Room Temperature Controller using LM35/TL 431 Circuit

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This circuit can be used to turn fan on automatically. When the temperature on IC LM35 is low, the output voltage also low. If the electric voltage of the transistor is below the transistor active area, the voltage will be blocked, LED will off, relay for the fan circuit in open condition, thus the fan is off.

When the temperature of LM35 is high, the output voltage is also high. If the transistor electric voltage in the transistor’s active area, the LED will on, relay on the fan in close condition, thus the fan will be turn on.

Room Temperature Controller using LM35 Circuit

Components :
Resistor R1 : 10k ohm
Resistor R2 : 4.7M ohm
Resistor R3 : 1.2k ohm
Resistor R4-R5 : 1k ohm
Resistor R6 : 330 ohm
Capacitor C1 : 0.1 uF
Polar capacitor C2 : 470 uF/25V
IC1 : LM35
IC2 : TL431
IC3 : LM358
Q1 Transistor PNP : BC557
Zener diode D6 : 13V/400mW
Diode D1-D2 : 1N4148
Diode D3-D4 : 1N4007
Relay K1 : 12V SPDT
12V power supply

Estimated price : $7