Thermostat Controller with Relay using LM35 and TL431
Thermostat Controller with Relay using LM35 and TL431


Thermostat Controller with Relay using LM35 and TL431


Here 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 AC or DC 12V supply or battery (100mA min.)

Thermostat Controller with Relay using LM35 and TL431

There are several versions of LM35 temperature sensors :
LM35CZ & LM35CAZ (in TO-92 case) --- −40C to +110C
LM35DZ (in TO-92 case) -------------- 0 ~ 100oC
LM35H & LM35AH (in TO-46 case) ------ −55C to +150C

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). 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. 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). The purpose of R2 is to provide a bit of hysteresis which helps to prevent relay chattering. Hysteresis is inversely proportional to the value of R2. Lower value of R2 gives higher hysteresis.

No special instrument is required. The relay can be set to "trip" (change state) at any temperature form 0 ~ 100oC. For example: To set a 70oC trip point ( switchover temperature):

Connect a precision digital volt meter or multimeter across the test points "TP1" & "GND". Slowly Adjust VR1 until you get a exact reading of 700 mV ( or 0.7V) on your voltmeter or multimeter.

Application example:
Here is a practical example showing how the circuit can be incorporated in an automatic temperature control system which maintains the temperature of water in a container at 70oC.

IC1 : 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)

Resistor: ( 1/4W or 1/6W)
R1 -- 10K
R2 -- 4.7M
R3 -- 1.2K
R4 -- 1K
R5 -- 1K
R6 -- 33Ω

C1 -- 0.1 F ceramic or mylar cap (# 104 or 100n)
C2 -- 470 F or 680 F electrolytic cap. (16V min)

Miscellaneous items:
8-pin socket -- x 1 pcs
Miniature relay -- DC12V DPDT, Coil=400 Ω or higher

IMPORTANT note on printing the artwork : Open the pdf file with Acrobat Reader. Use a laser printer to print the artwork on tracing paper * in true size (i.e., : do not enlarge or shrink the artwork). The printed artwork can be used for direct exposure with UV sensitized pcb such as the Kinsten PP-1530.
(* tracing paper is the type of translucent paper used by architect for drawing plans. It's much cheaper than OHP transparency)

Download PCB artwork for this project below.

Thermostat Controller with Relay using LM35 and TL431

Thermostat Controller with Relay using LM35 and TL431

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