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 Description
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 Schematic
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
Godson says
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.
Swag says
Hello Godson, Actually I misunderstood his question, and seem to have missed the 3 step switch in the question.
however for PWM you can try the following concept
https://homemade-circuits.com/pwm-air-blower-controller-circuit-for/
in the extreme left you can see a 10k preset, you can connect an NTC in series with its lower arm for the heat detection and fan control.
and make sure that the BC547 is made using two BJTs in a Darlington form.
dave windsor says
hi what value is R1 can see the value anywere
Swagatam says
R1 can be a 10k preset
Anurag Parla says
HERE THERE NO IS POTENTIOMETER SO HOW CAN WE FIX THE REFERENCE VOLTAGE ??
Swagatam says
R1 can be replaced with a pot for altering the reference value.
Kissan Kumar says
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
Swagatam says
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.
Kissan Kumar says
http://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
Henrik Lauridsen says
Dear Swagatam,
Can a NTC resistor be used instead of the thermistor?
Regards,
Henrik
Swagatam says
Dear Henrik,
I think both are one and the same, thermistor itself is a temperature dependent resistor, yes here it's a NTC thermistor
Henrik Lauridsen says
Thank you.