Incubator Thermostat Circuit with Hysteresis Control

INCUBATOR-THERMOSTAT-WITH-HYSTERESIS-CONTROL
The following post describes a simple yet accurate adjustable incubator temperature controller circuit which has a special hysteresis control feature. The idea was requested by Dodz, let’s know more.

The Request


Hi Sir,

Good day. I wanna say that your blog is very informative aside from the fact that you are also very helpful blogger. Thank you very much for such wonderful contributions in this world.

Actually, I have a little request to make and I hope this does not burden you that much. I have been researching on analog thermostat for my homemade incubator. I learned that there are probably dozen of ways of doing it using different sensors such as thermistors, bi-metallic strip, transistors, diodes, and so on. I want to build one using either of these methods but I find the diode method as the best one for me because of the availability of the components. However I could not find diagrams that I am comfortable experimenting with. This circuit http://www.zen22142.zen.co.uk/Circuit/Switching/therm_sw.htm is doable for me if only I can have thermistors but there’s none available in our locality. This one http://chemelec.com/Project/Temperature-Sensor/Temp.htm and your circuit http://homemadecircuitsandschematics.blogspot.com/2011/12/how-to-build-simple-electronic.html are also good but could not follow much regarding setting the high and low temp levels and adjusting the hysteresis.

My point is I want to make thermostat with sensor that is diode-based with adjustable hysteresis for a homemade incubator. This project is for personal use and for our local farmers that venture into duck and poultry hatching.

I am an agriculturist by profession by I studied (vocational very basic course) electronics as a hobby. I can read diagrams and some components but not very much. I hope you can make me this circuit. Lastly, I hope you can make simpler explanations especially on setting the temperature thresholds and the hysteresis.

Thank you very much and more more power to you.

The Design

In one of my previous posts I have already discussed an interesting yet very simple incubator thermostat circuit which uses an inexpensive transistor BC 547 for detecting and maintaining the incubation temperature. The circuit includes another sensor in the form of a 1N4148 diode, however this device is used for generating the reference level for the BC547 sensor.

The 1N4148 diode senses the ambient atmospheric temperature and accordingly “informs” the BC547 sensor to adjust the thresholds appropriately. Thus during winter, the threshold would be shifted on the higher side such that incubator stays warmer than during summer seasons.

Everything seems to be perfect in the circuit except one issue, that is the hysteresis factor which is completely missing there.

Without an effective hysteresis the circuit would respond fast making the heater lamp switch at rapid frequencies at the threshold levels.

Moreover adding a hysteresis control feature would allow the user to manually set the average temperature of the compartment as per individual preferences.

The following diagram shows the modified design of the previous circuit, here as we can see, a resistor and a pot has been introduced across pin#2 and pin#6 of the IC. The pot VR2 can be used for adjusting the OFF time of the relay as per the desired preferences.

The addition almost makes the circuit a perfect incubator design.


Parts List

R1 = 2k7,
R2, R5, R6 = 1K
R3, R4, R7 = 10K,
D1—D4 = 1N4007,
D5, D6 = 1N4148,
P1 = 100K,
VR1 = 200 Ohms, 1Watt,
VR2 = 100k pot
C1 = 1000uF/25V,
T1 = BC547,
T2 = BC557,
IC = 741,
OPTO = LED/LDR Combo.
Relay = 12 V, 400 Ohm, SPDT.


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