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.
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.
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:
- 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