0 ~ 100^oC Electronic Temperature-Controlled Relay
using LM35 and TL431

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Introduction :

In response to visitors requests, we have designed this 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 one-chip temperature sensor LM35. When the LM35 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.)

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

The one we supply with this project is the -DZ version with temperature range from 0 ~ 100^oC.

Circuit Operation : The heart of the circuit is the LM35 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 with temperature from 0V (0^oC) to 1000mV (100^oC). This greatly simplifies the thermostat 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 thermostat. The preset (VR1) & resistor (R3) form 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 LM35 and decides whether to energize or de-energize the relay (LED1 ON or OFF respectively).

Calibration : No special instrument is required. The relay can be set to "trip" (change state) at any temperature form 0 ~ 100^oC (LM35DZ). For example: To set a 70^oC 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 reading of 700 mV ( or 0.7V) on your voltmeter or multimeter.

Here is a practical example showing how he circuit is used to maintain the temperature of water in a container at 70^oC. Click here to view the diagram of the application example (Constant temperature Control).

Click on picture to view the schematic

Click on picture to view parts layout

PARTS LIST (the following components will be included in the package unless otherwise stated)
IC1 : LM35DZ Precision 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)	Resistor : ( 1/4W or 1/6W) R1 -- 10K R2 -- 4.7M R3 -- 1.2K R4 -- 1K R5 -- 1K R6 -- 33Ω Capacitors : C1 -- 0.1 µF ceramic or mylar cap C2 -- 470 µF or 680 µF 16V electrolytic cap Miscellaneous items : 8-pin socket -- x 1 pcs Miniature relay -- DC12V DPDT, Coil=400 Ω or higher
Download PCB artwork for this project (pdf) 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 PP-1530. ( tracing paper is the type of semi-transparent paper used by architect for drawing plans. It's much cheaper than OHP transparency)* Instruction on how to make customized PCB (pdf)
Components package available : Gathering parts for a project may sometimes be troublesome & time consuming. To help you get around this problem, we have packages of components ready for each project. Click here for a complete price list of the available packages.

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