[OpenTRV-dev] Mobile phone detector

Thu Nov 5 12:47:08 GMT 2015

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
>>>>> _______________________________________________
>>>>> OpenTRV-dev mailing list
>>>>> OpenTRV-dev at lists.opentrv.org.uk
>>>>> http://lists.opentrv.org.uk/listinfo/opentrv-dev
>>>>>
>>>>
>>>>
>>>>
>>
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