Build Your Own Contactless Temperature Sensor

By Dave Hoover for Mouser Electronics

Edited June 3, 2021 (Originally Published August 11, 2020)

The COVID-19 pandemic has presented hardships for businesses and consumers around the globe. Even businesses that have been categorized as essential have had to seek solutions to ensure they are operating safely and efficiently. Unfortunately, enterprises such as hospitals and warehouses prone to large gatherings run a heightened risk of contracting the virus. The solution? An economic contactless wrist temperature sensor that ensures essential workers can safely enter the workplace without endangering their co-workers or any manual temperature gauge operators.

The TTI Family of Companies (FOC), which includes Mouser Electronics and the TTI Semiconductor Group (TSG), has combined its resources to develop an open-source, contactless wrist temperature sensor that is cost-effective and efficient. The temperature sensor was designed and prototyped by a TSG company, Connected Development (CD), specializing in hardware and software design services. CD tapped into TTI FOC’s wide range of component distributors and services, with contributions from TTI, Symmetry Electronics, and Mouser Electronics, to develop this open-source design. CD’s temperature sensor design centers on a Silicon Labs microcontroller starter kit, a TE thermopile for temperature measurements, a TDK piezoelectric buzzer for audio feedback, and Cree LEDs for a visual cue.

TTI’s employees are using the temperature sensor at their warehouses. Employees simply hold up their wrist to the sensor to receive a reading of their temperature accompanied by a beeping noise and a red or green light that indicates whether they are at or above the acceptable temperature range.

The contactless wrist temperature sensor could benefit several use cases, including hospitals, warehouses, retail, and offices. Although the sensor is not available for sale, the design has been made open source to allow engineers to develop the product independently. The following is a detailed reference guide to walk intermediate-level engineers through the steps it takes to build this technology.

Project Materials and Resources

The following information pertains to the various components used in the contactless wrist temperature sensor project. The links can be accessed to order relevant parts and download helpful datasheets technical information or complete project bill of materials (BOM) link here.

Bill of Materials (BOM)

• 634-SLSTK3301A–Silicon Labs EFM32 Tiny Gecko TG11 Starter Kit
• 824-TSD305-2C55–TE TSD305 Digital Thermopile Sensor
• 810-PS1240P02CT3–TDK Piezoelectric Buzzer
• 941-CLX6FFKCCKNNQDGB –Cree PLCC6 3 in 1 SMD LED
• 932-MIKROE-511–Jumper Wires (To connect starter kit to sensor, buzzer, and LEDs. Note: gender and length can vary based on mechanical design. The current design uses eight separate jumper wires. (See Install Electrical section below.)
• 538-68768-0404–USB Cable (For powering the device. Note: Length and brand selection is not imperative–any USB mini cable will work.)

Software

• Simplicity Studio–Getting Started with EFM32 Tiny Gecko 11 Instructions
• Contactless Temperature Sensor GitHub Repository

Useful Links and Resources

• Silicon Labs EFM32 Tiny Gecko TG11 Starter Kit User's Guide
• TE TSD305 Digital Thermopile Sensor Data Sheet
• TDK Piezoelectric Buzzer Data Sheet
• Cree PLCC6 3 in 1 SMD LED Data Sheet

Mechanical

The mechanical design of the temperature sensor is beyond the scope of this project article. Care should be taken to provide a design that is ergonomically sound and capable of rapidly taking temperatures for individuals of various heights. The display and LED should be easy to see while holding your wrist within close proximity to the temperature sensor. The design should not require the user to have to keep the wrist at an uncomfortable angle during measurements, nor should it require additional personnel to operate.

Project Technology Overview

The contactless temperature sensor utilizes the TE TSD305 Digital Thermopile Sensor to measure object temperature. The Silicon Labs EFM32 Tiny Gecko TG11 Starter Kit is used to collect temperature sensor information from the thermopile and to notify the end-user of results.

The thermopile is also used to detect whether a human body is within close proximity to the measurement station. The Silicon Labs starter kit microcontroller constantly regulates the thermopile for temperature fluctuations. If the temperature rises above 90˚ Fahrenheit, it is assumed that a human body is close by, prompting a measurement cycle to begin. The microcontroller then awaits a consistent measurement, which is achieved by capturing three readings in a row within one degree. Once a stable measurement is captured, the temperature is displayed on the Silicon Labs starter kit LCD. The TDK Piezoelectric Buzzer sounds a short beep accompanied by a green light from the Cree PLCC6 3 in 1 SMD LED (if the temperature is below 100.4˚Fahrenheit), or a long beep with a red LED light (if the temperature is greater than 100.4˚Fahrenheit). All temperature thresholds mentioned here can easily be modified by changing a setting within the code.

Operational Flowchart

The following flowchart (Figure1) demonstrates the high-level operation of the temperature sensor.

Figure 1: Flowchart offering a high-level demonstration of how the temperature sensor operates. (Source: Connected Development)

Silicon Labs EFM32 Tiny Gecko TG11 Starter Kit

The Silicon Labs EFM32 Tiny Gecko TG11 Starter Kit includes the microcontroller for the temperature sensor, all necessary power regulators, a USB connector for power, and an LCD for displaying the temperature and user prompts. All peripherals (such as a thermopile, piezoelectric buzzer, and LED) are connected to the starter kit using jumper wires. (See the following Install Electrical section).

TE Digital Thermopile Sensor

The Silicon Labs microcontroller communicates with the TE thermopile via I²C reads and writes. The TE sensor includes an ambient temperature sensor and the thermopile used to calculate calibration offsets. The algorithm for reading measurements and applying calibration offsets to determine the final object temperature can be found in the TE TSD305 Digital Thermopile Sensor Data Sheet.

TDK Piezoelectric Buzzer

A PWM (pulse-width modulation)-capable GPIO (General Purpose I/O) pin on the Silicon Labs microcontroller is used to drive the piezoelectric buzzer.

Cree PLCC6 3 in 1 SMD LED

The temperature sensor only requires a green and red LED for visual cues (the temperature itself is displayed on the Silicon Labs starter kit LCD). The LEDs can be controlled with simple GPIO writes from the Silicon Labs microcontroller.

Simplicity Studio Software

The software development tool used for the project is Silicon Labs’ Simplicity Studio IDE (Integrated Development Environment). Instructions related to installing Simplicity Studio are found in the Set Up Silicon Labs Tools and Environment section below.

Developing the Temperature Sensor

Perform the following steps to develop the contactless temperature sensor:

1. Set Up Silicon Labs Tools and Environment
2. Retrieve Project Code from Github
3. Import Project Code into Simplicity Studio
4. Install Electrical
5. Set Up Mechanical
6. Calibrate and Tune

Set Up Silicon Labs Tools and Environment

1. Procure a Silicon Labs EFM32 Tiny Gecko TG11 Starter Kit.
2. Register for or Log In to the Silicon Labs site.
3. Follow the Getting Started with EFM32 Tiny Gecko 11 Instructions (Note: Run installer as administrator when installing Simplicity Studio per “Step 2: Download and Install Simplicity Studio”.)
4. Log in to your Silicon Labs account from Simplicity Studio. The login screen will pop up by default (Figure 2).
5. Select “Install by Device” when Simplicity Studio Installation Manager window pops up (Figure 3).
6. Plug your Silicon Labs starter kit into a USB port on your computer. (Note: The screenshot used in Figure 3 shows a Giant Gecko (instead of Tiny); however, the Tiny Gecko install will look similar.)

Figure 2: Silicon Labs login screen within Simplicity Studio. (Source: Simplicity Studio)

Figure 3: Install by Device. (Source: Simplicity Studio)

7. Double click on your starter kit that is now listed in the “Connected devices" pane and click "Next" (Figure 4).
8. Continue to click "Next" with default options selected.
9. Accept the MLA (License Agreement), click “Finish” and wait for installation to complete.
10. Click on the device and then the “Getting Started” tab (Figure 5).

Figure 4: Starter kit listed in the "Connected devices" pane. (Source: Simplicity Studio)

Figure 5: Getting Started tab. (Source: Simplicity Studio)

11. Choose a software example (such as “blink”) from the “Software Examples” section (Figure 6). (Note: While not the required selection, blink is a simple software that can easily prove that the Simplicity Studio installation, compiling, and the microcontroller code download are working properly.
12. After clicking on the “blink” project (see step 11 above), a pop-up dialog appears asking to confirm, create the new project, and switch to the Simplicity IDE perspective. Click “Yes”. (Figure 7).

Figure 6: Software Examples section. (Source: Simplicity Studio)

Figure 7: Confirmation Dialog Box (Source: Simplicity Studio)

13. Compile the project using the “hammer icon” (Figure 8).
14. Debug the project using the “bug icon” (Figure 9).
15. Run the project once the debugger is launched by clicking on the “run icon”. Breakpoints can be set, and single-stepping is enabled, etc. (Figure 10).

Figure 8: Compile the project. (Source: Simplicity Studio)

Figure 9: Debug the project. (Source: Simplicity Studio)

Figure 10: Run the project. (Source: Simplicity Studio)

16. Download and run the application by clicking on the “profile icon”. A dialog box will appear showing download progress. Once complete, the code will execute on the target (Figure 11).
17. Make a change to the blink rate in the code by modifying the src/blink.c file, line 75. The Delay() function call indicates the time between each LED blink cycle in milliseconds.  Modify this number to a different number of milliseconds and then re-compile and download (see steps 13-16 above), and observe the LED blink rate changes that take place on the target.

Figure 11: Debug the project. (Source: Simplicity Studio)

18. Figure 10: Download the project. (Source: Simplicity Studio)
19. Retrieve Project Code from GitHub: Contactless Temperature Sensor GitHub Repository
20. IMport Project Code into Simplicity Studio

Import Project Code into Simplicity Studio

Navigate to the Simplicity IDE tab in Simplicity Studio (Figure 12) and import the Contactless Temperature Sensor code downloaded from Github by selecting “File”, “Import”, Browse to the project code directory, and select the file. Click “Next>” and “Finish” to import the project.

Figure 12: Import the Contactless Temperature Sensor project. (Source: Simplicity Studio)

Install Electrical

Jumper wires can connect the thermopile, buzzer, and LEDs to the Silicon Labs starter kit.  Reference the Silicon Labs EFM32 Tiny Gecko TG11 Starter Kit User's Guide for the starter kit pinouts and header definitions. Access the TE TSD305 Digital Thermopile Sensor Data Sheet for the TE thermopile pinout.

• The code example provided uses the following starter kit pins:
• Thermopile VDD: Exp Header, pin 20 (3V3)
• Thermopile GND: Exp Header, pin 1 (GND)
• Thermopile I²C SDA: Exp Header, pin 16 (PD6, I²C_SDA)
• Thermopile I²C SCL: Exp Header, pin 15 (PD7, I²C_SCL)
• Piezoelectric Signal: J101, pin 7 (PC1)
• Piezoelectric GND: J101, pin 15 (GND)
• RGB LED (Red): J101, pin 6 (PC0)
• RGB LED (Green): Exp Header, pin 10 (PC8)
• RGB Gnd: J101, pin 15 (GND)

Be sure not to exceed the microcontroller’s maximum current draw specification. A resistor should be put in series with each LED to limit current. The resistor value can be chosen appropriately based on the desired LED brightness.)

Set Up Mechanical

The project’s mechanical design will vary depending on the application and beyond this article’s scope.

Calibrate and Tune

Differences in environment, distance/proximity to the measurement target, and target location of the measurement (forehead, wrist, palm, etc.) can make a difference in the detected temperature. An offset (in degrees) is applied in the code to convert from a typical wrist temperature detected by the thermopile to an internal body temperature. This offset can easily be changed in the compiled code to account for such differences. In addition, the design could be modified mechanically to accommodate different measurement targets such as the forehead.

Using the Device

Once the unit is assembled and coded per the instructions above, the device is ready to use. The temperature sensor will turn on automatically when power is applied. Once powered, the device waits for a body’s proximity to be detected per the flowchart in (Figure 1). The temperature is then measured per the same flowchart, and results are displayed. The device then waits for the next user/proximity detection.

Expanded Features

The following enhancements could be added to the project in the future:

Separate Proximity Sensor

A separate proximity sensor could inform a user of their distance from the sensor to provide a more accurate and consistent measurement.

Enhanced Features

Adding a larger display, better speaker, and other enhanced features such as voice feedback could further improve the device’s usability.

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Dave Hoover, VP Advanced Technology
With nine US patents to his name and over 25 years of experience in the wireless industry, Dave Hoover has the development experience to make your project a success. He has successfully brought many M2M and cellular phone projects to market while managing large projects and working with engineering teams across the globe. He can adapt to your tools and processes, or implement known best practices to deliver your project on time, on spec and on budget.