Temperature sensing or monitoring and control is the important process in industries and some temperature specific applications, for simple applications no need to put complex circuits to indicate the temperature level, here LM35 temperature indicator LED circuit designed with few components and its works without any microcontroller and indicates whether temperature above the threshold or below the threshold.
This circuit uses Green LED and Red LED to indicate the temperature threshold level and this level can be varied by using variable Resistor RV1, this circuit can be powered by 5V DC supply.
- Temperature Sensor LM35
- Operational Amplifier IC MC1458
- Transistor BC547 (NPN) = 2
- LED Green = 1, Red = 1
- Variable Resistor 10KΩ
- Resistor 8.2KΩ = 1, 10KΩ = 1, 680Ω = 3
Construction & Working
To Construct this circuit first connect the general purpose operational amplifier MC1458 with bias and connect the temperature sensor LM35 with bias as show in circuit diagram, then connect the LM35 sensor output to operational amplifier non-inverting input pin, Inverting pin of operational amplifier should be connected with the variable resister to set temperature threshold level.
LM35 Pin Details
Connect the output of MC1458 to the Q1 transistor base through R3 Resistor and collector terminal of Q1 is connected to the Q2 transistor base through R5 Resistor. Here LED Red connected with Q1 collector and LED Green connected with Q2 collector terminal through 680Ω Resistors.
If the temperature below the threshold level then MC1458 doesn’t generate output and hence the Q1 transistor becomes turn OFF so the Q2 transistor base gets supply through R2 Resistor and makes LED Green to Glow. When the temperature above the threshold level then MC1458 generates output and Q1 becomes turn ON and makes the Red LED to Glow and Q2 transistor wont get bias so it becomes turn OFF.