Hello, thanks for the answer. I finally found, what was the problem. The sensor was connected very close to the stabiliser 7805, which produced the temperature, and if even the sensor was outside the box, the temperature has been injected to it via the cu wires ( approx 1 cm ). So just I replaced the cu wires with the normal one and make them longer ( approx. 10 cm ) and the problem is not there any more. Thanks, Robert