How can I prevent these LEDs from blinking when turned off?

Any copper wire has an electrical resistance, which is added to the resistance of the sensor itself. A very long small-gauge wire can alter the apparent temperature that the sensor is reading. This effect is more prominent in lower value thermistors than in higher ones. So, when dealing with long runs of existing small-gauge wire, 10K sensors may be preferable to 1K. Follow the equipment manufacturer’s recommendation for wire size and maximum length. For example, a note with one of the Tekmar solar controllers mentions that 18-gauge wire up to 1,000 feet long will provide a plus or minus ½ degree F accuracy for their standard 10K sensor. 

Signal interference
The wire cable itself can affect the performance of a thermistor sensor. Long lengths of unshielded straight wire (commonly used for room thermostats) can act like an antenna. Interference from nearby power equipment can cause random "signals" to be induced in unprotected wiring such as this. This can cause the temperature reading to become unstable and jump around rapidly instead of settling on the actual temperature at the sensor.

To prevent these unwanted signals, use metal conduit, shielded wire and twisted pairs whenever you can. Connect the metal shield to the “Common” terminal in the control box (the low voltage or sensor common). It is interesting to note that sensors with higher resistance (10K) are more sensitive to this kind of interference. Sensors that operate with a lower resistance range (1K) are less likely to be affected. So, when choosing temperature controls for an unshielded and noisy electrical environment, equipment compatible with lower sensor values might be preferable.
 

HTH



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