Energy Harvesting RD Showcases Low Power, Wireless Sensor Node Applications
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Silicon Labs recently announced the Energy Harvesting reference design highlighting a low power wireless sensor node operating from a harvested solar energy source. This challenge of implementing a wireless node with an alternative energy source has now been addressed with the Si10xx single chip wireless MCU—which performs low power control and wireless interface functions. Energy collected by the solar cell charges an MEC thin film battery and allows the wireless MCU to measure temperature, light level and charge level. This data is then sent wirelessly to a USB wireless adapter that demonstrates Silicon Labs‟ USB connectivity solution. Silicon Labs’ wireless sensor energy-harvesting reference design accommodates a wide range of harvested energy sources. An on-board bypass connector gives developers the flexibility to tap other energy-harvesting sources such as vibration, thermal and RF.
The Si10xx wireless MCUs offer very low active current and sleep modes to conserve system resources, as well as fast wake-up times. The on-chip wireless transceiver, integrated dc-dc switching regulator and wide input voltage range make this an ideal solution for energy-harvesting applications. Included on the reference design is an MEC thin film battery with a capacity of 0.7 mAh developed by Infinite Power Solutions. In direct sunlight, the battery can be recharged fully in only two hours and is designed to constantly recharge itself at an appropriate level. Linear Technology’s LTC4071 battery charger IC provides additional power management and offers an easy-to-use shunt battery system for Li-ion/Polymer batteries, which integrates a charger and battery pack protection into one IC.
The amount of time it takes to fully charge the 700 μA/hour thin film battery will depend on how much light is available. In general, the following guidelines would apply:
Charging Source | Light level | Charging time required |
|---|---|---|
Electric light | 50 – 200 Lux | 24 hours |
Office window | 1000 Lux | 6 hours |
Direct sunlight | 100K Lux | 2 hours |
USB cable interface | N/A | 30 minutes |
Sample Applications
Industrial Controls — energy harvesting wireless sensor nodes are used to monitor systems that are inconvenient to access regularly to replace batteries. Such systems are common in factories and industrial plants and are often currently “run to failure.” Because downtime can be expensive, an energy harvesting solution allows the wireless sensor node to not be dependent upon a battery solution.
Infrastructure sensing systems — structural integrity sensors are often located in places where it is difficult to access to replace a battery. The sensor node can be embedded deeply in the infrastructure. Energy harvesting powered wireless sensor nodes are an ideal way to monitor structural data without having to regularly replace batteries in inconvenient places.
Home Security — a suitable application for solar powered wireless sensor nodes is window/glass break detectors for home security systems. These sensors are based on acoustic or shock-based technology to detect when a window is broken. A security alert signal is sent to a main control unit from the sensor. By using a solar cell to power the unit, batteries do not need to be replaced. This is particularly useful when there are lots of windows in buildings like schools, hospitals, etc.