Simple IC LM35 Temperature Indicator Circuit

Keeping track of temperature is important for a lot of things but sometimes you do not need a fancy computer to do it.

This article discusses how to build a simple temperature sensor with an LED light using a common chip called the IC LM35.

It only uses a few parts and does not need a complicated microcontroller.

The light will turn on if it gets too hot or too cold depending on how you set it up.

This circuit runs on a simple 5V power supply like from a USB charger.

Circuit Construction:

Parts List:

• Begin by connecting the versatile operational amplifier IC1 741 with correct biasing.
• Connect the IC2 LM35 temperature sensor with biasing as shown in the above circuit diagram.
• Wire the IC2 LM35 sensor output to the non inverting input pin of the operational amplifier.
• Connect the inverting pin of the operational amplifier to the variable resistor to set the temperature threshold level.
• Use the variable resistor preset to change the temperature threshold level according to specific requirements.
• Connect the output of IC1 741 to the base of Q1 transistor through the R3 resistor.
• Wire the collector terminal of Q1 to the base of Q2 transistor through the R5 resistor.
• Connect the red LED to the collector of Q1 and the green LED to the collector of Q2 through 1k resistors.

Working Principle:

• If the temperature is below the threshold level IC1 741 wont generate an output.
• As a result Q1 transistor turns OFF providing bias to the base of Q2 through the R2 resistor causing the green LED to blink.
• When the temperature exceeds the threshold level IC1 741 generates an output turning ON Q1 transistor.
• This steps flashes the red LED while transistor Q2 loses bias and turns OFF.

Formulas:

Here is easy formula to improve the IC2 LM35 temperature indicator LED circuits technical features.

A voltage that is linearly proportional to the Celsius temperature is output by the LM35 temperature sensor.

The LM35 provides an output voltage Vout​ that is directly proportional to the temperature T:

V_out = 10 × T

here,

• Vout is a millivolt (mV) which illustrates the voltage of IC LM35 sensor outputs.
• 10 is the LM35 sensor specific proportionality constant.
• T is the temperature that the LM35 measures which is expressed in degrees Celsius (°C).

The calibrated IC LM35 integrated circuit instantly transforms temperature into a proportional voltage.

This makes the sensor useful and easy to use various kinds of temperature measuring applications.

Remember:

Vout and T have a linear connection as indicated by the proportionality constant (10).

There is a 10 mV increase in output voltage for every 1°C increase in temperature.

Only the LM35 sensors operational temperature range which is between -55°C and 150°C depending on the model covered by the calculation.

You may rewrite the formula to convert the output voltage Vout to temperature T:

T = V_out / 10 (in °C)

To function the IC LM35 needs a power supply voltage usually around 5V.

In relation to ground the output voltage Vout should be measured.

Conclusion:

This LM35 temperature indicator LED circuit offers a simple yet effective solution for temperature sensing without the need for complex microcontroller based systems.

With connections of adjustable components such as the variable resistor boots the circuits flexibility making it suitable for various temperature specific applications in industrial uses.

References

LM35 Precision Centigrade Temperature Sensors

How does the LM35 Temperature sensor work?