[OpenTRV-dev] Mobile phone detector

Thu Nov 5 11:50:07 GMT 2015

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