[OpenTRV-dev] Mobile phone detector

Thu Nov 5 14:40:22 GMT 2015

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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