16f877 with LM335 problem

[flotulopex]

Thanks Ioannis,

You're right: less than that is almost impossible

Now drive the raised GND of the LM35 to another ADC input of your PIC

I need to ADC measure the raised GND only once since it will never change - or am I missing something?

And "yes", for sure I'll have values to measure that will be below 0°C; in fact, they will have to be always under 0°C or I'll have a problem with the food inside the freezer

[flotulopex]

The two diodes connected to the ground of LM35 will change their value due to temperature change.

So this is the reason for why two ADC channels are needed.

Since I'll have to locate the LM35 inside the freezer, shall I put these two diodes as close as possible to the LM35, meaning, the sensor and the diodes will be located inside the freezer? Or is it not necessary?

[Ioannis]

You got it right about the temp instability of the diodes.

And since both (diode and LM35) have positive coefficient regarding the temperature, i'd put both in the same thermal spot.

Remeber, NOT to use the LM35D

Ioannis

[richard]

if you wire a lm335 like this
would not a adc reading of 512 = about -22C and a adc read of 569 = about 0 C
not very much resolution but workable

or am I missing something

[Ioannis]

Of course it it workable if you accept the 2,59 counts per degree. Noise will be an issue also and especially in this case I would insist on using oversampling to 12 bits to iron out the noise as much as possible.

Ioannis

[richard]

having in mind the accuracy I need is not that critical (+/- 2°C is okay),

with a bit of averaging or oversampling 0.5 resolution is achievable, the software solution is at least cheap and easy to try out

[flotulopex]

Hello,

I have built the circuit and I'm now testing.

I'll send some pictures from the the values I read from the LM35 in a moment - very strange values, not stable at all... :-(