Cold Sensor Circuit

This circuit monitors the temperature level in Air-conditioned rooms, Labs, Cold storage etc. and intimate you whether the low temperature is maintained in the room or not. It gives audio visual indications when the Air-conditioning system or freezing system fails and temperature rises. This prevents the spoilage of stored materials. If the temperature is above the preset level, it gives LED indications first and after Four minutes it gives loud beeps inviting immediate attention.

The circuit uses NTC Thermister as temperature sensor. Its resistance decreases when the temperature in its vicinity increases above one degree or more.IC1 is wired as a voltage comparator with its inverting and non inverting inputs tied to two potential dividers. The non inverting input (pin3) receives a variable voltage from the potential divider comprising the NTC Thermister and VR1. Similarly, the inverting input (pin 2) also receives an adjustable voltage from the potential divider R1 and VR2. Preset VR1 adjusts the voltage level at pin 3 slightly lower than that of pin 2 in the idle state in which the room temperature is below 26 degree. VR2 adjusts the voltage level at pin2 of IC1 slightly higher than that of pin 3.

Cold Sensor Circuit diagram

Errata : Pin 8 of IC CD4060 is ground. It should be connected to negative rail.

Normally IC1 will give Zero volt output since its pin3 gets lower voltage than pin2.This is due to high resistance of Thermister in cold. When the Refrigeration system fails, room temperature rises above 30 degree and the resistance of Thermister decreases. This allows more current to flow into pin 3 and output of IC1 becomes high. This triggers T1 and Yellow LED turns on indicating that the room temperature is rising. If this condition persists, T1 remains conducting and the reset pin 12 of IC2 reach the ground potential and the IC2 starts oscillating. With the given values of C1 and R5, Q8 output becomes high after 2 minutes. This makes the Red LED flashing. If the condition continues, Q9 output becomes high after 4 minutes and buzzer starts beeping. In short, if the refrigeration system is not restarted within 4 minutes buzzer beeps to invite the attention.

NTC Thermister and IC CD4060


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  • jay

    i already test in in the breadboard. sir when i connect it in a 9v supply. the yellow LED is turn on and i wait for almost 3 mins. the red LED is not working also the buzzer is not. pls help me . i test it in a room temp. i also adjust the VR1 but still wont work.

  • jay

    thnx very much sir..

  • D.Mohankumar

    IC CA3140 is selected since its input is very sensitive and output swings down to zero volt.Any Op Amp IC like CA3130 or similar like TL071with same pin arrangement can be substituted. These ICs are common types

  • jay

    how about the ca3140?
    havent available ic here in the phil.
    what kind of ic can i replace?

  • D.Mohankumar

    It will also works in room temperature. Adjust the sensitivity of thermister using VR1. Pin8 of CD 4060 is ground. It is missing in the diagram. I will correct it. Thanks for pointing out the mistake

  • jay

    this ckt is an on-off control right?
    according to your ckt, it will works if the refrigiration fails., how can i test the ckt if i dont have a room like a cold storage room? it will works if i test it in a room temp?
    lastly, the cd4060 the pin 8 (VSS) where is it? its ok if i disregard the pin8?

  • D.Mohankumar

    The Thermister is very sensitive to temperature variations so presets are necessary to set the output of IC1.After adjusting the exact point, measure the resistance of the preset’s wiper and end, then you can replace it with an exact value of resistor.But the exact resistor value may sometime difficult to get

  • jay

    sir ill make it a prototype. can i ask a question.?
    vr1 and vr2 can be replace into a simple resistor only?
    pls response. thnx