Wireless sensor nodes (WSNs) play a crucial role in advancing the Internet of Things (IoT). The advantages of WSNs lie in their extremely low power consumption, small size, and ease of installation. For many IoT applications, such as those installed outdoors, WSNs can be powered by solar energy. When there is light indoors, the system is powered by sunlight, simultaneously charging tiny button batteries or supercapacitors to power the system in the absence of light.
In general, wireless sensor nodes are sensor-based devices responsible for monitoring conditions such as temperature, humidity, or pressure. Nodes collect data from any type of sensor and then wirelessly transmit it to a control unit, such as a computer or mobile device, where the data is processed, evaluated, and action is taken. Ideally, nodes can obtain operating power from energy harvesting mechanisms, becoming independently operating devices. In a general sense, energy harvesting is the process of capturing and converting minute amounts of energy from sources such as light, vibration, or heat into electrical energy.
Figure 1: Example of an energy harvesting system design
Figure 1 shows a block diagram of an energy harvesting system. Energy is harvested by an energy harvesting system (such as a solar panel) and converted into stable energy by a power management integrated circuit (PMIC), which then stores it using low-leakage, low-impedance capacitors. This energy can supply sensor interface loads (such as microcontrollers (MCUs), which are sensors that transmit data wirelessly. In this figure, the energy harvesting sensor (EHS) is a wireless sensor node.
Figure 2: Example of a wireless sensor node system
Figure 2 shows a block diagram of a wireless sensor node. Here, processed sensor data is transmitted wirelessly via Bluetooth Low Energy (BLE). BLE is a standard for short-range, low-power wireless applications for exchanging status or control information. BLE operates in the 2.4 GHz ISM band and uses Binary Frequency Shift Keying (GFSK) modulation, supporting data rates of up to 1 Mbps.
Power management ICs are used to stabilize the power consumption required by energy harvesting devices to support their ultra-low power operation. For example, the Cypress S6AE103APMIC device consumes as little as 280nA and has a startup power of 1.2uW (see Figure 3). Therefore, even in low-brightness environments of approximately 100 lux (lx), compact solar cells can still harvest a small amount of energy.
Figure 3: Block diagram of the S6AE103APMIC device for energy harvesting.
High-efficiency wireless sensor node design
Let's consider the steps involved in designing a wireless sensor node:
Step 1: Select Hardware
On the hardware side, you'll need appropriate sensors, an MCU and PMIC that can ultimately power an energy harvesting device. You may also need additional passive components, depending on the design.
Sensors can be analog or digital. Many sensors on the market today are digital sensors based on Integrated Circuit Bus (I2C), Serial Peripheral Interface (SPI), or Asynchronous Receiver/Transmitter (UART) interfaces. Extremely low-power sensors are also available. To keep device costs low and form factors small, MCUs with integrated BLE simplify design and shorten time-to-market. To further accelerate design, many manufacturers use fully integrated, fully certified programmable modules, such as Cypress EZ-BLEModules. A module consists of a main MCU, two crystal chips or tracking antennas, an expansion board, and passive components. Because these modules already have the necessary BLE certification, products can be launched quickly.
Figure 4: BLE module example: Solar-powered Bluetooth Low Energy sensor beacon CYALKIT-E02
Step 2: Design firmware and estimate power consumption
After selecting a programmable MCU, the next step is to write appropriate firmware. The basic functions the firmware needs to have include an interface for collecting sensor data, a BLE component or stack for wireless data transmission, and a CPU capable of handling firmware processing.
Since ultra-low power operation is critical, total current consumption needs to be considered from the outset. Total current consumption is the sum of the current consumed by the sensor and the current consumed by the MCU. Since sensors typically don't consume much total current, the focus should be on minimizing the current consumed by the MCU. Before optimizing current, consider the three main current-consuming components within the MCU: the CPU, sensor interface modules (such as I2C, SPI, etc.), and the BLE subsystem. Here, when the radio is powered on (e.g., BLETx and Rx), the primary current consumer is the BLE radio.
Embedded MCUs offer various low-power modes to reduce current consumption. Firmware designers need to consider these low-power modes and design code to minimize average current consumption. For example, sensor data does not change instantaneously; the firmware needs to scan the sensor data periodically (e.g., every 5 to 10 seconds, depending on the sensor). The sensor's read data is transmitted wirelessly via BLE.
Regarding BLE firmware, the sensor can send data along with BLE broadcast packets. We recommend against forwarding too much other data along with the broadcast packets, as this will further increase current. Between the broadcast interval and the sensor scan interval, the MCU needs to enter a low-power mode, such as "sleep mode". A low-power timer, like a watchdog timer, can wake up the device when the timer counts down. To enable low-power operation, the MCU is optimized to provide an internal BLE timer that wakes up the device in sleep mode when the broadcast interval ends. Figure 5 shows the firmware flow of the operation.
Figure 5: Firmware flow for high-efficiency wireless sensor node design
Once the firmware is designed correctly, you can measure the current. You can measure the current using a prototype board. Please note that the current consumption of the MCU in startup and low-power modes needs to be measured independently. As long as you know the time the MCU operates in startup and low-power modes respectively, the average current consumption is:
Once you have the average current figure, you can multiply it by the PMIC voltage to find the average power.
Step 3: Optimize firmware to minimize average current consumption.
The situation might be that the initially calculated design power was too high, and the solar PMIC couldn't support it. If so, you'll need to optimize the firmware. Here are a few effective ways to do this:
Executing optimized MCU startup code: When the MCU is booting up, you don't need to use a high-frequency external clock, such as a 24MHz crystal chip clock, to operate BLE. Turning this clock off initially saves energy. Furthermore, the clock crystal can use this time to stabilize, and it is also one of the components involved in startup. As these clocks gradually stabilize, the MCU can readjust to a low-power mode, and the internal low-frequency clock can wake the device when the clock is ready. In short, startup code execution time can be long, and firmware designers need to minimize startup current consumption.
a. Reduce the main CPU operating frequency
b. Control the drive mode before entering low-power mode to prevent current leakage from MCU pins.
c. If the MCU supports any debug interfaces, they should be deprecated.
These steps help reduce average current consumption.
Step 4: Design the hardware
With power-optimized firmware, it's time to design hardware based on a PMIC. Figure 6 shows a simple PMIC design based on energy harvesting.
Figure 6: Simple energy harvesting design
In the PMIC, solar energy is first stored in a storage device, VSTORE1 (VST1), in this case, a 300-μF ceramic capacitor. When VST1 reaches VOUTHV, the energy can be sent to the MCU. However, this simple energy harvesting design cannot operate 24/7 because it lacks a backup capacitor. Let's see how a backup capacitor can be added to the PMIC device and how a capacitor can help the MCU.
Figure 7: Energy Harvesting and Backup Capacitor
The energy required to operate the WSN is first stored in VST1, and the remaining energy is used to charge VST2. The energy stored in VST2 can continuously supply the WSN even in the absence of light. Additionally, an extra coin cell battery can be connected to the PMIC to increase reliability, as shown in Figure 8.
Figure 8: Energy harvesting from multiple power inputs
The PMIC switches between two power sources so that the WSN can operate under all conditions, even without lights. The switching is automatic, ensuring energy is supplied to the WSN only when needed. Therefore, this is likely the most suitable hardware design for a WSN.
Step 5: Design the User Interface
The user interface design for connecting to wireless sensor nodes can be a mobile application that receives data via WSN transmission—it's that simple. Since sensor data may appear at fixed locations in broadcast packets, BLE applications can be designed to extract relevant data from these locations and display the data on your phone. This technology can be used to manage complex networks composed of multiple WSNs.
