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4 LED Temperature Indicator Circuit

4 LED Temperature Indicator Circuit

The 4 LED temperature indicator circuit discussed here is very useful for getting a visual information regarding the state of temperature which is to be monitored.

Circuit Operation

In the circuit temperature status is displayed using four LEDs.



- A green LED, indicating that the temperature is in the desirable level
- Two yellow LEDs are included to indicate that the temperature is higher than normal, and the situation is unsafe.
- A red LED warning status tells that the temperature is very high and must be acted upon quickly.

To complement the high temperature red LED warning a buzzer is included in the circuit which emits an audible warning note to alert regarding the emergency.

The circuit is executed using four comparators inside the IC LM324. This is an outstanding chip which has four operational amplifiers on par with 741 type together in one package.

The first stage of the diagram shows a voltage divider network formed with the help of R2, R3, R4, R5 and R6 resistors.

Here the voltages are fixed referenced at 2.4V, 4.8V, 7.2V, 9.6V.

Each of these voltages is connected directly to the non-inverting pinout (+) of the operational amplifiers which is being used as comparators

The upper lead of the thermistor (R10) connects directly with all inverting (-) terminals of the opamps.

If the subjected temperature varies, the voltage also proportionately varies at the upper pin of the thermistor.

This induced responsive voltage is compared with the opamp comparators across their non-inverting terminals and in response to lesser voltages sends correspondingly high voltage comparator output activating the relevant LED.

As the temperature rises, conditions across the thermistor begins getting lower illuminating the LEDs in sequence.

When the lower most comparator is activated, the red LED lights and activates the "buzzer" giving an audible alert tune that may be considered crucial if the device needs to be safeguarded.

Circuit Diagram

BOM for the proposed 4 LED temperature detector circuit

- IC (integrated circuit): LM324
- Thermistor: 1 10K (R10)
- Resistors: 5 5K (R2, R3, R4, R5, R6), 1 10K, 4 220 (R7, R8, R9, R11)
- Diodes LED: 1 green, 1 yellow, 1 red
- 1 "buzzer"

Note: for the thermistor, you must keep the terminals long enough, so that it can be terminated across the place where the temperature is in question.



 

Submitted by: Shweta sawant

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About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!



16 thoughts on “4 LED Temperature Indicator Circuit”



  1. Sir,
    Please i need a solar heater circuit which i can warm my chicken coop in Winter season. And i want the circuit Design to come with Thermostat in oder to swich off when temperature reach 37fh and swich on at 35fh.Thanks

  2. Hello sir Swagatam,
    Can I wire this circuit like the one you posted in the link below?
    https://www.homemade-circuits.com/3-step-dc-voltage-level-monitor/
    I want connect it in such a way that I can vary the individual temperature levels for each LED to light up or go off. The LEDs will work in bar mode. I can reconfigure it to work in bar mode.
    For the circuit in the link, can I replace the zener diode with the NTC and replace R2-R6 with 10k pots (using four pots this time)?
    If I cannot, kindly suggest a way to wire the circuit so that it can work like the one in the link.
    I would really love to upgrade this schematic by using LM35 which is a more accurate temperature sensor. Is it possible to wire LM35 and LM324 together? If it’s possible, kindly suggest how to.
    Anticipating your usual prompt response.
    Thank you.

  3. Hello sir Swagatam,
    Thank you very much for this schematic. As Kumar asked above, I also want to use the schematic to control a 220V AC fan. You suggested that a PWM circuit should be included. Can you please suggest such PWM circuit and how it can be used with the above schematic?
    Thanks.

  4. Dear Sir. i want to connect a 3 speed fan to above circuit posted by you.instead of LED, it will control the speed of fan.LED 1 fan off, LED 2 fan speed low LED 3 fan speed medium and LED 4 fan speed high. the fan motor will be driven by 240V and the circuit will b driven by 12V..this is easy to do by my problem is how to connect this speed controlling thing..i got a 3 speed control motor.how can this circuit control the switch of the fan

    • Dear Kissan, it's possible but you'll need to include a PWM circuit also along with the above detector circuit….only then the mentioned application will work.

  5. 4.bp.blogspot.com/-dV_lqrOPthA/VPaBXrZtXmI/AAAAAAAAJnQ/-xmDZcKgRLQ/s1600/temperarture%2Bindicator.png

    Dear Sir. i want to connect a 3 speed fan to this circuit.instead of LED, it will control the speed of fan.LED 1 fan off, LED 2 fan speed low LED 3 fan speed medium and LED 4 fan speed high. the fan motor will be driven by 240V and the circuit will b driven by 12V..this is easy to do by my problem is how to connect this speed controlling thing..i got a 3 speed control motor.how can this circuit control the switch of the fan



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