Internet of Things (IoT) devices enable formerly unimaginable levels of remote monitoring and control. From locomotives to baby monitors, home appliance controls, wearables and interactive lighting, new applications are sparking the imaginations of designers. This article reviews the anatomy of wireless sensor nodes, explains how they usually function, and provides guidance regarding important considerations for selecting components for these types of applications.
Anatomy of a Wireless Sensor Node
Whether you’re designing a wearable device, an interactive lighting system, or even a jet engine, the building blocks of an IoT device are remarkably similar. Here are the three main components for a wireless sensor node:
Sensors – gather information about the environment and condition signals before transmitting to the microprocessor.
Microcontrollers – process the signal from sensors, determine appropriate responses, manage power consumption and local memory.
Communication – wireless chips, radio modules and protocols needed to transmit the information between devices and to the cloud.
The components in an IoT node will vary in sophistication, depending on the application. But the basic topology of a wireless sensor node always includes these elements.
Source: Electronic Design
For some applications, like a home thermostat, taking sensor readings and making adjustments several times an hour is sufficient. The room temperatures will not change very rapidly and it will take the sensors several minutes to register a change. Other applications, like a conveyor belt in an automated factory, could require frequent monitoring. A sudden change in the load could cause its motor to groan and smoke if quick adjustments are not made. No matter what you're designing, having a clear understanding of the application and the environmental factors is essential to selecting the right components for the job.
In each of these examples, the task of the sensors, microcontrollers and communications circuitry is very much the same:
Sensors take readings of the temperature produced by the HVAC system (heating, ventilation, air conditioning), or the conveyor belt speed produced by the motor drivers. The signal conditioning circuitry amplify (and, often, linearize) the output of the sensors so that it can be read by the microcontroller.
Microcontrollers process the sensor readings to monitor and react to a room’s temperature or to asses a conveyor belt’s speed and adjust the motor’s controls.
Communication components use its connection to cloud computing resources to further analyze the data generated by the conveyor belt sensors, or to share the updated temperature to a homeowner’s mobile app.
Regardless of what type of sensors are employed (ambient light, environmental temperature, motor speed, etc.), regardless of the processor bit-width of the microcontroller (8-, 16- or 32-bit), regardless of the communications loop (WiFi, ZigBee, or Bluetooth), the sequence of collecting and evaluating data from a remote sensor loop remains mostly the same.
How IoT Sensor Nodes Work
No matter what a sensor node is used for, it will likely spend much of its time asleep – as much as 95% of it, in fact. A change in the sensor’s environment – such as movement, temperature, pressure – will bring the sensor to life. This will wake the microcontroller, which will search its memory for something familiar to assess “What have we got here?”
Most of the time the answer will be something to the effect of “nothing to worry about.” The processor will inform whatever is attached to its communications port and go back to sleep. This chain reaction will be much pretty the same, regardless of whether a system is controlling a smart oven’s temperature, monitoring an industrial machine tool, or checking on your home’s security with a smartphone.
In addition to sensors, microcontrollers and communications ports, when identifying the key specifications that will guide your choice of components for IoT sensor nodes you’ll also need to consider signal-conditioning, and power management needs.
Sensor Signal Conditioning
In a large majority of IoT applications, there may be no separate signal conditioning semiconductors. The signal conditioning circuits are either integrated with the sensor module package, or with the microcontrollers. Whether visible or not as a separate component, the signal conditioning is necessary to ensure the sensor can be understood by the node’s microcontroller.
In motion detectors, a MEMS device will require at least two levels of signal conditioning: one to bias the micro-electromechanical system and amplify its output; another to make the sensor output easier for the microcontroller to interpret. MEMS suppliers frequently integrate signal conditioning into their sensors so that their output is easier to interpret.
The power consumption of the sensor signal conditioning package is critical in remote IoT applications where a device is battery powered. In these applications, energy harvesting devices are valuable additions to the sensor nodes.
Power Management Devices
These devices tend to be divided by function. One popular group includes custom-crafted voltage controllers (PMICs), which supply a feature-rich mobile device with a variety of voltage rails. These devices have traditionally supported mobile handsets. The other kind of power
What the power management devices have in common is their need to supply regulated (or tightly fixed) voltage rails to all elements of the IoT sensor node. To conserve battery life, the voltage regulators must consume little-to-no-power of their own.
This leaves the designer a choice between linear and switch mode regulators. The linear regulators are very inexpensive and easy to implement. The switch-mode regulators are more expensive, sometimes difficult to use in a circuit, but offer the highest energy transfer efficiency. A wide variety of battery-backed power rails in remote sensor nodes could be implemented with switch-mode regulators with 50- or 100mA regulators.
Selecting the Right Components
There are numerous specifications that you will need to map out in order to select appropriate components for your wireless sensor node. Here is a summary of some of the most important questions and specifications that you’ll need to address:
How much intelligence is required in the the sensor node? How much intelligence can be relegated to the cloud?
This will dictate whether you should use an 8-, 16- or 32-bit processor, how much memory will need to be attached, and what clock rate you’ll use to run your IoT node.
Where is the data concentrated and how often does it have to be communicated from the remote sensor node?
This will guide your choice between different communication technologies and protocols such as ZigBee, BTLE, WIFI, etc.
How much resolution and what kind of sampling rate is required for sensor signal conditioning?
Data converters for jet engines may require 16- or 18-bit resolutionm which digital fabrication tools could make good use of 12bits.
What is the power source? Batteries, offline power supplies, or energy harvesters?
This can impact your choice of sensors, microprocessor and communication module. For battery-powered and energy harvesting nodes, a key concern is energy transfer efficiency – you don’t want your voltage regulator tapping your supply voltage.
For decades, Avnet has helped large multinationals develop sensor networks for consumer and industrial applications. Today, Avnet is also helping thousands of small entrepreneurs, inventors, and creators design and manufacture IoT sensor networks. For assistance selecting the right components for your product, or to learn more about the efficiency of your product design, apply to the Hardware Studio Connection.