With the rapid growth of IoT combined with the high requirement of data storage, processing and transmission will become a big concern for the sustainability of the project. As such, any type of energy harvesting solution is welcome, if not absolutely mandatory.
When implementing these solutions, the designer must keep in mind that an IoT sensor device will not only have to measure a value (including temperature, moisture, pollution, light level), but also be able to communicate the value to its system host, often wirelessly, with a limited power supply.
To make this a reality, every single system-level component of the design must be considered thoroughly, including the sensor, receiver, energy source and the communication duty-cycle.
The challenge with battery-based systems is that they typically require a more complex energy management. This includes charge and discharge control as well as cell protection against over charge and over discharge. This increases the system complexity as well as the BOM cost as such energy management systems often involve switching regulators (additional passive components) and result in more complex ICs due to the functionalities required. The chip complexity in addition to the requirements of high efficiency and low quiescent currents often leads to rather expensive IC solutions.
In applications that do not require extended operation periods without being exposed to light a capacitor based solution can be a more cost effective solution. The storage capacitor temporarily accumulates the energy from the solar harvesting device until sufficient energy is available to perform a measurement and transmit the result. When a capacitor with adequate voltage rating is used, no charging circuit is required. The open circuit voltage of the used solar harvester when exposed to the expected peak brightness determines the maximal input voltage. If the capacitor has a voltage rating that exceeds the open circuit voltage, no charging circuit or protection is required.
For both, battery and capacitor based solutions an output voltage regulation is required to supply the attached circuits (sensors, microcontrollers, etc.) with the correct voltages. Systems that use lithium based storage options reach voltages above 4 V, which often exceeds the input voltage range of sensors and microcontrollers. To match the supply voltage of typically 1.8-3.3 V, a step down voltage conversion is required. In capacitor based systems the voltage is linearly dependent on the amount of charge stored. This can lead to great voltage variations over the discharge cycle, which cannot be accepted by all sensors or microcontrollers and hence requires some kind of regulator to stabilize the supply.
The RSL10 Solar Cell Multi-Sensor Board (RSL10-SOLARSENS-GEVK) is a comprehensive solution for battery-free IoT applications including smart building, smart home and Industry 4.0. Based on the industry’s lowest power Bluetooth Low Energy radio (RSL10), the board features multiple sensors for temperature and moisture-sensing (BMA400-a smart 3-axis accelerometer, BME280- a smart environmental sensor, and the NCT203 wide-range digital temperature sensor).
The board also features an ultra-low cost, low profile storage capacitor of 47 µF, a programming and debug interface and a ZIF interface for connecting the solar cell.
Since the device is harvesting energy from a low current source, it is important that the leakage of the overall system is small while operating and harvesting energy. Several smart devices are selected for that end, including an ultra-low quiescent current LDO (NCP170) which is populated on board.
With this solution, ON Semiconductor has demonstrated it is entirely possible to make a low cost, small form-factor sensor node powered entirely by solar energy and with features including continuous sensor monitoring and data transmission to the cloud gateway .Several use cases will take huge benefit of the new technology and capabilities of the RSL10 Solar Cell Multi-Sensor Platform, including Smart Buildings, City Management and Mobile Health. By using this platform to create new innovative sensor designs, developers can help revolutionize IoT by closing the gap in the energy needs created by implementing billions of smart sensors.
Read our full white paper here.
