IoT wireless sensor node design

Wireless Sensor Nodes (WSNs) play a key role in facilitating the development of the Internet of Things (IoT). The advantage of WSN is that it has extremely low power consumption, very small size and easy installation. For many IoT applications, such as outdoor installations, WSN can be powered by solar energy. When there is light in the room, the system is powered by sunlight, while charging small button cells or supercapacitors to power the system in the absence of light.

In general, wireless sensor nodes are sensor-based devices that monitor conditions such as temperature, humidity, or pressure. Nodes collect data from any type of sensor and then wirelessly pass the data to a control unit, such as a computer or mobile device, where it processes, evaluates, and takes action. Ideally, nodes can be powered by the energy harvesting mechanism and become independent devices. In a general sense, the process of energy harvesting is the process of capturing and converting a very small amount 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 the energy harvesting system. Energy is collected by energy harvesting systems (such as solar panels) and converted to stable energy by a power management integrated circuit (IC) (PMIC), which is then stored using low-leakage, low-impedance capacitors. These energy sources can be supplied to sensor interface loads (such as microcontroller MCUs), while MCUs 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 that communicate state or control information. BLE operates in the 2.4 GHz ISM band and binary frequency shift keying (GFSK) modulation, which supports a data rate of 1 Mbps.

Power management ICs are used to stabilize the power harvesting equipment required to support its ultra-low power operation. For example, the Cypress S6AE103A PMIC device consumes as little as 280 nA and has a startup power of 1.2uW (see Figure 3). Therefore, in a low-brightness environment of about 100 lux (lx), a compact solar cell can still obtain a small amount of energy.

Figure 3: Block diagram of the S6AE103A PMIC device for energy harvesting

Efficient wireless sensor node design

Let us consider the steps involved in designing a wireless sensor node:

Step 1: Select the hardware:

On the hardware side, you need the right sensor, an MCU and PMIC that can eventually be powered by an energy harvesting device. You may need additional passive components, depending on the design.

The sensor can be in analog or digital form. Many sensors on the market today use digital sensors based on integrated circuit bus (I2C), serial peripheral interface (SPI) or asynchronous transceiver (UART) interfaces. Sensors with extremely low power consumption are also available on the market. In order to keep equipment costs low, the compact form factor, MCU with integrated BLE can simplify the design and shorten the time to market. To further speed up the design, many vendors use fully integrated, fully certified programmable modules such as Cypress EZ-BLE Modules. The module consists of a main MCU, two crystal, chip or tracking antennas, expansion boards and passive components. Since these modules already have the necessary BLE certification, the product can be quickly introduced to the market.

Figure 4: BLE Module Example: Solar Powered Low-Power Bluetooth Sensor Beacon CYALKIT-E02

Step 2: Design firmware and estimate power consumption

Once the programmable MCU is selected, the next step is to write the appropriate firmware. The basic functions that the firmware needs to have are an interface to collect sensor data, a BLE component or stack that wirelessly transmits data, and a CPU that can handle firmware processing.

Since ultra-low power operation is the key, the sum of current consumption needs to be taken into account at the outset. The total current consumption is the sum of the current consumed by the sensor and the current consumed by the MCU. Since the sensor typically does not consume too much total current, the focus should be on how to minimize the current consumed by the MCU. Before optimizing the current, consider the three main components that consume current in the MCU: CPU, sensor interface modules (such as I2C, SPI, etc.) and the BLE subsystem. Here, when the radio is activated (eg BLE Tx and Rx), the main consumer of the current will be the BLE radio.

Embedded MCUs offer a variety of low power modes to reduce current consumption. Firmware designers need to consider these low-power modes and design codes so that average current consumption is minimized. For example, sensor data is not instantaneously changed, and the firmware needs to scan the sensor data in between (for example, every 5 to 10 seconds, depending on the sensor). The sensor's read data is transmitted wirelessly via BLE.

In the case of BLE firmware, the sensor can send data along with the BLE broadcast package. We recommend not transferring too much other data along with the broadcast package as this will further increase the current. Between the broadcast interval and the sensor scan interval, the MCU needs to enter a low power mode, such as the "sleep function." The low-power timer, like the watchdog timer, wakes up the device when the timer counts down. In order to use low-power operation, the MCU is optimized to provide a BLE internal timer that wakes up the device that has entered the sleep function when the broadcast interval is over. Figure 5 shows the firmware flow for the operation.

Figure 5: Firmware flow for efficient wireless sensor node design

As long as the firmware is designed, you can measure the current. You can use the prototype board to measure current. Note that the MCU startup and low current mode currents require independent measurements. As long as you know the time the MCU is operating in the startup and low-power modes, the average current consumption is:

With the average current figure, you can multiply it by the PMIC voltage to find the average power.

Step 3 optimizes firmware to minimize average current consumption

It may be that the initial calculated design power is too high and the solar PMIC cannot support it. If so, you will need to optimize the firmware. Here are a few valid ways to do this:

Execution Optimized MCU Startup Code: When the MCU is starting up, you don't need to use a high frequency external clock such as a 24MHz crystal clock to operate BLE. Turning off this clock initially saves energy. Furthermore, the clock crystal can be stabilized with these times, and it is also one of the components that are activated. These clocks are gradually stabilized, and the MCU can be adjusted to the low-power mode again. The internal low-frequency clock can wake up the device when the clock is ready. In short, the startup code can take a long time to execute and the firmware designer needs to minimize the startup current consumption.

a. Reduce the main CPU operating frequency

b. Control the drive mode before entering the low power mode to prevent the MCU pin from leaking current.

c. If the MCU supports any debug interfaces, abolish them.

These steps help to reduce the average current consumption.

Step 4: Design the hardware

With power-optimized firmware, it's time to design hardware based on PMIC. Figure 6 shows a simple PMIC design based on energy harvesting.

Figure 6: Simple energy harvesting design

The PMIC first stores solar energy to the stored device VSTORE1 (VST1), a 300-μF ceramic capacitor. When VST 1 reaches VOUTH V, energy can be sent to the MCU. But this simple energy harvesting design does not work all day because there are no backup capacitors. Let's see how backup capacitors are added to the PMIC device and how the capacitor can help the MCU.

Figure 7: Energy harvesting and backup capacitors

The energy required to operate the WSN is first stored in VST 1 and the remaining energy is used in VST 2 charging. The energy stored in VST 2 can be continuously supplied to the WSN without light. In addition, you can connect an extra button battery to the PMIC to increase reliability, as shown in Figure 8.

Figure 8: Energy harvesting for multiple power inputs

The PMIC converts two sources of power so that the WSN can operate under all conditions, even in the absence of light. The conversion is automatically generated so that the energy is supplied to the WSN when needed. Therefore, this may be the most appropriate hardware design for WSN.

Step 5: Design the user interface

The user interface design connected to the wireless sensor node can be a mobile application that transmits with WSN to receive data, which is as simple as that. Since the sensor's data may appear in a fixed location in the broadcast package, the BLE application 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 of multiple WSNs.

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