The proliferation of sensor solutions based on MEMS technology provides designers with an increasingly wide range of sensor function and precision options. Their application is made easier with the integration of circuitry for calibration, linearization, FIFO buffers, and low-pin-count digital interfaces, together with software drivers and example code to ensure access to all features.

When coupled with a suitable wireless connectivity solution, these tiny sensor devices bring tremendous potential to markets such as health care, smart transportation, home automation, and all aspects of IoT. With this primer, insights are provided regarding wireless sensor building blocks as well as examples of their application 

Let’s start by identifying some trends in sensor technology:

• Increased offerings in digital form, with two or three wire serial interfaces for communication.
• Use of MEMS technology for sensing elements.
• Ease of interface and low processing requirements, permitting low-cost, low-power host MCUs to be used for control and sampling of sensors.
• Additional value extracted from sensor data by sending it to internet based cloud services, using one of several wireless connectivity standards, depending on application.

Wireless Sensor “Tiles”

These versatile modules are an exciting new development, providing powerful new building blocks for system designers. Despite their limited footprint, some can be as small as a postage stamp, yet these modules can integrate multiple sensors, processing functionality, plus Bluetooth LE or other low-power wireless-connectivity.

ST Micro “SensorTile” and Intel “tinyTILE” are two recently released examples.

Both of these “Tiles” feature Bluetooth LE (BLE) wireless connectivity and multi-axis motion sensors.

The ST SensorTile, though smaller in size, includes multiple MEMs sensors for:

• 9-axis of motion sensing (3D accelerometer, gyro, magnetometer functions)
• Barometric pressure
• Audio sensing (digital microphone)

Combo Sensors

Combo devices feature multiple sensors integrated into a single IC device package. Examples include:

• 6- and 9-axis IMU sensors (Inertial Measurement Units) available from multiple suppliers typically include 3D accelerometer, gyro, and magnetometer in the same package.
• Combo environmental sensors (e.g. temperature plus relative humidity),  with shared footprint and communication interface, simplify some aspects of the hardware and software design.       

Wireless Connectivity: BLE / WiFi Bridge to the Cloud

Combining low-power, local wireless connectivity plus WLAN Internet connectivity on a pre-certified, easy-to-deploy module or SOM can extend the scope and scale of your design beyond what is achievable using isolated sensor nodes. These smarter systems can offer cloud-based fusion or aggregation of measurements from multiple different sensors, data storage, analytics, visualization, alerts, management, and control.  

For example, SOMs used as BLE to WiFi-to-Cloud bridge, fully exploit this capability with:

• Multiple BLE-to-cloud reference designs connected via power-efficient MQTT publish/subscribe transactions with AWS IoT (Amazon Web Services) or Watson IoT (IBM Bluemix) cloud services.
• Demo material using HTTP/HTTPS protocol REST API access to multiple ThingSpeak read/write channels, used for storage and shared access by browser-based custom dashboard for User Interface system control and data visualization.     

Wireless Connectivity: Cellular Network

Wireless Wide Area Network (WWAN) connection to a cellular network provides a powerful alternative in remote applications where WiFi Internet is not a viable option. IoT starter kits that function as both a versatile development platform and as a cost-effective module suitable for use in volume production (with regulatory and network certifications and easy application) are efficient for designers. Additionally, designer should look for IoT kits that support multiple sensors onboard (for GPS location sensing, 3D-accelerometer, temperature, and ambient light sensors), and off-board (custom sensor combinations). 

Power Management

Power management is a critical aspect for untethered wireless sensor applications. Extra care is required when attending to design factors such as:

• Choice of wireless protocol/technology
• Measurement sample rate and optimization of sleep/active duty cycle
• Operational temperature range
• Battery technology and energy harvesting technologies

General Observations on Wireless Connectivity in Sensor Systems

Choice of wireless solution can be narrowed down by categorizing and comparing key design factors. Use of a spreadsheet is recommended for side-by-side evaluation and ranking of the multiple considerations a designer needs to grapple with for each specific use-case.

Manufacturers typically categorize their wireless solutions by network range.  Broad categories and examples include:

• Short Range—NFC, BLE 4.x
• Medium Range—BLE 5.x, Zigbee, Thread, WiFi
• Long Range—Sub-GHz, 3G/4G Cellular

When comparing solutions, designers need to consider:

• Network range / RF performance
• Network topology and size
• Maximum throughput
• Frequency bands and WW regulations
• Communication protocols and the trend towards standards-based interoperability
• Wireless coexistence
• TCP/IP support
• Power consumption, sleep current, energy efficiency and battery life
• Software enablement/effort
• Total cost
• Latency and robustness
• Level of integration (eg. Multiprotocol SoC, certified module)
• Mechanical size and operating temperature range
• Challenges and constraints of your unique application use-case

Generalizations such as the following also need to be re-evaluated against the rapid advances and refinements in the capabilities of the various wireless protocols:

• Sub-GHz is best for long range low-power systems.
• Zigbee is best for mesh network topologies and lighting applications.  
• Bluetooth LE is best for interfacing with smartphones and tablets (but is rapidly evolving into much more).
• WiFi is best/easiest for interfacing to the Internet (TCP/IP).
• Thread is best for lower power, small software access to TCP/IP networks (using 6LoWPAN protocol and 802.15.4 transceivers).
• Cellular 3G/4G is best for long range and overall coverage.

Multiprotocol SoCs / Wireless MCUs are one exciting new trend to watch. Available from a number of manufacturers are solutions where the SoC can be configured for eg. BLE, Thread, or Zigbee operation.

Putting It All Together

Small footprint I²C or Serial Peripheral Interface (SPI) digital sensors, together with low power wireless connectivity and optimized power management techniques, provide fresh options for extending the intelligence and utility of embedded hardware, enabling better context awareness, new application opportunities, and valuable differentiation from competing products. 

Next Steps

When considering wireless sensor implementations and the potential these bring to make your next design smarter and more context aware, it is critical to stay current with latest developments in component level solutions. Your local distributor FAE can be a great resource, providing vital insights in this regard and sourcing key information on what is, or soon will be, available from different suppliers.