I've setup an Arduino Yun to provide the following graphs :
- ambient light, with a photo resistor and a voltage divider, which work well : http://malheur.goddess-gate.com/light/
- ambient temperature, with a TMP36 sensor, which has a strange behavior : http://malheur.goddess-gate.com/temp/
As you may see, there is a regular temperature / voltage drop which happens every 31/32 hours, and I can't explain it, and this voltage drop only happens with temperature measure, not light measure.
The photoresistor and TMP36 are powered by the same +5V pin, photoresistor output is measured with A0 pin, and TMP36 with A1 pin. There's a 100 µF capacitor near VCC and GND pins of the TMP36 sensor.
Light and temperature values are gathered every 10 minutes by a cron job on Linux side via a REST query. Arduino sketch just wait for the REST query and return A0 and A1 values. Temperature measurement is not a single shot : when temperature is asked, I take 10 measure each 1 second, then return the median value.
I can't really explain :
- Why the voltage drop
- Why it only happens to temp measure
- Why it happens regularly, every 31/32 hours.
If someone here have an idea, (s)he's welcome :-)
I'm sorry I don't have an idea on how to help you with your problem, but would you mind if I asked you how you made sure that cron is running when the board boots up?
I have created a crontab entry, but don't see cron in the ps (process) list. I can see a script for it in /etc/init.d which is linked from /etc/rc.d ....
I love answering my own question:
found here, http://wiki.samat.org/CheatSheet/OpenWrt
Now hopefully the next person to google this will find the answer quicker. :)