Temperature Controlled Relay Circuit Diagram

Description

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.) 

 Circuit Diagram

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 (0^oC) to 1000mV (100^oC).

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).

Part 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

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