I'm developing an incubator which includes a hot air pump.
I'm generating the heat with 2 40W-12V ceramic resistor from 3D printer conected to a transistor (IRF540N) which is controlled by an Arduino (using PID library.)
For safety, I added an LM35 to check whether the temperature rises over the plastic box melting temperature. The problem I have is that every time I do a measurement (I'm trying to tune the PID parameters) the same strange noise appears almost at the same time (between 200 and 700 seconds.)
Here you can see the schematics for the measurement system:
A0 is the Arduino's analog input. The LM35 and the Arduino are conected with a 1m long mesh cable.
On the other hand, the PID circuit is as follows:
D3 is the Arduino's PWM output.
The error I'm talking about can be seen in the following pictures. It repeats in EVERY measurement but more or less the same shape.
The blue line represents LM35 measurement over time.
If you were wondering what the PWM signal looks like (I suspect it might have something to do with the ascending signal of the PWM) here it is in a percentage representation:
This signal may change with the PID configuration but the noise is almost the same.
Last but not least, it is worth mentioning that the LM35 is mounted on the same small PCB board where the resistors are.
EDIT: Here you can see other measurements to ilustrate the repetition of the problem (in shape and in time length):