Keeping track of temperature is important for a lot of things, but sometimes you do not need a fancy computer to do it.
This guide shows you 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 basic 5V power supply like from a USB charger.
Circuit Construction:
Parts List:
| Type | Component | Quantity | Details |
|---|---|---|---|
| Resistors | 10k | 3 | 1/4 W CFR |
| 1k | 3 | 1/4 W CFR | |
| Semiconductors | Transistor BC547 | 2 | |
| IC741 | 1 | ||
| 1CLM35 | 1 | Assuming this is a typo, it’s corrected to LM35 | |
| Preset 10k | 1 | ||
| LEDs | Red 20mA 5mm | 1 | |
| Green 20mA 5mm | 1 |
Below are the circuit constructions details:
- Begin by connecting the general purpose operational amplifier IC 741 with appropriate biasing.
- Connect the IC LM35 temperature sensor with biasing as illustrated in the circuit diagram.
- Link the IC 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.
- Utilize the variable resistor RV1 to vary the temperature threshold level according to specific requirements.
- Connect the output of IC 741 to the base of Q1 transistor through the R3 resistor.
- Link 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 IC 741 does not generate an output.
- Consequently, Q1 transistor turns OFF providing bias to the base of Q2 through the R2 resistor, causing the green LED to illuminate.
- When the temperature exceeds the threshold level IC 741 generates an output turning ON Q1.
- This action illuminates the red LED while Q2 loses bias and turns OFF.
Formulas:
Below are some basic formula to enhance the technical aspects of the LM35 temperature indicator LED circuit.
The LM35 temperature sensor outputs a voltage linearly proportional to the celsius temperature.
The LM35 provides an output voltage Vout that is directly proportional to the temperature T:
Vout = 10 × T
here,
- Vout: This is a millivolt (mV) representation of the voltage that the LM35 sensor outputs.
- A voltage that rises linearly with temperature is produced by the LM35.
- 10: This is the LM35 sensor specific proportionality constant.
- It means that for every degree Celsius (°C) that the temperature varies, the output voltage changes by 10 millivolts.
- T: This is the temperature that the LM35 is measuring, expressed in degrees Celsius (°C).
Temperature is immediately converted into a proportional voltage by the calibrated integrated circuit known as the LM35.
Because of this, it is a practical and simple to use sensor for a range of temperature measuring uses.
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 typically between -55°C and 150°C, depending on the model is covered by the calculation.
You may rewrite the formula to convert the output voltage Vout to temperature T:
T = Vout / 10 (in °C)
To function, the LM35 needs a power supply voltage, usually 5V.
In relation to ground, the output voltage Vout is measured.
Conclusion:
To conclude, this LM35 temperature indicator LED circuit offers a simple yet effective solution for temperature sensing without the need for complex microcontroller based systems.
The incorporation of adjustable components such as the variable resistor enhances the circuits versatility, making it suitable for various temperature specific applications in industrial settings.