(First of all, sorry for the approximative english translation)
I have made a circuit in order to get the luminous value of a photodiode, the most basic circuit in that case :
The measure (Mesure analogique) is made by pin 11 on an Arduino MEGA board, I average the value on 10 measures done with a delay of 20ms between each, everything works fine, except that every 6 seconds (5.7s more precisely), there is an increase of the value read, and measuring the input with a voltmeter, that's actually the voltage that increases on the electronic board.
But I can't understand the origin of this instability. It is periodic and acurate, goiing smoothly from 0V to 0.1V then goes down, in about 2 seconds.
I search quit a bit on the internet, and I couldn't find any clue about it, does anybody have an idea what may be the origin of that ? I just checked, it's not due to an instability of the 5V supply nor from the ground.
Thanks in advance ! See you !