[Tinkerphones] Fun with BME280 inside GTA04A5

Andreas Kemnade andreas at kemnade.info
Tue Sep 19 18:38:56 CEST 2017


On Tue, 19 Sep 2017 08:42:21 +0200
"H. Nikolaus Schaller" <hns at goldelico.com> wrote:

> Hi,
> 
> > Am 19.09.2017 um 08:01 schrieb Andreas Kemnade <andreas at kemnade.info>:
> > 
> > Hi,
> > 
> > On Sat, 16 Sep 2017 21:57:32 +0200
> > "H. Nikolaus Schaller" <hns at goldelico.com> wrote:
> >   
> >> Hi,
> >> since we a re a tinkerphones community, I just had
> >> some fun with the GTA04A5 I want to share :)
> >> 
> >> Maybe you know that there is a BME280 inside. It
> >> is a combined barometer, thermometer, hygrometer
> >> sensor chip.
> >> 
> >> And it has full Letux kernel support so that it
> >> works out of the box.
> >>   
> > Well, to produce nice input there still needs to be some logic,
> > Probably some power management logic. like suspend for some minutes to let the
> > gta04 cool down and read then the sensor directly after suspend.  
> 
> That is easy:
> 
> ./measure-suspend 300; cat /sys/bus/iio/devices/iio:device5/in_humidityrelative_input
> 
> Or integrate it into a new script that does similar things as ./measure-suspend
> for suspend + wakeup by RTC.
> 
> > 
> > So the question is: does the hygrometer produce sane output when the gta04 case
> > is closed?  
> 
> Yes, that would be nice if someone could find out.
> 
> >   
> >> It was simple to do some readout:
> >> 
> >> 	cat /sys/bus/iio/devices/iio:device5/in_humidityrelative_input
> >> 
> >> Here is the full sequence when breathing upon
> >> the sensor (of a board not installed in the case):
> >> 
> >> root at letux:~# while true; do cat /sys/bus/iio/devices/iio:device5/in_humidityrelative_input; sleep 1; done
> >> 35.765625000  
> > root at letux:/sys/bus/iio/devices/iio:device1# cat in_temp_input
> > 39100
> > root at letux:/sys/bus/iio/devices/iio:device1# cat in_humidityrelative_input
> > 78.563476562
> > 
> > Hmm, that does not make sense. 78% at 39 C. It was not in
> > my pocket. It is in a train. So why it is so humid?  
> 
> Hm. Maybe the sensor needs calibration? More likely it is too much
> influenced by the device temperature.
> 
> Maybe printing temperature and humidity in a loop right after booting
> shows some drift/correlation?
> 
That should clearly be monitored. Of course I measure some innner device temperature and not outdoor temperature but I do not understand why a high device temperature would create a wrong humidity. Higher temperature -> air can store more vapour -> RH will go down. If I measure something I like to understand what I am actually measuring and how it is influenced.


Regards,
Andreas
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