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

How to Select the Resistors

If you wish to change the LED switching range across a desired input detection range, you may adjust the reference resistor values as per your requirement, as I have explained below:

As we can understand that the 4 op amps of the LM324 are set up as comparators, wherein the non-inverting pins 3, 5, 10, 12 are clamped to the corresponding fixed reference levels determined by the resistors R2----R6.

The inverting inputs of the 4 op amps are joined in common and connected with another resistive divider formed by R1/Thermistor. The potential across this resistive divider junction varies depending on the variation in the temperature.

This varying temperature dependent potential across the inverting inputs of the op amps is compared with the relevant reference voltage levels across the non-inverting pins 3,5,10,12.

When the thermistor potential divider on the inverting pins go higher than the corresponding non-inverting pin reference levels, the output of the specific op amp becomes high, illuminating its connected LED.

This implies that by appropriately changing the reference resistor values of R2----R6 we can change the gaps between the LED illumination and thus the change the input detection range suitably across the 4 LEDs, as per a desired specification.

This may be done by using the formula:

Vout = Vin x R1 / (R1 + R2)

Where Vin is supply voltage which must be constant.

Vout becomes the desired reference level on a the given non-inverting pin.

R1 is the total value of the resistor(s) on the positive side of the relevant non-inverting pin

R2 is the total value of the resistor(s) on the ground side of the relevant non-inverting pin.

IC LM324 Pin Diagram

BOM for the proposed 4 LED temperature detector circuit

Resistors (1/4 watt 5% CFR)

• R2, R3, R4, R5, R6 = 5K
• R1 = 10K,
• R7, R8, R9, R11 = 220 Ohms
• LEDs: 1 green, 1 yellow, 1 red
• Buzzer = 1no
• IC LM324 - 1no
• R10 = 10K Thermistor (as shown below)

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

UPDATE from the Admin

The accuracy and reliability of the above 4 LED temperature indicator circuit can be further improved by adding discrete presets to the 4 op amps and by replacing the thermistor with LM35 IC. The complete circuit is shown below:

Parts List

• All presets are 22K (linear)
• All resistors are 1K 1/4 watt
• ZD1 is 6V 1/4 watt zener diode
• LEDs are red, green, yellow, white 5mm 20mA
• Op amps are from the IC LM324
• Temperature sensor is LM35 IC