An electronic thermometer circuit diagram with high measurement accuracy, memory and beep tips for human body temperature.

The electronic thermometer consists of a temperature sensor, an LCD monitor, a button battery, a special integrated circuit and other electronic components.

Compared with the traditional mercury glass thermometer,electronic thermometer can measure the body temperature quickly and accurately,has the advantages of convenient reading, short measurement time, high measurement accuracy, energy memory and buzzing cues. The circuit of the electronic thermometer is shown as below, thermal power RT stares and R1, R2, R3 and RP1.When the temperature is 20 ℃, select R1, R3 and adjust the RPL to make the bridge a balance.

Introduction

A digital thermometer circuit diagram illustrates the electronic components and connections required to construct a thermometer that provides temperature readings in numeric format. Unlike analog thermometers that use a mercury column, bimetallic coil, or other mechanical sensor, digital thermometers convert temperature into an electrical signal that is digitized using analog-to-digital conversion and displayed.

Understanding the circuitry allows constructing simple to more complex digital thermometers. The key components include temperature sensors, analog signal conditioning, microcontrollers or display drivers, displays, and power supplies. Common sensing elements used include thermistors, RTDs, and thermocouples.

This article will provide an overview of several typical digital thermometer circuit implementations and their diagrams.

How Digital Thermometers Work

Here are the key steps in how a digital thermometer functions:

• A temperature sensor converts the ambient temperature into an analog electrical signal that varies based on temperature.
• The analog signal is conditioned and amplified through circuitry. This improves the signal quality.
• The amplified signal is fed into an analog-to-digital converter (ADC) to digitize the temperature value.
• A microcontroller or display driver chip reads the digitized temperature from the ADC.
• The chip outputs the numeric temperature reading on a digital display like an LCD.
• Power supply circuitry provides regulated voltage to the components.

By understanding this sequence, we can now examine some sample digital thermometer circuit diagrams.

Simple Thermistor Thermometer

Here is a basic digital thermometer circuit using a thermistor as the temperature sensing element:

This circuit uses just a few low cost components:

• Thermistor – Temperature dependent resistor. Resistance drops non-linearly as temperature rises. Provides analog temperature signal.
• Comparator – Produces a digital output by comparing thermistor voltage against known reference voltage.
• 555 Timer – Configured in astable mode to produce clock pulses for the counter. Clock frequency varies based on thermistor resistance.
• Counter – Counts clock pulses from 555 timer. Higher temperature causes higher clock frequency and thus higher counter value.
• Decoder & Display – Translates counter output to drive the 7-segment LED display. Shows 2-digit temperature reading.
• Reference Voltage – Provides stable voltage threshold for comparator. Set based on thermistor specs.

This simple circuit can measure over a narrow range of ~50°C with 1-2°C accuracy. Wider range thermometers require additional components.

LM35 Based Thermometer

The LM35 linear temperature sensor makes an excellent choice for more accurate digital thermometers:

• LM35 Sensor – Integrated analog temperature sensor with linear +10mV/°F output. High accuracy and low self heating.
• Op-Amp – Configured as amplifier to boost LM35 output to readable voltage levels. Gain sets resolution.
• ADC – Built-in analog-to-digital converter transforms amplified signal into digital values.
• Microcontroller – Reads digitized temperature from the ADC channel and calculates temperature based on ADC reference voltage.
• LCD – Microcontroller outputs temperature reading directly to LCD display. Can add °F or °C symbol.

This circuit allows much wider range and better than 1°C accuracy with proper calibration. The LM35 enables a very linear and stable thermometer.

Multi-Sensor Thermometer

Here is a circuit for a versatile thermometer using multiple sensor types:

• Instrumentation Amp – Differential amplifier subtracts voltage across RTD sensor to produce amplified temperature signal. High common mode noise rejection.
• Thermocouple Amp – Amplifies small thermocouple voltage. Cold junction compensation circuit removes reference junction error.
• Thermistor network – Voltage divider circuit produces thermistor signal readable by ADC.
• ADC – Digitizes signals from all three sensors using input multiplexer and single ADC.
• Microcontroller – Manages ADC measurements. Applies linearization and conversion algorithms specific to each sensor type.
• Display – Shows current reading. Can display temperature from selected sensor.

This flexible design supports thermocouples, RTDs, and thermistors. The microcontroller software provides cold junction compensation and linearization for accurate readings from multiple sensor types.

Isolated Thermocouple Thermometer

For very high temperature measurements with thermocouples, an isolated circuit is required for safety and noise rejection:

• Thermocouple – Acts as the temperature transducer based on differential voltage produced by dissimilar metal junctions.
• Isolation Amplifier – Provides electrical isolation and protects low voltage circuitry from high common mode voltages. Also removes noise.
• Cold Junction Compensator – Specialized integrated circuit measures reference junction temperature and compensates voltage to eliminate reference errors.
• ADC – Digitizes the compensated thermocouple voltage for the microcontroller. High resolution ADC improves accuracy.
• Microcontroller – Applies linearization algorithms specific to the thermocouple type. Calculates and displays temperature.
• LCD Display – Shows current temperature reading up to 3-4 digit precision.

The combination of isolation amplifier and cold junction compensation allows this circuit to accurately measure very high temperatures in industrial environments.

Summary

• Digital thermometers convert temperature into an electrical signal and then digitize it to display a numeric temperature reading.
• Key components include temperature sensors, amplifiers, ADCs, microcontrollers, displays, and power supplies.
• Popular sensors used include thermistors, RTDs, thermocouples, and integrated sensors like the LM35.
• Circuit techniques like isolation, linearization, and cold junction compensation help improve measurement accuracy.
• A microcontroller running sensor-specific code allows a single circuit to support multiple sensor types.
• Digital thermometer circuits can be very simple or highly complex depending on the required measurement range and precision.

FAQ

What are the main components of digital thermometer?

The core components are the temperature sensor, signal conditioning circuitry, analog-to-digital converter, microcontroller/display driver, display, and power supply. Additional components improve linearity, isolation, accuracy, etc.

What temperature sensor is most accurate?

Platinum RTD sensors provide the highest accuracy and stability. However integrated sensors like the LM35 also offer excellent linearity and precision. Thermocouples are highly versatile for wide temperature range but need compensation.

What communication protocols are used in digital thermometers?

Many industrial digital thermometers use HART, Profibus, or Modbus for digital communication overtwisted pair wires or RS-485 links. This allows remote temperature monitoring and digitization.

How does software improve digital thermometer accuracy?

Linearization algorithms running on the microcontroller improve sensor accuracy by correcting non-linearities inherent in devices like thermistors and thermocouples over their operating range.

How does isolation improve digital thermocouple thermometers?

Isolation amplifiers prevent ground loops, reject common mode noise, and protect internal low voltage circuitry from the high voltages that can be produced by thermocouples monitoring very high temperatures in industrial processes.