Explanation The circuit is nothing more than two light emitting diodes (D1 and D2), its position is controlled by the temperature of the surrounding environment. Here the tempe
rature sensor known as the IC LM35. LM35 increase the output of 10mV per degree of temperature rise. You can use the desired reference potentiometer R2 to the output voltage of the LM35 given the non-inverting input is connected to the same operational amplifier inverting input of the operational amplifier CA3130.The. If the reference voltage is 0.8V, then the non-inverting input terminal (LM35 output) voltage becomes 0.8V, when the temperature is 80 degrees Celsius. Then the output of IC3 to positive saturation. This makes the transistors Q1 and D1 LED lights glow. Since the basis of the second quarter, Q1 is connected to the collector of the second quarter will close LED D2 remains closed. When the temperature is below 80 degrees Celsius reverse happens.IC1, produce a stable 5V DC working available9V DC supply voltage. If you already have a 5V DC power supply, then you can use it directly. Schematic Precautions The circuit board assembly in Verona. IC3 holder must be installed. You can adjust the settings POTR2 temperature trip point. Type no Q1 and Q2 is critical. Any common NPN transistors will do it.