire

Joined Apr 5, 2006
13
Hmmm... maybe I'm not askin' the question right....

...You know those cheapo, $10, off-the-shelf, universal vibrating belt clips that you can buy for any ordinary cell phone, and not only can you clip your phone to your belt with it, but it (the clip, not the phone) vibrates whenever you get a call....?? ... how do those things (the clip, not the phone) know when to start vibrating, without actually being wired to the phone?? I'm pretty sure there's no telephone parts inside the clip... the darned thing just seems to know when somebody is calling. Who knows...maybe it's magic. I'm just a car mechanic. Here's a couple of examples: [attachmentid=1359] [attachmentid=1360] #### paultwang Joined Mar 8, 2006 80 Originally posted by walkonfire@Apr 5 2006, 05:38 PM I hate to be stupid, but I'm no circuit guy. I just know that somehow these old vibrators picked up on some transient phenomenon that occurrs when the phone gets a call. My guess is its a change in inductance, reluctance, RF or magnetic field. [post=15876]Quoted post[/post]​ Is the phone GSM or CDMA? GSM phones can easily induce currents in nearby speakers and electronics. #### Papabravo Joined Feb 24, 2006 13,689 Originally posted by walkonfire@Apr 5 2006, 10:13 PM Hmmm... maybe I'm not askin' the question right.... How about this... ...You know those cheapo,$10, off-the-shelf, universal vibrating belt clips that you can buy for any ordinary cell phone, and not only can you clip your phone to your belt with it, but it (the clip, not the phone) vibrates whenever you get a call....??

... how do those things (the clip, not the phone) know when to start vibrating, without actually being wired to the phone??

I'm pretty sure there's no telephone parts inside the clip... the darned thing just seems to know when somebody is calling. Who knows...maybe it's magic. I'm just a car mechanic.

[post=15886]Quoted post[/post]​
Sorry, I've never seen one of those devices. I couldn't tell you how they work but inductive coupling sounds plausible. I guess you could do an experiment to see what happens. With a coil of wire positioned in proximity to the phone see if your scope can measure some kind of signal above the noise floor. If you can find something non-random them we might have something to work with.

#### Papabravo

Joined Feb 24, 2006
13,689
Originally posted by dtiger2k@Apr 6 2006, 02:30 AM
Your cell phone searchs for towers when it is on (not continuosly but often) It sends a signal. When someone calls your cell phone the last tower that recieved a signal sends a signal to see if your phone is still in it's cell area. The phone signals back and the call is put through. This signal is picked up by the vibrating device and switches on the vibrations. I'm not sure if it's the frequency range or the small signal and relevant proximity.

I have to agree with papabravo in that your best direction would be to purchase one of these \$10 devices and hack it. I believe it has everything that you are wanting built into it. Instead of the motor that vibrates you could wire the device that you want to run.
[post=15892]Quoted post[/post]​
The frequencies of these signal are in the 1-2 GHz. range. RF design at these frequencies takes experience, equipment, and knowhow well beyond the reach of the average experimenter.

#### walkonfire

Joined Apr 5, 2006
13
Thanks guys for trying, anyway. I figured the off-the-shelf option was where I'd end up at. I'll let you know what works out.

#### electro-man

Joined Apr 9, 2006
4
Originally posted by walkonfire@Apr 7 2006, 01:17 AM
Thanks guys for trying, anyway. I figured the off-the-shelf option was where I'd end up at. I'll let you know what works out.
[post=15905]Quoted post[/post]​

WHOAH - slow down - i had a similar thing which flashed when a call came (currently destroyed). on close examination, it had, what appeared to be, a TINY transformer that i think picked up the electromagnetic feild of a call. I never got the chance to try it on my electric toothbrush, but i think its true. I duno - try dismantling one anyway to make sure

Cheers guys

#### danejo

Joined Apr 9, 2006
1
Hello i read in Nuts n Volts mag a circuit a guy built ..so his hearing impaired wife could tell if the cell phone rang without having it cliped to her ..anyways the cell phone does call out before it recieves a call to let the tower know its available where its at etc etc.. the circuit he built turned a light on in the house to let her know a call was about to come in .. i think it used rf sensing to a relay so you would be able to turn dc or ac on or off, i dont know if this will help the orig poster here ..but i have all the back issues and will search for it today and post the circuit
sorry if im off topic here ...thanks Dan