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Accurate Fridge Thermostat Circuit Using a Single IC LM324

A simple yet effective electronic fridge thermostat circuit has been explained in the following article. The post is based on the request sent to me by Mr.Andy. The proposed idea incorporates just a single IC LM 324 as the main active component. Let's learn more.The email which I received from Mr.Andy:

 Technical Specifications


I am Andy from Caracas. I have seen that you have experience with thermostats and other electronic designs, so i hope you can help me. I need to replace the mechanical fridge thermostat which is not working anymore. I'm sorry i didn't write directly on the blog. I think it's too much text.

I decided to build a different schematic.

It is working well, but only for positive temperatures. I need the schematic to operate from -5 Celsius to +4 Celsius (to use VR1 to set the temperature inside the fridge in the range of -5 Celsius +4 Celsius as the old thermostat knob used to do).

The schematic is using LM35DZ (0 Celsius to 100 Celsius). I’m using LM35CZ (-55 Celsius to +150 Celsius). To make the LM35CZ send negative voltage, I put an 18k resistor between pin2 of LM35 and the negative from the power supply (pin4 of LM358). (as in page 1 or 7(figure7) in the datasheet).

https://www.ti.com/lit/ds/symlink/lm35.pdf

Because i’m using a 5,2v stabilized power supply, i operated the following modifications:1.ZD1, R6 are out. R5 is 550 ohm.

2.VR1 is 5K instead of 2,2K (i couldn’t find a 2,2K pot)The design is not working at temperatures below 0 Celsius. What else should i modifiy?I did some measuring.

At 24 Celsius, LM35CZ is giving 244mVAt -2 Celsius, LM35CZ is giving -112mV (at -3 Celsius is -113mV)At -2 Celsius the voltage between TP1 and GND cand be set from VR1 from between 0 to 2,07vThank you!

Circuit Assessment:


The solution is probably much simpler than it may appear to be.

Basically the circuit is responding only to positive temperatures because it incorporates a single supply. For making it respond to negative temperatures. the circuit or rather the opamps needs to be fed with dual supply voltages.

That will most certainly solve the issue without the need of modifying anything in the circuit.

Though the above circuit looks superb, new hobbyists may find the ICs LM35 and TL431 quite unfamiliar and difficult to configure.A similar type of circuit of an electronic fridge thermostat can be built using just a single IC LM324 and by an ordinary 1N4148 diode as the sensor.

The figure below shows the simple wiring done around a quad opamp IC LM324.

A1 produces a virtual ground to the sensing circuit opamps, thus creates a dual voltage supply very simply avoiding complicated and bulky wiring.A2 forms the sensing stage which utilizes the "garden diode" 1N4148 for doing all the temperature sensing.

A2 amplifies the differences generated across the diode and feeds it to the next stage where A3 is configured as a comparator.

The final result obtained from the output of A4 is finally fed to another comparator stage consisting of A4, and the subsequent relay driver stage. The relay controls the fridge compressor On/OFF switching as per the settings of the preset P1.

P1 should be set such that the green LED just shuts off at -5 degrees or any other lower temperatures, as per the users demands.Next P2 should be adjusted so that the relay just triggers at the above condition.

R13 should be actually replaced with a 1M preset. This preset should be adjusted such that the relay just deactivates at around 4 degrees Celsius or any other closer values again depending upon the users preferences.



 

Need Help? Please leave a comment, I'll get back soon with a reply!




Comments

  1. can we use ic (F72) with lm324 for remove the relay can it possiable i need help.

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  2. How will it help? can you please explain?

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  3. Hi,

    Could you elaborate on the power source? Do we need a 12V positive and negative power source? Also, the ground of R2 is the ground of the power source?

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  4. Hi, a single 12V power supply would be enough, the (+) will go to the 7805 input and the negative or the zero terminal of the supply may be connected to ALL the ground marked points, we have two of those in the diagram.

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  5. so the Lm78l05 acts as its sensor becoz u had a doide 1n4148

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  6. the 1N4148 diode acts as the sensor...

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  7. sir on the circuit diagram i see only to leds where can i place that diode

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  8. diode D1 is the sensor, and this must be attached with the heat source to be controlled, and the power to the heat source must be applied through the shown relay's N/O contact

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  9. sir there i no where lebaled as D1 only d2 and green led and red led only

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  10. Am sorry for disturbing u i have got every thing let me start off with building my circuit becoz its the only thing i was stil looking for will update thanks

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  11. OLupot, build it only if you have understood the concept and know how to test it.

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  12. hello sir am done with building the circuit now left with setting.
    variable P1 and P2 how do i set them

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  13. Olupot, initially keep R12 disconnected from the green LED, keep P1 slider at the center position, heat some water to boiling point, and bring D1 near the vapors rising from the boiling water, and simultaneously adjust P1 such that the green LeD lights up with a good illumination. Do not touch D1 ends while implementing this.

    now, removing the D1 away from the vapors must quickly shut down the LED. At this point your circuit is set for 100 degree temperature, you can repeat the procedure similarly for setting up other temperatures also.

    If the above does not happen, then definitely something's wrong in your circuit or in your component.

    first complete this, then I'll explain how to set the last opamp with P2.

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  14. yes first my 78L05 is heating up imediately i place the batt on
    next i have two leds red and green i disconnected green and heat up D1 but red did not light up. so do not know why my 78 L05 heats up yet am using 12v batt

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  15. it could be due to a faulty IC, a wrongly connected IC, or a short circuit at the output, check the 7805 separately without load, and check its output voltage.

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  16. hello sir this lm78L05 seems to be faulty so can i use lm7805 becoz i have asked they diferent apperances may be it would work

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  17. yes you can use 7805, no problem!

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  18. l have got a new 7805 am replacing it and do the setting then wil update u thanks

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  19. sir acording to your explanation i should disconnect the green led from R12. and use hot water where D1 is placed above then vary P1 until the red led lights brightly

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  20. I have already explained it elaborately, please read my previous comment carefully...

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  21. hello sir i have done the test u told me to do using hot water. but its not responding. i have to check as i vary pin#5 which connects to P1 has no votage

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  22. Hello Olupot, what is not responding?

    the green LED should slowly light up as D1 is heated, and it should slowly shut off as D1 cools down. This threshold can be adjusted with P1

    I had tested this circuit 10 years ago and i could see the results immediately on testing. I had used my soldering iron as the heat source on D1...but too much heat can damage D1.

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  23. sir for me i placed red led with the emitter of BC547 and green led connected it with R12. so i expected the red led to illuminate wen i heat becoz i had disconnected green led from R12.
    i connected the relay as per the circuit so it should also turn on as D1 heats up hope am right

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  24. Olupot, If you did not read by instructions carefully then I can't help, I have already explained the procedure in the earlier comments.

    I had clearly told you to first set-up the green LED and keep the 4rth opamp isolated initially.

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  25. hello sir i have done some simple tests on the doide seperately using a batt while heating the doide it responds but it has failed to respond while connected to to the circuit becoz pin#6 has 0.78v and pin#7 has 0.39v only. i have followed the circuit wiring is ok but only that issue of D1 not responding

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  26. i have come to know that the system is ok.
    one problem am facing is that i did al the tests while measuring the voltage at the led. the voltage is lower than that requred by the led to iluminate properly. its around 1.5v only yet the led am using needs atleast 2.5v to give deem light. so wat can i do now

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  27. if your green LED is responding to heat changes on D1 then your circuit is OK, P1 must be tweaked to increase the brightness while applying heat on D1

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  28. gud morning sir. i want now to set the last variable resister P2. thanks

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  29. good morning, first let me know how you set the green LED?

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