having a problem with LM35 temperature sensor

How is your sensor connected ?

Schematic, R value and a picture are needed !

It looks as if your readings are very noisy, but possibly there could be another issue.

When you say that 5 different sensors give the same result, what do you mean. Is it exactly the same result (which would not be a noise issue) or similar results (which could be noise.) In engineering you need to be very careful when describing results.

MK

This means that with 5 LM35s I get strange results, they are not 100% the same values ​​but they are strange and imprecise , i have joined the circut and also the data that i get from serial monitor 

it seems that for LM35 1 st. C translates to 10mV of output voltage. Considering that (for Atmega) one bit of ADC result at Vref = 5V is about 5mV, typical ADC conversion accuracy is about 1 st.C (2 LSB), with max error of about 2 st. C.

Maybe switching to internal voltage reference of 1V would bring better accuracy? and adding some Vcc filtering in case when observed variability is caused by Vcc (used as reference) noise? 

how can i do that ?

docs.arduino.cc/.../

and change 5.0 to the value of the reference in code...

can you please do it for me idk how to do it

It looks as if you have no resistor from the output to a negative bias voltage - this means that you will get poor results as the temperature drops towards zero but you can ignore that for now.

The sensor output is 10mV/degree C. The resolution of your ADC is 5000/1024 = 4.88 mV which is equivalent to 0.488 C.

You would expect to see the readings jump about by at least half a degree. You are taking a reading once per second so I suggest that you filter the signal going in to the ADC. Put a 2k resistor between the sensor output and the ADC input and a capacitor between the ADC input and ground. The capacitor value should be chosen for a 0.5 second time constant C = 0.5/2000 = 25uF (use 22uF).

This will ensure that no noise above a few Hz will get into the ADC and may well help.

If you don't have the right parts pretty much any value for R between 1k and 10k should work and any value from 1uF to 100uF for C. (Obviously if you use 10k and 100uF it will respond rather slowly but this probably won't matter.)

Do you have access to an oscilloscope - if you do then you could look at the signal and see how noisy it is. 

What power supply are you using ?

MK

put 

analogReference(INTERNAL);

in setup, then change sensorValue * 5.0 to something like sensorValue * 1.1

(reference voltage is about 1.1V (min 1V, max 1.2V) - could be measured for better accuracy)