4 Temperature controlled switch When the LM35DZ(An IC included in a circuit) detects a temperature higher than a preset level, the LED is switched on.So basically the circuit can be used in circuits where a device can be made operational or non-operational when temperature variations occur.This is a circuit which can be used in applications where automatic temperature control is needed.
5 Components usedIC1 : LM35DZ Precision Celsius (Centigrade) Temperature sensor IC2 : TL431 +2.5V precision voltage reference IC3 : LM358 Dual single supply Op-amp. LED1 -- 3mm or 5mm LED Q1 -- General purpose PNP transistor ( A1015,...) with E-C-B pin-out) D1, D2 -- 1N4148 silicon diodes (or 1SS133) D3, D4 -- 1N400x (x=2,,,,.7) rectifier diodes ZD1 --- Zener diode, 13V, 400mW Preset (trim pot) : 2.2K (Temperature set point) (# 222 or 2k2)
6 Components used Resistor : ( 1/4W or 1/6W)R1 -- 10K R2 -- 4.7M Capacitors :C1 µF ceramic or mylar cap (# 104 or 100n)
7 Working…..The heart of the circuit is the LM35DZ temperature sensor which is factory-calibrated in the Celsius (or Centigrade) scale with a linearDegree->Volt conversion function.The output voltage (at pin 2) changes linearly with temperature from 0V (0oC) to 1000mV (100oC). This greatly simplifies the circuit design as we only need to provide a precision voltage reference (TL431) and an accurate comparator (A1 of LM358) in order to construct a complete thermal-controlled switch.
8 Working….The preset (VR1) & resistor (R3) from a variable voltage divider which sets a reference voltage (Vref) form 0V ~ 1.62V. The op-amp (A2) buffers the reference voltage so as to avoid loading the divider network (VR1 & R3). The comparator (A1) compares the reference voltage Vref (set by VR1) with the output voltage of LM35DZ and decides whether to switch on or switch off the LED.We can set a specific temperature according to our wish with the help of voltage across the variable resistor.When the temperature in the surroundings is higher then a specific value, then it gets switched on. And when temperature is again low it is switched off.