Temperature Display Circuits and Sensor Technology

I was always told to keep the fridge door shut, and so I comply. But when the fridge door chose to stay slightly open on one fine day, I got into massive trouble as the water vapor built on literally every item inside. Thankfully, none of the vegetables were spoiled, or I’d have suffered unbearable naggings. 

In certain industries, runaway temperature results in worse consequences than rotten fruits or melted ice cream. Cold rooms need to have precise temperature control or the goods, often fresh produce risk being contaminated. The temperature in a hospital is also kept below a certain threshold to prevent bacteria growth. 

At the core of the sophisticated controller is a temperature sensing circuit. Often, a display is included in the design to provide the real-time value of the temperature. 

How a Temperature Display Circuit Works 

A temperature circuit measures the temperature of the immediate environment and presents the value in a digital display. It’s just like an analog thermometer, except that no mercury is used to determine the temperature value.

Instead, a temperature display circuit involves a temperature sensor, microcontroller, and a digital display. The temperature sensor uses electrical principles to detect the temperature value and converts it to an analog signal. 

A microcontroller is used to pick up the analog signal and convert it into digital values. Often, an op-amp is needed to amplify the analog value of the temperature before being fed into the ADC pin of the microcontroller.

A display, usually an LCD or segment-LED, is used to show the current value of the temperature. Depending on the resolution of the sensor, the display may show the temperature value in decimal points. 

Types Of Sensors Used For A Temperature Display Circuit

The general concept of a temperature circuit is simple. But that doesn’t mean you could start building one by grabbing any temperature sensors you could find from a supplier. There are various types of temperature sensors available, you’ll need to choose one that fits your requirement. 

Here are some common types of temperature sensors. 

• Thermocouple: A thermocouple operates by measuring the offset in voltage between two different types of metal. It is non-linear, has low accuracy but could operate across the widest range of temperature. 

• Resistance Temperature Detector (RTD): The RTD is usually made of a platinum wire wrapped over a ceramic or glass core. As the temperature changes, the resistance of the RTD changed in a generally linear scale. It’s highly accurate but not an economical option.

• NTC Thermistor: An NTC thermistor works by varying its resistance according to its temperature. It is highly sensitive to small changes in temperature. The value changes on an exponential scale compared to the temperature.

• IC Sensors: Semiconductor-based sensors use diodes to determine temperature values. Temperature-sensing ICs often have built-in op-amp and provides the temperature value in a digital or analog signal. 

A thermocouple covers the largest temperature range but has the lowest accuracy.

Challenges When Designing A Temperature Display Circuit

Depending on the types of temperature sensors, you’ll have to deal with specific challenges in the PCB design. Thermocouple, RTD and NTC thermistors produce analog signals that correspond to the temperature value. This means that you’ll need a good separation between the analog and digital parts of the PCB.

Keeping both analog and digital grounds separate is important to ensure no digital noises are coupled into the analog temperature signal. Of course, you’ll also want to ensure that the analog signal has a clear and direct return path to the analog ground. 

Obviously, using a semiconductor-based temperature sensor will simplify the PCB design. However, that doesn’t mean you should take anything for granted. If the IC sensor produces an analog output, you’ll still need to adhere to the best practice of analog-digital separation.

Getting the temperature sensor design right results in accurate displayed value. 

Some IC-based sensors provide an I2C or SPI interface, which means you don’t have to deal with the analog part of the sensor. This leaves you dealing with another issue that often affects the IC temperature sensor. You’ll need to keep the sensor away from heat-generating ICs, such as the microcontroller if you intend to measure the ambient temperature.

Thankfully, designing the display interface is a rather straightforward process. As long as you get the sensor design right, you’ll have no problem getting the right value on the display panel. 

It helps to use a flexible PCB design software, such as OrCAD Allegro for digital-ground separation or setting clearance for the temperature sensor. Allegro provides a clear layout solution that manages a collaborative design process with ease and pairs potential for analysis and simulation.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.