What kind of module can detect RF from a cell phone? It's OK if it also detects other RF too, a false positive. It does not need to be sensitive. I'd like it to work up to 5ft away. Would be great if it had an output which goes high when there is a signal. This way I could use Arduino to differentiate between a burst of data like an SMS compared to a voice call. I do not need to know the frequency. Extra credit for a rough signal strength, 2-3 levels. Links please? I've tried Googling mostly I get stand alone units.
Something like this maybe:
And it's datasheet: http://www.analog.com/static/imported-files/data_sheets/AD8312.pdf
Analog Devices make great chips for the RF and linear market. Here is a list of RF detector chips:
Lefty
In my experience, almost anything with a speaker or earphone will detect a cellphone operating nearby 
Boffin1:
In my experience, almost anything with a speaker or earphone will detect a cellphone operating nearby
Not here, I have lots of ham radio and other stuff with speakers and I wouldn't tolerate it if cell phone noise came through them. But then again I'm in the northern hemisphere, so perhaps things are different here, like with toilet swirl direction? 
Lefy
We've all heard it come thru our stereo speakers. Because of the long wire?
There must be an easy way to take advantage of this issue. Ideas?
Well if you wish to experiment with simple circuits just do a search on 'signal strength meters'. The simple ones are usually just based on a simple diode detector circuit. Radio hams have been building and using them for decades. Here is one example:
Lefty
Would this circuit be sensitive enough from a few feet away from a cell phone?
I bet it is easier at HF frequencies.
I'd assume you would make it more directed to cell phone transmissions by using the right style antenna and probably an op amp to increase the sensitivity.
" I'm in the northern hemisphere, so perhaps things are different here, like with toilet swirl direction? smiley-grin "
I think you are on to something there, we get the leftover toilet grade celphones down here
I was involved with scrolling advertising machines in University cafeterias for some years , and the original method of sensing the end off each poster had a coil near the edge of the poster, and some metalic tape on the back of the poster where it should stop. ( it looked really horrendous )
Whenever they complained about the poster stopping half way, the tech would go out after hours and all was fine, so I went there during the day , and all the students were messaging on their phones at the tables around the scrollers !
I changed it to an optical system and it was fine.
Can anyone verify this is sensitive enough from a few feet away from a cell phone? I think this is what I need!
Simple...
sbright33:
Can anyone verify this is sensitive enough from a few feet away from a cell phone? I think this is what I need!
Simple...
I think that is part of the experimental part of the project, only you will be able to verify that. I don't see how anyone else could verify it. A cell phone puts out between 2 watts and a few milliwatts depending on the distance to the nearest cell tower.
Lefty
I understand that it is my responsibility to build it and test it. But I'm traveling right now and I need to estimate costs for the project so I'm asking for help. I need to order the needed parts, so far none. Not for someone to verify it. Just your thoughts. Will it work at all for my application? If I need more sensitivity an LM358 will be more than enough? Thanks for your help so far!
sbright33:
I understand that it is my responsibility to build it and test it. But I'm traveling right now and I need to estimate costs for the project so I'm asking for help. I need to order the needed parts, so far none. Not for someone to verify it. Just your thoughts. Will it work at all for my application? If I need more sensitivity an LM358 will be more than enough? Thanks for your help so far!
Well as cell phone use microwave frequencies I would at a minimum try and find a suitable diode and transistor rated for such frequency, as I'm sure those shown are just not very effective at cell phone frequency. Note also that such 'field strength' meters are not frequency selective without additional front end filtering so will respond just as energetically to cordless telephones, garage door openers, and other common consumer RF generators.
Lefty
I was hoping to use the instructable link above with only a single resistor? Maybe 2ft away without Op amp?
I understand the power output of a cell phone varies with distance to the tower.
sbright33:
I was hoping to use the instructable link above with only a single resistor? Maybe 2ft away without Op amp?
I understand the power output of a cell phone varies with distance to the tower.
That method is just relying on the very high input impedance of a 'floating' analog input pin and would be better called a EMI (ElectroMagnetic Interference) detector. It will respond to all radiated electrical 'noise' above a certain amplitude, with common 60Hz AC house power wiring probably being the biggest detected signal. However that doesn't mean it won't respond to cell phone RF output, and is worth a little experimentation if for no reason then it's so cheap.
Lefty
retrolefty:
Boffin1:
In my experience, almost anything with a speaker or earphone will detect a cellphone operating nearbyNot here, I have lots of ham radio and other stuff with speakers and I wouldn't tolerate it if cell phone noise came through them. But then again I'm in the northern hemisphere, so perhaps things are different here, like with toilet swirl direction?
Lefy
Its typically only GSM phones that do this, may be why you havn't heard it in NA, we're mostly CDMA where everyone else is GSM.
Its my understanding that GSM phones have to "ping" the towers every once in a while, so the tower has an idea how many clients it has in range. This ping uses more power than normal, so speakers pick it up even when they don't pick up normal phone use. Whenever I had friends over in my shop, my speakers would buzz about 5 seconds before one of them would get a text message or call. It almost sounded like Morse code, a series of distorted beeps, then a pause, then more beeps.
Has anyone built any RF detector? A different frequency? Let's compare results of different methods!
I've been an Amateur Radio operator for 40 years (WA7EMS is my call sign, a "Vanity call" and yes, the phonetics DO mean what they say... )
What you are intending to do is difficult because of the Average "noise" level where noise is defined as all detectable energy. Providing selectivity at 900 MHz (Approx) can be done by several different methods, the simplest is the antenna and a 1/4 wave antenna is usually the choice for several different reasons that any RF engineer can point out. There are several problems with that issue too. The principal one is the selectivity of a 1/4 wave antenna isn't the greatest. A "Hotcarrier" diode... HP 5082-28XX series diodes or an OLD 1N23 microwave diode will make a great detector... for anything from 400 Mhz and up, the "sorta' limit is the anrtenna length as it is the only selectivity in circuit. Convert the RF to DC with the detector and apply some DC gain with an op-amp... You could use an LM358 here but I am real partial to R-R I/O Op-Amps and I would use an LMC662. It is a single or dual power supply type of op-amp that can swimg from Vcc to ground in single ended types of circuits and can operate to ground on the inputs whare some types of bo-polar types will latch up. So now you have an amplified broad band noise detector with a small antenns (1/4 wave @ 900 MHz is about 3 inches) and even that is do-able but to differentiate between SMS and phone conversations is nearly impossible and totally so when you consider the other sources of EMI (a simple message might well be 100 mS for the phone to receive the information and respond to the cell site for the message before announcing the call to you as the data rates are rather high < 100K baud. IMO you will have to be real clever to accomplish your stated goal.
Bob
No, I'm not trying to differentiate between those 2, that's the easy part. There is no other noise within 3ft. The existing noise is weaker coming from broadcast bands most significantly FM in the city. Has anyone tried anything related to this? I mean actually built it? It's only one passive component....
KB3SF Ham since 1978
Still nobody has tried this? I'm headed home to test it myself...