[OpenTRV-dev] Mobile phone detector

Damon Hart-Davis dhd at exnet.com
Thu Nov 5 14:56:54 GMT 2015 

Hi,

Scanning will (I think) be listen-only unless you habitially connect to a WiFi system that does not broadcast an SSID.  Could well be wrong, again, though.

Rgds

Damon


> On 5 Nov 2015, at 14:40, Deniz Erbilgin <deniz.erbilgin at gmail.com> wrote:
> 
> Given how quickly my phone seems to detect networks whenever I look at it, it may be scanning on events such as waking the screen (which seems like a fairly natural thing for someone bored waiting for the bus to do).
> 
> Regards,
> 
> Deniz
> 
> On 05/11/15 12:51, Damon Hart-Davis wrote:
>> Deniz:
>> 
>> The feedback from a (Bluetooth) engineer at the Cambridge meetup was that for both power and security reasons WiFi and BT chatter a lot less from idle phones than they used to, contrary to some of my other sources.
>> 
>> Which is why we’re on this track.  But I’d be happy to be wrong!
>> 
>> Rgds
>> 
>> Damon
>> 
>> 
>>> On 5 Nov 2015, at 12:47, Deniz Erbilgin <deniz.erbilgin at gmail.com> wrote:
>>> 
>>> Thanks Kevin,
>>> 
>>> I have a couple of extra questions:
>>> - Do you have any idea what the most popular bands are in London?
>>> - The higher frequencies you mention have fairly small wavelengths. How feasible would it be to use a directional antenna to discriminate between, say mobiles directly under the bus shelter and those nearby/the cell site?
>>> - As far as I'm aware, a lot people walk around with their wifi left on. Would it be more practical have a similar system checking for wifi connections instead/as well?
>>> 
>>> I guess it would be worth going around bus stops with a spectrum analyzer and a couple of different antennas...
>>> 
>>> Regards,
>>> 
>>> Deniz
>>> 
>>> On 05/11/15 11:50, Damon Hart-Davis wrote:
>>>> Hi,
>>>> 
>>>> Thanks for that analysis!  Wow!
>>>> 
>>>> Key points to note: our initial solution does not have to be perfect nor work at every shelter (nor all of the time).
>>>> 
>>>> I would be delighted if we were right on occupancy 50% of the time; combining the outputs of multiple sensors in a sensible way is part of our aim.
>>>> 
>>>> We also can in principle record typical sensor output levels over time to set a noise floor or even to disable a sensor entirely if it hears loud continuous chatter from a nearby base station!
>>>> 
>>>> We can also choose only to fire up the cellular detector if other sensors are currently drawing a blank, ie we can selectively throw more energy at some sensors dynamically.
>>>> 
>>>> ***
>>>> 
>>>> So, what is the simplest thing that could we do to have a (say) 50% chance of catching an average Londoner’s mobile chatting to the base station, initially ignoring energy consumption?
>>>> 
>>>> Could we say have a big rectenna that we put in the seat and is even powered by the energy from the phone itself and drives a GPIO or more?
>>>> 
>>>> Next option up we have a nice current sense op amp in our valve motor driver circuit with VERY low quiescent current which can give a decent boost on a voltage from the rectenna without killing us.
>>>> 
>>>> Rgds
>>>> 
>>>> Damon
>>>> 
>>>> 
>>>>> On 5 Nov 2015, at 10:21, Kevin Wood <kevin at the-wood-family.com> wrote:
>>>>> 
>>>>> Hi All,
>>>>> 
>>>>> As promised, a few thoughts:
>>>>> 
>>>>> Mobile phones are also battery critical devices that like to sleep as
>>>>> often as possible. Making 2 such devices detect each other is always going
>>>>> to be a challenge!
>>>>> 
>>>>> We have quite a few technologies operating at a few different frequencies
>>>>> in the UK:
>>>>> 
>>>>> GSM/GPRS:
>>>>> 880 - 915 MHz
>>>>> 1710 - 1785 MHz
>>>>> 
>>>>> WCDMA
>>>>> 1920 - 1980 MHz
>>>>> Some in the GSM bands too
>>>>> 
>>>>> LTE
>>>>> 832 – 862 MHz
>>>>> 1710 – 1785 MHz
>>>>> 2500 – 2570 MHz
>>>>> 3400 - 3800 MHz (TDD)
>>>>> 
>>>>> The above are uplink frequencies (mobile TX) but there will be adjacent
>>>>> downlink bands where cell sites will be constantly transmitting. Places
>>>>> like bus stops are a popular location for cell sites, so discriminating
>>>>> between them and the mobile stations might be a challenge.
>>>>> 
>>>>> With TDD network configurations, cell site transmissions are on the same
>>>>> channel as mobile transmissions!
>>>>> 
>>>>> What is transmitted is too complex to even try to demodulate, so a basic
>>>>> rf detector is about all you can do to detect presence.
>>>>> 
>>>>> Such a simple detector is prone to false triggering from any other radio
>>>>> signal it receives. About the only defence we have is to make it frequency
>>>>> selective around one or more of the above bands.
>>>>> 
>>>>> During use (speech call or internet activity) the mobile will regularly
>>>>> transmit. When idle, the timer that determines when it "phones home" is
>>>>> set in 6 minute intervals, and a typical value would probably result in a
>>>>> timer running for an hour or two between updates, so an idle phone is not
>>>>> easy to detect! Especially if you don't intend to sample 100% of the time.
>>>>> 
>>>>> If you're looking for a reasonably strong field strength, the detector
>>>>> needn't consume any battery power. A simple diode detector fed from a
>>>>> resonant antenna and tuned circuit could just wake the CPU using an
>>>>> analogue comparator channel when it "sees" RF.
>>>>> 
>>>>> But! Another issue is that power control of a mobile's transmission is
>>>>> very tight, again, because battery life of mobile is critical, and also
>>>>> because, in WCDMA and LTE technologies, received signals for each device
>>>>> must be as close to equal strength as possible at the cell site in order
>>>>> that one mobile doesn't swamp the signal from others. For this reason, if
>>>>> a cell site is close, we will have a double whammy of a strong signal from
>>>>> the cell site to reject, and low signals from any mobiles transmitting to
>>>>> it.
>>>>> 
>>>>> That's it for this "brain dump". Hopefully there's some useful stuff there!
>>>>> 
>>>>> 
>>>>> Best Regards
>>>>> 
>>>>> 
>>>>> Kevin
>>>>> 
>>>>> 
>>>>>> Fab, thanks!
>>>>>> 
>>>>>> All as Deniz says, and with the circuitry to be capable of running on at
>>>>>> most tens of microwatts average (we might be able to sample for a few
>>>>>> seconds every few minutes) at ~2.4V to work nicely with some variant of
>>>>>> our V0p2 board running from 2xAA NiMH or other similar low-power
>>>>>> microcontroller.
>>>>>> 
>>>>>> (We could even harvest a small amount of power to inject back into
>>>>>> batteries or a supercap as a secondary consideration!)
>>>>>> 
>>>>>> Rgds
>>>>>> 
>>>>>> Damon
>>>>>> 
>>>>>> 
>>>>>>> On 8 Oct 2015, at 22:16, Kevin Wood <kevin at the-wood-family.com> wrote:
>>>>>>> 
>>>>>>> Hi guys,
>>>>>>> 
>>>>>>> Currently on holiday but my day job is developing systems to test mobile
>>>>>>> phones.
>>>>>>> 
>>>>>>> I'll give this some thought.
>>>>>>> 
>>>>>>> Kevin
>>>>>>> 
>>>>>>> Hi All,
>>>>>>>> Damon has asked me to do a bit of research into using a mobile phone
>>>>>>>> detector as a presence sensor for the bus shelters.
>>>>>>>> 
>>>>>>>> The basic idea is an antenna, and an amplifier/filter tuned to the
>>>>>>>> appropriate frequency. Additionally we'd need some kind of conditioning
>>>>>>>> to get the output into a microcontroller readable format.
>>>>>>>> 
>>>>>>>> Does anyone have any thoughts on how practical this would be for
>>>>>>>> sensing
>>>>>>>> whether there are people waiting?
>>>>>>>> 
>>>>>>>> Regards,
>>>>>>>> 
>>>>>>>> Deniz
>>>>>>>> 
>>>>>>>> P.S. A quick google for how the detectors work gave me the following
>>>>>>>> results:
>>>>>>>> http://www.eeweb.com/project/circuit_projects/cell-phone-detector
>>>>>>>> http://www.electroschematics.com/1035/mobile-bug-detector-sniffer/
>>>>>>>> http://www.instructables.com/id/Free-Energy-Cellphone-detector-From-Cellphone-An/
>>>>>>>> http://www.instructables.com/id/VHF-UHF-RF-Sniffer/?ALLSTEPS
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>>>>>>>> 
>>>>>>> 
>>>>>>> 
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