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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

    ask a question

    8 Responses to "Temperature Controlled Relay Circuit"

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

    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.

    3. 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

    4. sorry, but what is K1?

      thanks regards kesy

    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

    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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