Looking for a cell phone detector circuit ...  - Page 1
Forum section: electronics / general

author: LEECH666
author_rank: Frequent Contributor
timestamp: January 29, 2013, 04:29:27 pm
content: Hi there folks,does anyone of you know of a DIY cell phone detector circuit that can detect all kinds of cellphones/smartphones?I've looked around the web a little, but I couldn't really find any good circuits. The background is, that my mother is a teacher and nowadays every kid and their dogs have cellphones available that can be used to cheat during tests, and that the use of cellphones distracts the pupils from the lessons. So my mother asked me if I could find/make her a cellphone detector of some kind. I am not proficient enough to design smth. like this myself.Total cost for the project shouldn't be more than 100€ (135$).I know these specs are pretty loose, but I am just looking for options right now ...If anyone has any ideas I would like to hear em.Cheers,Florian

author: SeanB
author_rank: Super Contributor
timestamp: January 29, 2013, 05:31:42 pm
content: There was one using a broadband amplifier adjusted so that it was just short of oscillating, but I cannot remember where I saw it.

author: AlphZeta
author_rank: Regular Contributor
timestamp: January 29, 2013, 08:46:52 pm
content: Depend on the range you want to be able to detect. If it is for short distance, LT5534 RF power detector can be used. I made an RF strength meter a while ago and could be used to detect the presence of a cell phone (http://www.kerrywong.com/2010/11/01/wide-band-rf-field-strength-meter/).Because cellpohnes use spread spectrum, the circuit works only in a short distance (several feet for instance). But for devices that are narrow band (e.g. walkie talkies, the detection range could be extended significantly).

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 30, 2013, 06:18:14 am
content: Rewrite the tests such that cheating would be difficult and/or pointless.

author: amyk
author_rank: Super Contributor
timestamp: January 30, 2013, 09:40:06 am
content: Quote from: NiHaoMike on January 30, 2013, 06:18:14 amOr supervise them better and fail them if you catch them cheating... unless you're in a shielded room you'll be surrounded by RF, not necessarily from cellphones.

author: JuKu
author_rank: Frequent Contributor
timestamp: January 30, 2013, 09:44:51 am
content: The phones spend most of their time just listening the network, and transmit only when necessary (signal conditions change, switch masts). That is why the batteries last so long, and that is why you can't build a detector. In theory, you can build a detector for a phone that is actively transmitting, but the students would find out about the detector and start using pre-stored material.The problem is not solvable by electronics engineering, you need social engineering solution.

author: SeanB
author_rank: Super Contributor
timestamp: January 30, 2013, 03:52:06 pm
content: Big metal box in front of the class, with a lot of clear plastic bags and cable ties and labels. Each student is offered one to place phone in, seal with a tag and with their name on it, then all are placed in box in front of students. Have a big sign saying if you are caught in the room with a phone there is no excuse, you are out and will only be able to write the next year if that. Have a list of names under that ( and a lot of blank space) for the names of those caught disregarding this rule.

author: AlphZeta
author_rank: Regular Contributor
timestamp: January 30, 2013, 04:16:54 pm
content: Or instead of trying to detect cellphone signals, just use a jammer

author: ve7xen
author_rank: Super Contributor
timestamp: January 30, 2013, 06:57:23 pm
content: Quote from: AlphZeta on January 30, 2013, 04:16:54 pmI hope this is a joke.

author: tom66
author_rank: Super Contributor
timestamp: January 30, 2013, 08:13:51 pm
content: Hmm, idea: What if you were to fake a GSM signal (like how a phone receives a text - mast sends it a signal saying there is a message) to wake them up, then listen in for this signal? Thing is, I have no idea how to do this, short of multi-thousand dollar GSM test equipment.

author: ve7xen
author_rank: Super Contributor
timestamp: January 30, 2013, 08:53:22 pm
content: Quote from: tom66 on January 30, 2013, 08:13:51 pmThis would be all kinds of illegal. If you want to do something like this (detect phones) legally, your only viable option is a completely passive system. Anything active means transmitting in bands that are strictly license-only, and you're not going to get a license for this purpose. For your scheme to work, you'd also have to impersonate the user's carrier and thus intercept their traffic, which would likely fall under hacking or wiretap type laws as well.Probably doable with OpenBTS on a USRP, though.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 31, 2013, 05:52:38 am
content: Quote from: JuKu on January 30, 2013, 09:44:51 amOr they'll turn on "Airplane mode" and be "undetectable".

author: adam1213
author_rank: Regular Contributor
timestamp: January 31, 2013, 11:35:26 am
content: You could use something like a lot of am/fm radios to detect phones transmitting. When the phone transmits radios generally pick up interference.Quote from: NiHaoMike on January 31, 2013, 05:52:38 amEven if you can detect the use of phones students could still get around this. e.g. smartphones have wifi which could used. (A second phone elsewhere could be used as an internet connection)Quote from: JuKu on January 30, 2013, 09:44:51 amPost on the internet promising but wrong answers slightly before and during the test. If the answers are convenient enough they might end up on cheating students papers. This could then be used as proof of cheating. (*answers would need to be posted such that one couldn't claim they used them for studying rather than during a test)Alternatively there might be an even better approach which actually works:Make the test open bookAsk questions that require putting together information / problem solving(NiHaoMike mentioned something similar in an earlier post)Assist in teaching team workMake the content fun so that students will want to learn, will learn and won't have any temptation to cheat.Have learning activities that involve the use of phones (be creative)

author: Unknown
author_rank: Unknown
timestamp: January 31, 2013, 03:39:58 pm
content: Here is a circuit I designed some years back; good sensitivity so, if a transmitter mast is nearby, you need to cut the aerial wire length to make the detector less sensitive, by going up the band for the school room.What you hear is just the data cell noise, there are two switched positions between the centre-off... one side produces high level audio, the other, a low audio level.See Image:

author: bitwelder
author_rank: Super Contributor
timestamp: January 31, 2013, 05:53:50 pm
content: phase 1) Invite every student to turn off their cell phonephase 2) Put a cheap radio or amplified loudspeaker with the volume turned high near each 'candidate': it will be normally silent, but you'll hear the 200 Hz GSM 'burp' as soon as a nearby non-powered off phone will update their status to the GSM network.

author: 8086
author_rank: Super Contributor
timestamp: January 31, 2013, 06:20:38 pm
content: Take the phones away. They don't need them in exams.Easy.

author: lapm
author_rank: Frequent Contributor
timestamp: January 31, 2013, 06:36:52 pm
content: Quote from: AlphZeta on January 30, 2013, 04:16:54 pmHope you do realise that in many countries its illegal to jam cellphones. For example in mine, Finland. If they officials catch you doint that, they will fine you heavily under, law that says something about emergency calls must always be awailable...If your teacher, im sure head of your institution will not priciate if authorities show up not in good mood.

author: lapm
author_rank: Frequent Contributor
timestamp: January 31, 2013, 06:40:19 pm
content: Quote from: 8086 on January 31, 2013, 06:20:38 pmOr even easyer, instruct students turn off their phones and put them in their backpacks, jacket pocket, etc... little away from table they work on. Anyone thats phone rings and causes disturbance is welcome in 6 to 12 months to try again...

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 31, 2013, 07:41:41 pm
content: Quote from: ve7xen on January 30, 2013, 08:53:22 pmNot true, first of all to be illegal signal must exceed certain power level. All RF signals below certain power level considered as "noise" and therefore outside of regulation. There are some products that exploit that "loophole".In some shopping malls in US I noticed that they send SMS messages with ads for local stores to cell phones directly using their own equipment, and that as far as I know allowed since they own the radio space confined within the mall and as soon as signal is confined to YOUR flat, house, garage, movie theater, office or shopping mall you should be fine.People are so quick to spread unhealthy sensations about what is "illegal" :-)

author: 8086
author_rank: Super Contributor
timestamp: January 31, 2013, 07:51:41 pm
content: Quote from: Alexei.Polkhanov on January 31, 2013, 07:41:41 pmIt appears you're not any differentIf the power level is sufficient to be effective, they're going to either be legal or illegal based on their purpose, not the power output.In Germany, they're illegal. OP is apparently in Germany.http://en.wikipedia.org/wiki/Mobile_phone_jammer#Legalities

author: ve7xen
author_rank: Super Contributor
timestamp: January 31, 2013, 08:06:54 pm
content: Quote from: Alexei.Polkhanov on January 31, 2013, 07:41:41 pmWhile I'm fairly sure this would still be illegal under RF management legislation, regardless of power level (it's an intentional radiator after all), the particular approach suggested by the poster I was replying to would also be illegal for several other reasons not directly related to unlicensed transmission.Jamming is not quite as contentious as it doesn't involve trying to masquerade as a legitimate BTS and capture device associations, though I still think you're wrong that jammers can exist legally within the spectrum regulations, especially when it comes to mobile phones where you've also got emergency services disruption to consider.Just build a Faraday cage around the classroom. I don't think the RF is the problem though, the devices are the problem, and OP needs a way to detect them not block them from getting on the network.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 31, 2013, 08:57:55 pm
content: Quote from: ve7xen on January 31, 2013, 08:06:54 pmSome movie theaters in Toronto do that. I definitely remember they used to do that back in 2001. Way things going I think we will see more of that kind of signal blocking in future, but it has to be included into original construction plan. RF blocking wallpaper comes in snowflakes patternhttp://whatsnext.blogs.cnn.com/2012/07/18/signal-blocking-wallpaper-stops-wi-fi-stealing-and-comes-in-a-snowflake-pattern/

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 31, 2013, 09:25:25 pm
content: BTW here is the link to a graph describing power levels below which no license is required. I think is for Japan. I have similar table for US somewhere.http://www.telec.or.jp/ENG/testing_services/03.htmlIf you building device that operates within range of 3-5 meters it may be useful to know that you may not have to license it regardless of spectrum used.Canadian regulation states for example:(http://www.ic.gc.ca/eic/site/smt-gst.nsf/eng/sf10218.html#s3.5)"3.5 Transmitters with Input Power of 6 Nanowatts or LessAny transmitter that has a power consumption (total input power into the device) not exceeding 6 nanowatts is excluded from any Industry Canada requirements and may operate on any radio frequency, including in the restricted frequency bands listed in RSS-Gen."Is it possible to disable cell phone communication and stay within 6 nanowatt? I am talking about2-3 meter radius.

author: 8086
author_rank: Super Contributor
timestamp: January 31, 2013, 09:41:16 pm
content: It's illegal to possess or use a jammer in Canadabecause of it's functionalityhttp://www.ic.gc.ca/eic/site/smt-gst.nsf/eng/sf10048.html

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 31, 2013, 10:20:39 pm
content: Quote from: 8086 on January 31, 2013, 09:41:16 pmI think all together it means that if emitted/consumed power is below given threshold it cannot be even classified as jamming device since it lies below threshold covered by this regulation. It must be true because it cannot be applicable only in one part, but not in other - in other words how can it be that frequency spectrum regulations are not applicable for ultra low power device but its functionality is covered ?

author: 8086
author_rank: Super Contributor
timestamp: January 31, 2013, 10:33:21 pm
content: Quote from: Alexei.Polkhanov on January 31, 2013, 10:20:39 pmIt's prohibited due to it's function, not it's power. So you don't even need to take the power into account.I don't have time to go through the legislation, but I suspect you have missed something along the way.