Home Projects Motor, light and power control Analog Temperature Controller - Temperature Relay



Analog Temperature Controller - Temperature Relay
author: Thomas Ciciyan - thomasciciyanyahoo.com - electronic student @ UAST - Iran




1. Completely analog
2. 2 definable temperature to connect output relays
3. Low error
4. Noise resisted
5. Capability to be adjustable or fixed connecting point outputs
6. Ability to define centigrade or Fahrenheit graduate
7. 0 to 150 centigrade sensor
8. Sensor disconnecting reaction

This project, provide one design, with more than three functions!


I designed that, to control diesel engines cooling system, through just one sensor, to keep outlet cooling liquid temperature at ideal temperature. For a diesel engine, it would be 70 to 80 in centigrade degree. This analog controller may reduce cooling fan work time and let it to continue being aside diesel without emergency service need, because it will work less than summers, at winters. On the other side, diesel engine generates a lot of smoke, when they are out of ideal temperature. This circuit will keep cooling system disable, until achieve ideal temperature on outlet cooling liquid.

But, this was my target, to design this circuit. I created some changes on circuit, to make it consistent for some other projects and have this capability for other people to use that. Then it can be share with others on internet. Another function of my project is active kitchen air condition system, when you forget to start that! Just place it sensor, up on the kitchen heater or cooking dish with appropriate height. Then connect sensor to the kit, through wires. Connector 4 is designed for this function. When dish upper air temperature exceeds defined or pre-defined value, kitchen hood or fan or air condition system, will activate by kit relays.






Third function may be providing enough temperature for change an egg, to a chicken! This function is so exciting! Being as a hen, for an egg! After 21 days, or a little bit more, you may get a beautiful chicken. As you know an egg, needs 42 in centigrade degrees for 21 days, to exchange to a chicken. A mover mechanism is needed also, to rotate egg every two hours. The project will keep egg temperature at 42 degrees.

Other functions concerns to your heuristic mind. For example, control an electrical kettle, or home boiler or room temperature controller. If you made this project and used it in another project, get me informed about your heuristic and report that to me, please.

The kit, have two relays. It relays can set to active at two different temperatures. Depended to component mounting, kit can have two different modes, adjustable or un-adjustable. On the component list, some components are star marked components. If you need an adjustable analog temperature controller, mount them instead of zener diodes. This means that, you have to not mount any zener diodes. Else, if you need an un-adjustable analog temperature controller, you have to mount two zener diodes, and do not mount star marked components. Zener break down voltage is exactly the relay activation temperature. By default, zener break down voltages are 3.7v and 4.2v for zener 1 and zener2. This means relay 1 will active at 37 degrees in centigrade temperature. Second one, will active at 42 degrees in centigrade temperature, to start second speed of cooling system fan or activating an
temperature exceed alarm depend on your request.

To prevent control system going to oscillate, I assigned an integral circuit that causes a 10 seconds delay to turn off relay, after achieve defined temperature. Integral circuit contains R11, R13, C3 and D1 for relay1. R10, R12, C2, and D2 are for relay2. This is necessary, when system is exactly on threshold of decision temperature. By hypothesis down coming system temperature, delay circuit assumes relays control. At a time as this, output LED turns off. But related relay will continue being on after about 10 seconds late. There are also reset keys to turn off relay manually, when LED got off and you need turn off relay immediately. If you need increase this delay, you should increase C3 and C2 capacity. This time follows this equation:

R is R10 or R11, and C is C2 or C3.

Because of silicon transistors, 0.7 volt mines on the end of graph.

Reason of double star resistors:

You may install your sensor far with board. In this situation, sensor has 3 wires that may be longer than 10 meters. And sometimes we need a system to have a reaction about sensor disconnecting. If you want to turn both relays off, after sensor disconnecting, mount R25. And if you want to turn both relays on, after sensor disconnecting, mount R26. I don't recommend mounting both resistors. It's ambiguous for the circuit, to have R25 and R 26, synchronously, after sensor disconnecting. But if you do, relay 1 and 2 may turn on by sensor disconnecting. This reaction is look like one of previous reactions that I don't recommend. You better select one of resistors singly. Be aware, these resistors causes error when temperature sensing by sensor.

When remote sensor installing, to prevent cable-circuit interact or noise reciprocal, use shielded wires and connect circuit ground to wires shield. Also, if sensor is installed far with circuit, mount L1 to reduce noise. Furthermore, if you are using a switching power supply instead of regular transformer and rectifier, L1 have to mount and its induction can be more than 100microhenry, till 2milihenry.at this situation, L1 should be a coil with ferrite core.

How circuit works?

LM35 has linear output. Its output concerns its environment temperature. Complete linear 10mv per centigrade degree. I'm Iranian. Iranian people, uses centigrade scale. So, I designed this project for work with centigrade scale. If you prefer to use LM34, instead of LM35, to work with Fahrenheit graduate, you have to know LM34 has same output, but per Fahrenheit degree. So you may need to define P1 and P2 for appropriate value to connect and disconnect relays at your desire temperature.

Temperature convert formulas, to convert Fahrenheit to Celsius and reverse, are mentioned below:

Output of LM35 is connected to first op-amp of LM324. It gain is equal 2. It obeys below formula:

This is obvious; output voltage cannot exceed over IC3, 7812 output.


But, why did I place extra 0 resistors in series, instead of Rin and Rf?

Sometimes it's hard to access precision resistors for op-amp operations. You can calculate your gain and output voltage freely, and get require resistor with 3 or 2 series resistors. For example, to get 1410 resistor for R5, you can set it with a 1000 resistor+330 resistor+82 resistor, as series. So, sum of them will be 1412. If you have precision resistor, use it and place jumper for 0 resistors.

Second op-amp of LM324 is doing the same thing that previous dose. But output of first op-amp is not linear. Second op-amp makes a linear signal, from first op-amp output. It performs this to make a comparable analog signal on its output. Second op-amp gain is 5.28.

According to my calculations, tolerance of circuit is below:

  • At 25 centigrade degree, is 0.13 degree.

  • At 50 centigrade degree, is 0.277 degree.

At 100 centigrade degree, is 0.544 degree. I think total error will be less than 1 degree, between 0 to 150 degrees in centigrade graduate.

 Tow remained op-amps are in compare mode. They have infinite amplification. If their condition confirmed, they provide +VCC on their out puts. Otherwise, output will connect to VCC or ground. Output of these two op-amps, are connected to two different ways. First way, goes to an LED to show output electrical situation. Second way, goes to an integral circuit, to perform delay time and oscillate preventer circuit, and then transistors bias and on command. Transistors drive two relays, to realize temperature controlling commands.

Based on kit capability, you can select whether to mount components. Adjustable and un-adjustable mode is available. This specifies by Comparator reference voltage provider. For have un-adjustable kit, mount zener diodes and for have adjustable circuit, mount TL431 peripheral components. They are R6, R7, R23, R24, P1, P2, IC4, IC5 . For this, let zener diodes situation to be vacant.

This design offers an external loop control system. This means, default thought is to control an ascending temperature system. For example, irrespective cooling fans of an engine, a running engine temperature, always increase. This is an active system. Circuit will turn on fans, after temperature growth, and will turn on second fan or second fan speed or start alarm, when temperature exceeds, by second relay.

But, when you like control a room temperature, at different months, control system cannot be external loop or open loop. For this, you have to make some changes. First relay have to control room heating system, by N.O contacts and second relay have to control room cooling system by N.O contacts. For as much as this target, observe specify first relay connecting temperature lower than second one, is necessary. Also they have to define at a little bit different temperatures. For room temperature control, I recommend first relay connecting at 22 degree in centigrade graduate and second relay connecting at 26 degree in same graduate. I repeat, defining P1, P2, D7, D8, means defining temperature.

If its winter, just heating system available in home. Temperature usually comes down. So we can set a better circuit to control room temperature. To obtain desire temperature and spend less energy, and prevent dissipating energy for a cooling system at winter, it's better to control heaters by N.O contact of first relay that had set on 22 degree, when is serried by N.C contact of second relay, set on 26 degree.


Now, if room temperature to be on defined confine, kit will keep temperature in span forever. For this loop, room temperature has to be in defined confine, on the start. Temperature increase causes relay 2 off. Then heaters will off. Temperature starts decrease because its winter. When temperature comes from 26 degree to 22, relay 1 turns on immediately after 22 degree and heaters starts. Then temperature starts coming up, to 26 degree. Now relay 2 turns off again and this loop continues forever.

Some tips when mounting components:

Till IC3 is 7812, output of second op-amp, can not represent more temperatures more than 120 degree in centigrade graduate. If you want to use this project for temperatures over 120 degree in centigrade graduate, replace 7812 with a 7815. Then replace 12 volts relays by 15 volts products. Other components don't need to change.

For regular mount, j1 have to not mount. It connects input of 7812 to its output to for direct supply. When you have a 24 volts regulated supply, mount j1 and remove 7812. Then replace 12 volts relays, by 24 volts relays.

You may face with some components that are exactly up on themselves. Do not worry. You have to mount jumpers on the first, then resistors, then ic suckets and then capacitors. For example, this capacitor is up on some resistors. This technique makes PCB smaller.


Also, for some readers who do not have protel programs to open PCB file, here is size and PCB photos.

Size of PCB is: 63mm*102mm.

These pictures help to make project without protel programs and helps to mount components.

But I recommend you to use PCB file and open it with protel 2.7 or later versions as DXP. PCB and schematic files has been attached to the PDF file. Also they are inside RAR file.


Part list:

C1 (100nF)          C2 (100uF)          C3 (100uF)         
C4 (100nF)          C5 (100nF)          C6 (1000uF)        
C7 (100uF)          c8 (100nF)          c9 (100nF/250V)
CN2 (Connector)     CN1 (Connector)        
CN3 (Connector)     CN4 (Connector)     D1 (1N4148)        
D2 (1N4148)         D3 (1N4001)         D4 (1N4001)        
D5 (Red)            D6 (Green)          D7 (3V7)           
D8 (4V2)            D9 (Yellow)         D10 (1N4001)       
D11 (1N4001)        D12 (1N4001)        D13 (1N4001)       
F1 (1A)             IC1 (LM324)         IC2 (LM35)         
IC3 (7812)          IC4 (TL431*)        IC5 (TL431*)        
J1 (Jumper*)        J2 (Jumper)         J3 (Jumper)              
J4 (Jumper)         L1 (100uH***)       P1 (10K*)           
P2 (10K*)           Q1 (BC547B)         Q2 (BC547B)        
R1 (1K)             R2 (330R)           R3 (330R)          
R4 (330R)           R5 (1410R)          R6 (1K)            
R7 (1K)             R8 (4K7)            R9 (4K7)           
R10 (33K)           R11 (33K)           R12 (1k)          
R13 (1k)            R14 (1K)            R15 (0)            
R16 (0)             R17 (0)             R18 (0)            
R19 (0)             R20 (0)             R21 (100R)         
R22 (100R)          R23 (3K3*)          R24 (3K3*)          
R25 (470K**)         R26 (470K**)         RL1 (12V)          
RL2 (12V)           S1 (Push Switch)    S2 (Push Switch)              
TP1 (Set Point1: 3.7v by default)   
TP2 (Set Point2: 4.2v by default)  




If you are not able to detect parts on these photos, use this technique: open circuit photos by mspaint.exe, and press ctrl+I, or go on tool bar on image menu. Then select invert colors command to have better view of PCB. To open paint, press windows key+R, then type mspaint.exe and press enter.

if you do that, you must see something like this:



If you found problem in design, please get me informed to edit and remove problem.

If you need more information or have questions, do not hesitate to call me, or send E-mail.

My phone number in Iran is: +98-912 80 11 244

And my E-mail is: thomasciciyan @ yahoo.com

Good luck, Ciciyan.






Download project in PDF format (English)
Download project in PDF format (Persian)
Download parts list
Download Protel design files



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