A very simple indicator for radiator temperature can be designed using electronic scheme in the figure below.
As long as the radiator temperature not exceeding 50 ° C, thermal indicator will flash a green LED, one orange will be provided for temperatures of 50 ... 75 ° C and a red LED, for temperatures above 75 ° C.
Temperature indicator consists of two special zener diode, D1 and D2, connected in series to ensure accuracy of 5.96 V Zener voltage at 25 ° C. Zener voltage will increase by 20 mV for each temperature increase of a degree Celsius temperature ¬. Radiator temperature corresponding voltage level is compared with two reference voltages, IC1 and IC2 using. When the temperature reaches 50 ° C, IC2's output goes to logic state "1" so that T3 leads and following ignition with diode D4. At 75 ° C, IC1's output is in logic state "1" and, therefore, T2 and T3 will, so that D3 and D4 lights are off.