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    This temperature controlled relay circuit is a simple yet highly accurate thermal control circuit which can be used in applications where automatic temperature control is needed. The circuit switches a miniature relay ON or OFF according to the temperature detected by the single chip temperature sensor LM35DZ.

    When the LM35DZ detects a temperature higher than the preset level (set by VR1), the relay is actuated. When the temperature falls below the preset temperature, relay is de-energized. The circuit can be powered by any DC 12V supply or battery (100mA min.)

    Electronic Temperature-Controlled Relay Schematic

    temperature controlled relay circuit schematic

    How it works?

    The heart of the circuit is the LM35DZ temperature sensor which is factory-calibrated in the Celsius (or Centigrade) scale with a linear Degree->Volt conversion function. The output voltage (at pin 2) changes linearly with temperature from 0V (0oC) to 1000mV (100oC).

    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 energize or de-energize the relay (LED1 ON or OFF respectively).

    Source: http://www.escol.com.my/Projects/Project-03%28Thermostat-1%29/Proj-03.html

    Components list:

    IC1 : LM35DZ
    IC2 : TL431
    IC3 : LM358

    LED1 – 3mm or 5mm LED

    Q1 – General purpose PNP transistor ( A1015,…) with E-C-B pin-out)
    D1, D2 — 1N4148
    D3, D4 — 1N400x (x=2,,,,.7)

    ZD1 — Zener diode, 13V, 400mW

    Preset (trim pot) : 2.2K (Temperature set point)
    R1 – 10K
    R2 – 4.7M
    R3 – 1.2K
    R4 – 1K
    R5 – 1K
    R6 – 33Ω

    C1 – 0.1 µF ceramic or mylar cap
    C2 – 470 µF or 680 µF electrolytic cap. (16V min)
    Miniature relay – DC12V DPDT, Coil = 400 Ω or higher

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    8 Responses to "Temperature Controlled Relay Circuit"

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    1. How can i use this circuit to protect an amplifier from overheating ?

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    2. What is the function of IC2? This is a viable project and I need functions of every component so as to design it well.

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    3. Rkphadke says: on July 18, 2013 at 12:43 pm

      Dear sir

      This is fine I shall try wish every things works well and wish that you make us smarter

      Thanks Kind regards Ramesh Phadke

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    4. sorry, but what is K1?

      thanks regards kesy

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    5. Can you please provide us a humidity controller circuit witch will provide humidity to a set point and turn of the humidity provider after reaching the set point as like temperature controller, and it has to be simple and easy … Thanks

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    6. do you have the video tutorial of this circuit making.. ????if u have any thing related to video please mail me..

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