How to Build a Simple Egg Incubator Thermostat Circuit

An electronic incubator thermostat circuit shown in this article is not only simple to build but is also easy to set and acquire exact tripping points at various different set temperature levels. The setting may be completed through two discrete variable resistors. The sensing range is quite good from 0 to 110 degrees Celsius.
Switching a particular load at different threshold temperature levels doesn’t necessarily need complex configurations to be involved in an electronic circuit. Here we discuss a simple construction procedure of an electronic incubator thermostat.
This simple electronic incubator thermostat will very faithfully sense and activate the output relay at different set temperature levels from 0 to 110 degree Celsius.

The conventional electromechanical temperature sensors or thermostats are not very efficient due to the simple reason that they cannot be optimized with accurate trip points.
Normally these types of temperature sensor or thermostats fundamentally use the ubiquitous bimetal strip for the actual tripping operations. When the temperature to be sensed reaches the threshold point of this metal, it bends and buckles.

Since the electricity to the heating device passes through this metal, it’s buckling causes the contact to break and thus power to the heating element is interrupted – the heater is switched off and the temperature starts falling. As the temperature cools, the bimetal starts straightening to its original form. The moment it reaches its previous shape, the electricity supply to the heater is restored through its contacts and the cycle repeats.

However, the transition points between the switching are too long and not consistent and therefore not reliable for accurate operations.

The circuit presented here is absolutely free from these drawbacks and will produce comparatively high degree of accuracy as far the upper and the lower tripping operations are concerned.

Parts List
R1 = 2k7,
R2, R5,
R6 = 1K
R3, R4 =
D1—D4 =
D5, D6 =
P1 = 100K,
VR1 = 200
Ohms, 1Watt,
C1 =
T1 =
T2 =
IC = 741,
LED/LDR Combo.
Relay = 12 V, 400 Ohm, SPDT.
Circuit Description

We know that every semiconductor electronic component changes its electrical conductivity in response to the varying ambient temperature. This property is exploited here to make the circuit work as a temperature sensor and controller.

Diode D5 and transistor T1 together form a differential temperature sensor and interact greatly with each other with changes in the respective surrounding temperature.

Also since D5 acts as the reference source by staying at the ambient temperature level should be kept as far as possible from T1 and in open air.

Pot VR1 may be used externally to optimize the reference level set naturally by D5.
Now assuming D5 is at a relatively fixed temperature level (ambient), if the temperature in question around T1 starts rising, after a particular threshold level as set by VR1, T1 will begin to saturate and gradually start conducting.

Once it reaches the forward voltage drop of the LED inside the opto-coupler, it will start glowing correspondingly brighter as the above temperature rises.
Interestingly as the LED light reaches a particular level, further set by P1, IC1 picks this up and instantly switches its output.

T2 along with relay also respond to the IC’s command and respectively actuate to trip off the load or the heat source in question.

How to Make an LED/LDR Opto-Coupler?

Making a homemade LED/LDR opto is actually very simple. Cut a piece of general purpose board about 1 by 1 inch.
Bend the LDR leads near its “head.” Also take a green RED LED, bend it just as the LDR (See figure and Click to Enlarge).

Insert them over the PCB so that the LED lens point is touching the LDR sensing surface and are face to face.
Solder their leads at the track side of the PCB; do not cut off the remaining excess lead portion.
Cover the top with an opaque lid and make sure its light proof. Preferably seal off the edges with some opaque sealing glue.

Let it dry. Your home made LED/LDR based opto-coupler is ready and may be fixed over the main circuit board with its leads orientations done as per the electronic incubator thermostat circuit schematic.


After some careful investigation it became evident that the above opto-coupler can be totally avoided from the proposed incubator controller circuit.

Here are the modifications which needs to be made after eliminating the opto.

R2 now directly connects with the collector of T1.

The junction of pin#2 of IC1 and P1 hooks up with the above R2/T1 junction.

That’s it, the simpler version is now all ready, much improved and easier to handle.

Please check-out the much simplified version of the above circuit:

196 Replies to “How to Build a Simple Egg Incubator Thermostat Circuit”

  1. hi..what is the main output of this circut,whether to make constant temperature at output or to switch the heater just on and off…

    1. To switch the heater ON and OFF with respect to the ambient temperature….as the ambient temperature increases the heater is kept switched OFF for longer periods of time and vice versa.

  2. sir,i would like to do a mini project on this topic and i would like to know hw to set the temperature to a 35-38 degree celsius used in neonatal incubators.pls rply fast….
    thank u

  3. sir, you are a very talented guy when it comes to electronics but i got a lil problem with regards to my project to pass for my accelerated electronics course. how can i possibly wired this these three and lay out it in pcb hence im no whiz on electronics:

    water circuit level a controlfor humidity
    heater sensor to heat the incubator
    motor timer to turn the eggs 4 times aday
    if you could hel me howto get this work it would be an honor

    1. Thank you very much!

      A simple Incubator heater sensor circuit is given in the above article, so you can try the shown circuit.

      I'll try to design your other needs soon, and post it in my blog.

      Stay in touch.

    2. dear swagatam,
      thanks for the reply and for your concern actually rigth after i sent you a message i design it already and put to pdf file for printing in one paper the (three circuit boards)i run drc test on each board it passes but some pins are not connected due to im no whiz in finding the rigth ic's on my software it's quite hard but im enjoying plotting drawings…
      by way im using a microcad for pcb layout.
      thanks and happy new year

  4. Good day Sir Swagatam, do you have a PCB layout for this incubator project? I am currently downloading the orcad software for me to make the pcb layout of this but takes a day i think for me to wait for the download to finish. your blog really helps a lot of people especially ECE students. Good job. can i have a copy of your pcb layout please sir, you can email me at… Thank you so much sir

    1. Gooday and Thank you Randolph,

      As you know designing a PCB is a hugely time consuming task, so presently it would be difficult for me to design it, but I'll try to update it in my free time soon.

  5. thank you very much sir Swagatam!!! i just realized my mistakE!

    actually, I just committed a "PERSONAL ERROR" of "printing out your design" leaving the "IC = 741" uncopied!!

    Sorry for this mistaken question! 😀 hahaha

    By-the-way… i still have another question sir,

    1.) SInce P1 is also a variable Resistor.. which has 10k,
    Do i need to consider its Wattage???

    2.) is P1 the adjustable part? or VR1?

    TY PLEASE KEEP THIS UP! :thumbsup:

  6. i really have problem on my circuit, is there any alternative on what to use as replacement for 200ohms potentiometer as variable resistor? my circuit isnt working since i used 2k ohms potentiometer as VR.

    1. try making a potential divider network by joining 56 ohm/100 ohm resistors, the free end of 56 ohm goes to R1, the free end of 100 ohms goes to ground, the center joint goes to the base of bc547…..hopefully this might work in place of the pot

  7. Hello Swagatam
    Please i want to build this cercuit for an incubator but i don't understand how D5 1N4148 and T1 BC547 measures the temperature. the datasheets for this two components does not talk about how they react on temperature.
    please explain it to me.
    Best regards,

    1. Hello Giniti,

      The datasheet of all semiconductor mention one of the characteristics (power dissipation typically)) with reference to ambient temperature, referred to as Tamb(oC), which indicates that these components vary their power output in response to ambient temperature.

  8. Hello Swagatam,
    I did build the circuit without TR1 D1,2,3,4 and C1, finaly i wanted to test it bij using an external power of 12v DC. By mistake i connected the power wrong (12v becomes the mass) and the IC burned.
    What should i do? (replace all the transistors and diodes)
    thanks in advance

  9. Hi Sir,

    Thank you for this project, I am really interested of making this one for my incubator. Just few queries if I may: (a) How can I set the low and high temperature levels? What pot/vr to tweak to?; (b) How can I set the hysteresis value?

    Thank you very much for your considerations.

    1. This circuit is without hysteresis.

      For adding a hysteresis control just add a 100K preset with a 10K series resistor across pin#2 and pin#6 of the IC.

      This preset may be suitably adjusted for getting the desired hysteresis.

  10. Hello Swagatamk,

    I want to upload the scheme like you explain on 11, 2013 at 10:21 PM to prevent mistakes and later if this circuit works goed for me i will make the PCB and share it here.


  11. Good day sir, im a noob when it comes to electronics, i just want to ask if what kind of resistor do i need to use as p1 so i can get a 37.778 degrees celcius temperature, i am planning to make a homemade incubator for my quails and i am assuming they will lay eggs in less than a week from now. i hope to get a reply soon from you sir, thank you in advance

    1. Hello, good day,

      P1 is a preset or a variable resistor, you will have to adjust it manually through some trial and error for obtaining the correct threshold cut-off , this will require a good prior knowledge of electronic basics.

  12. Thank you for your quick reply sir, sorry for asking too much but i badly need your help regarding the thermostat. When you said that d5 must be placed away from ti, does that mean the only part that should be exposed inside the incubator to sense the heat would be T1? i hope to hear from you again sir. Thank you once again and i hope you continue helping noobs like us when it comes to electronics.

  13. thank you for your help sir. i really appreciate everything and i hope to read more articles regarding updates and improvements about the homemade thermostat and maybe incubator in the future. i was hoping to read an article from you about the auto egg roll and also i hope you can invent an electronic auto feeder for my quails. thanks in advance!

    1. Thanks, however my expertise lies in the electronic field, so a mechanical design would be difficult to think and produce….still I'll try to do it if possible…


  15. good day sir, i already build the circuit above and it is working well. my project, the relay it creates a buzzer like sound during turn on and turn off it is normal?i notice that it only create a buzzer like sound because of the led that is gradually glow. is there is any solution for this?

    1. Good day aian,

      connect a 33uF/25V capacitor across the base and positive of the transistor, this will stop the relay from buzzing.

      any other higher value capacitor will also do.

  16. Well really its a informative post..
    I have a couple of questions. will yu please tell how to set the max and min temperature, which variable resister is to set max and which pot is tuned to set min temp..
    And why bc547 transister is near to heating device?
    Is it the thermal transister?

    1. Thanks!

      VR1 may be used for setting-up the cut off threshold, which must not exceed 110 degree Celsius (max), while P1 may be used for fine tuning the set cut off point.

  17. Hi! Swagat, as this is more informative , can I know the heater size or rather the element size and if I think of afan tell me the size and the connection points. Thanks

  18. Hay sir I have built it, its working good.
    8/10 Egg's hatched.

    But I gotta little trouble here, in the day time the temperature will be maintained as desired, but in the nights its on/off range will be lower by almost 5degrees.
    Ex: day= 37-39(perfect)
    Night= it goes 32-34(problem).
    All these days I use to tune for the desired temp twice a day.
    i guess its because of the room temp which will be lower in night at the ambient temperature diode.
    Suggest some replacement for this diode to make it constant all the time sir.

    1. That's great, if you look carefully you will find that actually the 1N4148 is not required, so you can simply remove it, and make VR1 into a 1K preset. Finished, now adjust the 1K preset for the desired actions.

      However in this case your power supply must be regulated, so add a 7812 IC after the bridge for making the supply stabilized.

  19. Dear Swagtam
    I like the modified circuit version which I intend to build soon.Would appreciate if you could kindly explain how to add a LED to indicate when the system is on and off.

    1. connect one LED from pin6 to positive, another from pin6 to ground, both LEDs should have individual series 10k resistors.

      one of them will indicate ON, the other one as OFF.

  20. sir, is P1 a 10k? or 100k? and how do we tune the presets? is it VR1 or P1? and BC 547 is the only one who should be kept near the heat source? or is there any other components? thank you sir..

    1. for second diagram P1 = 1k,
      BC547 is the only sensor near heat source.
      keep P2 at center, set VR1 roughly to activate the relay at about 70 degree temp, after this only P2 may be used for getting other desired thresholds.

    1. Connect an LED in between R2 and collector of BC547…..keep pin2 of 741 disconnected with R2.
      Switch ON power and heat BC547 slightly at about 60 degree and adjust VR1 such that the LED starts glowing dimly…that's all your circuit is set.
      Now connect pin2 of 741 back in position, repeat the procedure and adjust P1 to activate relay at any desired temperature within 110 degree.

    1. It basically depends on Ub (supply voltage), R1 and R2.
      R1 and R2 may be selected with the following formulas:

      R1 = (Ub – 0.6)/5 (kohms)
      R2 = (Ub – 1.5)/15 (kohns)

      these are the only computations available.

  21. Hey sir..
    I was the one who said 8/10 egg's
    hatched, please read that
    comment again before answering
    i did 4 boards with the modified
    circuit with the following
    1. gave LED in series with 10k.
    2.hyst 100k preset in series with
    10k resister.
    3. A capacitor across base and
    Done everything as yu said.
    Still problem is'nt solved, all
    boards giving same problem.
    It works good and fine first day
    (completely accurate), but after a
    day the temp on/off range
    changes again.
    Sometimes it never turns OFF or
    ON, couldn't findout what is the
    i think the room temperature
    again affecting its behaviour.

    1. Hi Rithesh,

      Remove the 100k hyst preset link, we do not need it at all because the heat will never rise or fall quickly, so a hysteresis feature is not required.

      Did you try the last circuit as given in the above article??

      Make it exactly as it's given…don't forget to use a 7812 IC with the circuit.

      Use good quality preset for both VR1 and P1.

      Make the above mods, it will work.

      With 1N4148 removed room temperature will not affect the circuit in anyway.

      In future you could think of replacing VR1, P1 with fixed resistors.

    1. Hi Ngoni,

      It will depend on how many heat sources you are using, for each heat source you will have to incorporate an individual circuit.

      Hatching period is not known to me.

    1. It will depend on the egg hatching specifications, you can get the information online regarding the right temperature required for hatching eggs. Then you can adjust the above circuit for maintaining the particular temperature range, throughout.

  22. hello sir swagatam, i already made the simplified version of your design, but it wont turn off the heat source ,,, if i adjust the vr1 manually, it will initialize turn on/off the relay but it wont turn off/on automatically to my desired temp, will you email me the step by step procedure in setting up my desired temp, cut-off,, tanx in advance

    1. hello Oliver, i am sorry it's not that easy.
      you can include an LED immediately after R2 and in series with it.
      Allow the transistor BC547 to heat up to around 70 degrees, and adjust VR1 to just make the LED glow.
      At this point you can try adjusting P1 for the required switching of the relay, or if it's already switched ON adjust P1 to just switch it OFF and then readjust to just switch it ON back.
      The circuit is set now and will repeat the actions every time the temp reaches the set threshold
      I hope you got it.

  23. sir, i have a temperature controller built-in-type, in its user guide denote that use a snubber circuit for long life of relay of temperature controller, how can select the values of resistor and capacitor which involved in snubber circuit? i'm waiting for your suggestion….

  24. Good day Sir Swagatam, i will be making this projet and i will base on your circuit as a reference =) I only have samome basic concepts questions.

    What will happen to the Light (heater) if the heat goes up in the sensor? Will it dim or will it turn off?
    Which pots are assigned to set the temperatures from 0-120? Can it be made in a way that it that if (assuming) i set the min temperature to 20 degrees it will light up and shine brighter if lower temperatures are achieved? And 35 degrees it will dim?

    Or i got it all wrong? Hehehe sorry for asking a lot of question. I'm kinda new to these.

    1. Good day Lloyd, the heater will shut off for a moment if the heat exceeds above the set high threshold, and will switch ON back as the temperature falls below the lower threshold.

      The setting up adjustments could be done in the following manner:

      connect a RED LED in series with R2, and adjust VR1 until this LED begins glowing at the required high temperature from the heater (not to exceed 100 degrees C), ignore the relay response during this setting.

      the heater should not be connected through the relay rather directly through a manual switch

      Now once the above position is reached, carefully adjust P1 so that the relay just deactivates, if it's already in a deactivated position, force it to activate by adjusting P1 and immediately adjust it back to deactivate it, all these must be while the red LED is still glowing bright.

      your setting up procedure is complete now, connect the heater via the relay contacts as shown in the diagram, check and enjoy the automatic response as per the above settings.

    2. Hi Sir, I don't know if my comment have been published. But just to be sure, i want to thank you again and i appreciate it very much for replying to my questions!
      I cannot find any way to private message you so i will put my request here to you..

      I changed my mind, instead i wanna make a 12v dc temperature controlled fan.

      I want it to make as such, it will turn on at 20 degrees and it will turn off below 20 degrees. If it reaches 20 degrees above, the speed of the fan will go faster aswell. The heat source will be the surrounding. And can we possibly add a LED, that will glow brighter aswell? I need as an example of a closed loop feedback control system. Please..

      Thank you again sir! I'll wait for your reply. Regards! (my gmail is here :

  25. Sir I need help on how to construct a DC thermostat with LCD display , which can equally trigger on a fan at 50degree centigrade ….. Pls this is my project … I need urgent help … Any link or help will be highly appreciated …

  26. Hi sir! Im trying to make the simpler version of this circuit. The one that doesn't need the OPTO coupler. However I'm having trouble at the Variable Resistors, you said that the P1 and the VR1 are both presets but my teacher said that we should make the P1 as the Rotatory Potentiometer. So I bought a Potentiometer 100k as the P1 and 200ohms as the VR1. Is there a problem that might occur in the circuit? Or is it all the same? However We tried to design the simpler version that we considered the P1 as a potentiometer and the VR1 as the preset. And the source heat doesn't turn off.

    1. P1 should be a preset because a pot can be too sensitive and cannot be held tightly in a set position, this can make the circuit unstable.

      The simpler version is not practically verified and might need some practical measurements and adjustments for getting proper results. and remember while testing if the BC547 is heated too much it might get damaged permanently, therefore things could be slightly difficult with the simpler version unless you exactly know how to proceed, so I would recommend you to go for the first circuit and build it as it's shown, because this circuit has the facility of an LED lamp which gives a direct indication of the BC547 response to heat.

      Therefore it becomes possible to first fix the amount of glow on the LED by correctly adjusting VR1 at the specified heat level and after that it can be sealed with an LDR for the relay activation.

    2. …in the simpler version the relay is supposed to be switched ON initially without heat application,

      when heat is applied and it crosses above the required threshold, the pin2 voltage is supposed to go just below pin3 to cause the relay to just switch OFF…

      the above will strictly depend on the precise setting of P1 at the specified heat threshold which should not be above 90 degree C max

  27. Hi sir! Im trying to do the the simpler version of this circuit. The one that doesn't need the OPTO coupler. However I'm having trouble knowing the use of the Variable Resistors. You said that the P1 and the VR1 are both presets. But my teacher said that we should use the P1 as a Rotatory Potentiometer. So I bought a 100k Potentiometer and used that as the P1 and a 200 ohms preset as the VR1. Is there going to be a problem in the circuit? Or is it just the same? However we designed and applied the simpler version using the P1 as the potentio meter and the VR1 as the preset it turned out that the Heat source doesn't turn off.

  28. My comment was posted twice, I'm new to blogs, I didn't know that the first one was published. Anyway thank you, I'll make sure to keep that in mind. I'll design a new one using the first circuit where the D5 and the T1 interact with each other as the sensor.

  29. Sir, I designed and built everything using the first circuit, I even followed how the assembly of the Led or Ldr should be, when you say sealed is there a proper way for the Led and Ldr to be permanently stick with each other? When I adjust the VR 200ohms, the glow in the Led also changes. I don't think that the Relay was even turned on because when I tried to put the BC547 near the heat source, still nothing has changed, It doesn't turn off. When I touch the IC it does produce heat, but not even a single reaction from the relay, What might be the problem? Is it the wattage in the resistors? I just used a 0.25 watts resistors. Or is it the P1? Im a little bit confused about that, because when I bought the variable resistors the 200ohms Variable resistor is a little blue and white preset with a number indicated 202, and when I looked at the P1 100k it is the same little blue and white preset but the number indicated is 104, so I was thinking if it was just 100ohms, and not 100k ohms, its confusing because there is no name indicated unlike the pot. What might be the problem that the heat source doesn't turn off.

    1. Christian, I told you to first set the LED glow at the preferred heat level and after that you need to seal it, by sealing it means that the assembly should be protected from the ambient light…this is clearly mentioned in the article…did you follow this??

      Once you set and seal the LED/LDR in a lightproof enclosure VR1 must NOT be touched anymore, otherwise the LED glow (temp setting) will get disturbed and you will get wrong results.

      The relay will operate when the opamp output goes low, and this in turn will happen only when its pin2 voltage gets slightly lower than its pin3.

      after integrating the LED/LDR assembly to pin2 of the opamp, introduce the controlled heat to the sensor BC547 and simultaneously check the voltage at pin2, you will find it dropping and stabilizing at some point, now adjust the pin2 preset such that this voltage is still reduced slightly below pin3 voltage…which will immediately cause the relay to click.

      If this

  30. Hi Sir Swagatam, I've got another problem, since there's no regulator, the voltage at pin 3 goes up to 7 to 7.2 voltage instead of 6 due to the Voltage Divider so I tried making the voltage at pin 2 slightly near below the voltage at pin 3 at around 6.8 volts hoping for the op amp output to be low and for the relay to operate.

    As I use the multimeter, when I introduced the controlled heat to the sensor BC547, The voltage at pin 2 unexpectedly increases instead of dropping, what causes the increasing of voltage at pin 2 as I introduce the heat to the sensor?

    1. Hi Christian, replace R4 with a 6V zener diode, that will take care of the unstable reference at pin3.

      I am sorry I told you the opposite by mistake….pin2 should witness a rise in voltage as soon as heat is introduced near the sensor.

      that means your circuit is working perfectly as per the expectations.

      so the moment pin2 voltage goes above pin3, the relay is supposed to switch ON (pin6 should go low)….you just need to confirm this now…

    2. Okay so I removed the R4 and replaced it with a 6V zener diode, now the Voltage at pin 3 is now stable at 6 volts, I set the voltage at pin 2 slightly near below 6 volts, I did set the voltage by adjusting the P1. When I set a LED glow I didn't touch the VR1 anymore, I just set the voltage at pin 2 by adjusting the P1, however, when I introduced the heat to the sensor, The voltage at pin 2 increases but as soon as I checked the voltage at pin 6 which is the output of the op amp, The Voltage was at 15 volts and doesn't show even just a slight dropping of voltage, considering the 15 volts as 12 volts if its regulated, I can't figure out the problem so please tell me my mistake.

    3. touch and remove the pin2 with the positive rail manually and simultaneous check the pin6 voltage it should switch from 0 to 12V/15V each time this is done…if this not happening then your IC may have gone faulty.

      1. Sir Swagatam, did you mean that when I remove the pin 2 along with the positive rail manually, the pin 6 voltage should switch from 12/15v to 0? I tried to do it, and you're right sir, I think the IC may have really gone faulty, When I removed the pin 2 along with the positive rail, the voltage at pin 6 still remains at 12V/15V. So I replaced the IC with a new one and tried it again but still the problem remains the same, However, As I checked the voltages. I discovered a new problem and maybe its the real problem that causes the Relay not to operate. When I checked the voltages of the pins that were not included in the circuit, The voltage at pin 5 remains at 0, the voltage at pin 8 is steady at 14V, and the Voltage at pin 1 goes to 13V, the interesting part is when I introduced the heat to the sensor I simultaneously checked the voltages. The voltage at pin 2 increases but the moment that the voltage at pin 2 goes above pin 3, the voltage at pin 1 suddenly goes to 0, but when the voltage at pin 2 goes back below the voltage at pin 3 the voltage at pin 1 goes back also to 13V the others voltages remains the same, The Pin 6 should be the one doing this not the pin 1.

        I really need your help sir, I'm just a student trying to finish this circuit as my project in Electronics.

        1. Christian, I meant to say that without disturbing any connections just short circuit pin2 with the positive rail, this should instantly turn pin6 from 12V to 0V, and on removing the situation at pin6 should revert from 0 back to 12V.

          The other pinouts of the 741 IC are irrelevant and should not be considered, only pun3/2/6/7 and pin4 are the ones that are used here, the rest must be ignored.

          Pin6 is the one that must change its response to the pin2 pin3 voltage differences, if this is not happening there could be something seriously missing or wrong n your IC connection.

          just verify the above accordingly. Connect an LED in series with the base of the transistor to get an instant indication regarding the pin6 response…LED anode should be connected to the base, and the cathode to the resistor junction.

  31. Sir thanks for this project
    I have followed all the steps you gave carefully but i am still having problems with setting of the thresholds
    When I connect red LED in series with R2 the relay doesn't on/off
    What can be the problem

    1. Patrick, if you have followed everything correctly then it should have worked.

      everything's pretty straightforward.

      Initially do not seal the LED LDR.

      first set P1 such that the LED just lights up at the preferred heat (below 100 degree).

      once the optimal glow is achieved you need to seal P1 with some glue.

      Next seal the LED with the LDR carefully inside a lightproof box.

      After this just do stepwise as suggested to Mr. Christian in the above comments.

      1. both are good, but the first one allows you to set the heat level by looking at the LED glow….but it involves the hassle of making the LED/LDR assembly.

        for newcomers I recommend the first one but the LDR/LED sealing is crucial.

  32. Hi Sir Swagatam, I want to ask if what I did was correct, I put a 10k resistor in series with the LDR and P1 and 100k POT namely the "Rf" to the rail of pin 2 and pin 6, I saw in the book that the Inverting op amp has a resistor and Rf for the voltage output at pin 6 so I thought that maybe I could try to calculate and put a resistor there for the output voltage to drop, when I tweaked the POT namely the "Rf", the voltage at pin 2 also changes, but as I tweak it again the relay did click and the heat source turned off, So I tried to learn the adjustments, however I'm having trouble with the variable resistors, what I did was, I tried to adjust the P1=100k and it gives me a change in voltage at pin 2 like how the "Rf" does, the only difference between them was if I tweak the "Rf" theres a bit of a change at the output voltage at pin 6, so I set up a fix point of P1, and focused on the "Rf", I wrote every details of what I observed, if I tweak the "Rf" and the voltage at pin 2 increases, the voltage at pin 6 drops and vice versa, so when I tried to find the point where the relay clicks, for example when the output voltage at pin 2 increases up to 7V and the output voltage at pin 6 drops from 12v to 10v the relay clicks and the heat source turns off. So when I found the point where the relay clicks at 7V I adjusted the "Rf" slightly down to turn on the heat source like about 6.5V, but when I introduced the controlled heat to the Sensor, the voltage at pin 2 increases, but when it reaches the 7V where I assumed that the heat source should turn off, it sadly didn't, so in other words the relay clicks when I adjust the "Rf" to 7V, but when I let the sensor do it on its own to reach the voltage at 7V, the relay won't click, please tell me that I didn't ruin the entire circuit.

    1. Hi Christian, all these are not required.

      you are unnecessarily making a simple concept too complex.

      previously you said that pin2 voltage was rising above pin3 in response to heat…so that is correct.

      but if the above response is not making pn6 low then the IC is faulty ….I also told to connect an LEd in series with base of the transistor for getting an indication…may be you did not do that as well.

      you can refer to the following article to learn how an opamp works like a comparator:

  33. Sir Swagatam, Thank You so much for your continuous help, I wrote the details of the point where the relay clicks when the sensor is being heated inside the incubator, but I'm curious at some point, for example if the sensor is heated and the relay clicks at 7v so the moment that the voltage at pin2 increases up to 7V the heat source will turn off, and it slowly cools the sensor which makes the voltage at pin 2 dropping, so the moment that it drops the voltage below 7volts the heat source will again turn on, This will make the desired temperature disturbed, is there a way that the heat source will just turn on again when the voltage at pin2 reaches the initial point? If the sensor is being heated I can notice easily the change of voltage at pin 2 but when the time of cooling comes, it takes too long for the heat source to turn on again, By the way the relay creates a sound like buzzing and the heat source starts to dim continuously before it turns on/off. What should I do, Is this normal?

    1. Chritian, leave it on the sensor, because he sensor will switch ON the load only when it cools down itself at the earlier point or when the ambient temperature goes below the set temperature by 2 to 3 degrees, that is what the sensor is supposed to do and it will do this correctly.

      In order to force the BC547 to cool quickly you can attach a small aluminum strip over it, so this might help to heat it faster and cool it faster…

      connect a 100uF/25V capacitor right across the relay coil to prevent its buzzing at the thresholds

      1. I'll definitely do that Sir Swagatam, uhmm Relay Coil do you mean to the rails of the N/C and the Central Common Terminal? or to the rails of the pins who went to the collector and the ground? And which rail should I put the positive and negative of the 100uf/25 capacitor?

        1. connect the capacitor between collector and ground of the T2, or in parallel with D6

          positive to collector, and negative with ground

          alternatively you can connect a 10uF/25V capacitor across base emitter of T2 for getting the same results

  34. Hello Sir Swagatam , i have some doubts in this design
    for my project
    1. i want to control the temperature cutoff at 75 celsius and on the voltage at 45 degree celsius . how i can adjust the cutoff and on temperature
    2. i want maximum temperature at 75 degree celsius for that which resistor i want to change and what range i want change the resistor and same as for minimum temperature as 45 degree celsius

    1. Hello Unknown,

      the 75 degrees can be set by the P1 preset, and for setting the 45 degrees, you will need to connect a 220K preset across pin6 and pin2 (hysteresis control, this preset can be used for adjusting the 45 degrees restoration.

      or alternatively you can use another opamp with its PIN#2 connected with pin3 of the existing opamp, and PIN#3 configured with a separate preset but attached to the same points as the existing opamp preset pin2.

      the output of this opamp then can be connected with the base of the transistor for the cut off action, with its PIN#3 preset adjusted for cutting off at 45C.

    2. thank u for your reply
      i have some query about your reply
      1. how to set p1 as 75 degree . when we set the preset value of the resistor before or after the power supply given.
      2. if we use 220k preset for setting 45 degree then if we want to set our minimum temperature as 35 degree then which resistor value we want to use. Is there any special formal for setting resistor preset

    3. Actually I have already explained the procedures in the earlier comments, I'll repeat it briefly for you.

      initially do not insert the LED with the LDR.
      heat the BC547 at 75 C and adjust VR1 until the LED illuminates jut moderately, once VR1 is set now you can seal this LED with the opamp LDR.
      after this repeat the heating procedure and now adjust P1 until you find the relay just clicking…seal P1 now…the circuit is now set for 75 C.

      repeat the procedure to confirm the activation threshold.

      do this without connecting the 220K preset.

      first confirm this result, afterwards we can go ahead with the 220k integration.

  35. Hello Sir Swagatam , i have some doubts in this design
    for my project
    1. i want to control the temperature cutoff at 75 celsius and on the voltage at 45 degree celsius . how i can adjust the cutoff and on temperature
    2. i want maximum temperature at 75 degree celsius for that which resistor i want to change and what range i want change the resistor and same as for minimum temperature as 45 degree celsius

  36. Hai Swagatam,

    I have few queries could you please explain to set the minimum and maximum temperature of the circuit.
    2.can i use 24vdc as input voltage for incubator Thermostat Circuit (cause in input side we have used a 7812 IC on that additionally 0.33micro farad capacitor as input and 0.1 micro farad capacitor as output side).
    3.In relay you have connected 2 pin is input side and output side also connected 2 pin only one pin is remaining in o/p side,can you please explain that relay connection.
    4.Transistor Bc547 Emitter terminal line is ground connection am i right.

    1. Hi Udhayakumar,

      the above circuit will switch ON/OFF sharply within 2 degrees range, meaning if it's set for tripping at 50C then it will keep toggling ON/OFF within the range of 50C and 52C, for setting the range apart you may have to include a resistor across pin2 and pin6 of the opamp, as explained in the previous comment.

      24V will do if 7812 is employed in the circuit.

      in the relay the blue box ends correspond to the relay coil, while the unused contact represents N/O

      BC547 emitter is connected to ground or the negative line.

    2. Hi Swagatam,

      Thanks swagatam for your valuable comments also i am need your help please help me on this,

      I have tried the second circuit(without opto LED/LDR compo),on that am used a 24vdc as input to the Ic 7812 circuit used 12v and my load is 24v dc.

      Now switch on the input and checked with the multimeter across input of IC 7812(+) and relay centre pin(-) am getting 24v, also heated the BC 547 more than 60degree but cut-off function not working.

      at the same time check the voltage with respect input of IC 7812(+) and circuit ground as(-) am get the zero voltage of load, i don't know where is the mistake please help me on this.

    3. Hi udhayakumar,

      It will be difficult for me to explain the same procedures again and again to each member individually.

      please go through the above comments to exactly understand how the circuit needs to be handled.
      unless you thoroughly understand how an opamp works and how the temperature indicator stage needs to be integrated with the opamp circuit you would find succeeding with this design extremely difficult.

  37. Sir,how it possible bc547 how to control temperature. The transistor can't trigger relay.
    Can I replace LM35 in BC547 place?
    Please reply me!!

  38. Please tell me If I can Use a small coli heater instead of the lamb in this circuit?
    I am aiming to get heat in the range of 45 deg. to 50 deg in my small Incubator.

    and if possible, please provide me the coli type.

    And im really sorry for bothering you,

    I need more information on how I can do my small project.

  39. Hi Swagatam,
    I´ve built this circuit based on the second schematic, and I´m having some problems setting the temp trip point. After re-reading your explanation I realized that there is no D5 in my circuit. Is it missing from the second schematic or not necessary?

    1. Hi Dan, D5 provides a reference level for T1, they both interact with one another and decide the collector voltage of T1.

      but I thought that since 7812 is already there, the T1 preset would be enough to provide the reference level for the collector, and therefore removed D5…you can try putting it back or you can also try replacing the area with a 4k7 preset.

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