Simple Temperature Meter Circuit

A temperature meter circuit is like a tiny detective for heat.

It uses a special sensor, like a thermocouple or a thermistor to feel the temperature around it.

This sensor can be a simple component or even a clever chip like the LM35 or LM34 that turns heat into electricity.

The circuit then takes that electrical signal and figures out how hot or cold it really is.

Finally, it shows the temperature on a cool display like tiny lights or even a digital readout.

So next time you are curious about the temperature you can build your own mini weather station.

Circuit Working:

Parts List:

This temperature meter utilizes the LM35 precision temperature sensor IC which provides a linear output of +10mV per degree Celsius.

The temperature reading is shown using an LED display.

The LM35 is a three pin IC that behaves like a transistor and offers a linear output of +10mV per degree celsius increase in temperature.

It can measure temperatures ranging from -4 degrees to +110 degrees Celsius.

Its counterpart, the LM34 is a fahrenheit sensor that provides an output of -10mV per degree fahrenheit.

The output of IC1 is connected directly to the input of the LM3915 display driver IC.

The LM3915 is a monolithic integrated circuit with 10 active-low outputs capable of directly driving 10 LEDs without needing a current limiting resistor.

The internal circuitry of the LM3915 adjusts the current flowing through the LEDs.

The LM3915s input is highly sensitive and its outputs 18 to 10 sink current sequentially as the input voltage increases by 125 millivolts.

In this setup, only six outputs are used to drive six LEDs but more LEDs can be added using the remaining outputs if necessary.

As the LM35 detects a temperature increase LEDs one to six illuminate.

If the sensitivity is not sufficient, VR2 can be excluded.

In that case, the output of IC1 should be connected directly to the input of IC2.

When power is applied, some LEDs will light up.

To calibrate the circuit expose IC1 to different temperatures using a thermometer and hot water.

Dampen some cotton with water around 37 degrees celsius normal room temperature and gently apply it to IC1.

Adjust VR1 and VR2 until LED1 lights up.

Formulas:

Below is the formula for LM35 temperature sensors output voltage:

This formula basically says that the ambient temperature, expressed in degrees Celsius (°C), and the output voltage Vout of the LM35 sensor are exactly related.

For every degree Celsius that the temperature rises, the output voltage rises by 10 millivolts, according to the constant value of 10 mV/°C.

Vout = 10mV/°C × Temperature (°C)

where,

• C stands for “Celsius” in the formula, representing the chosen temperature unit.
• Vout is the voltage produced by the LM35 sensor is referred to as the output voltage .

LED Resistor Calculation:

Calculate the resistor value RLED​ for each LED using ohms law:

R_LED = V_CC−V_LED / I_LED

where:

• V_CC​ is the supply voltage for 9V.
• V_LED is the forward voltage drop across the LED.
• I_LED​ is the desired current through the LED which is 20mA for 5mm LEDs.

Note:

You may create a temperature meter circuit that drives LEDs through the LM3915 display driver by utilizing these calculations and recommendations and the LM35 temperature sensor.

To achieve accurate temperature indication, test the circuit thoroughly and adjust component values according to individual needs.

How to Build:

To build a Simple Temperature Meter Circuit follow the below mentioned steps:

• Connect the output pin of the LM35 IC to the input of the LM3915 IC.
• Connect the Vcc and ground pins of both ICs to the appropriate power supply.
• Connect the LEDs to the output pins of the LM3915 IC, with current limiting resistors in series.
• Connect VR1 and VR2 as per the circuit diagram for sensitivity adjustment.
• Place the LM35 and LM3915 ICs on the breadboard or PCB.
• Connect the power supply to Vcc and ground pins of both ICs.
• Connect the output pin of the LM35 to the input pin of the LM3915.
• Connect the LEDs to the output pins of the LM3915, with current limiting resistors.
• Connect VR1 and VR2 as per the circuit diagram.
• Ensure all connections are secure and correct.

Testing:

• Apply power to the circuit.
• Use a thermometer and hot water of different temperatures to calibrate the circuit.
• Adjust VR1 and VR2 until the LEDs light up sequentially based on the temperature detected by the LM35.

Note:

• It’s important to handle the ICs and components carefully to avoid damage.
• Double check the connections and ensure they match the circuit diagram.

Conclusion:

In conclusion, a temperature meter circuit is an essential electronic circuit used to measure and display temperature accurately.

It consists of components such as temperature sensors, signal conditioning circuits and output displays.

These circuits play a vital role in various industries and applications where temperature monitoring is critical for ensuring safety and efficiency.

References:

LM35 Based Digital Room Temperature Meter