Silicon Labs drafted its wireless, low power Si1012 microcontroller to be the heart of their new energy harvesting reference design (Fig. 1). A 0.7 mAh thin film battery is used to store energy from the solar cell. It can also take power from other sources attached to the board.
The battery can run the sensor node for up to 7000 hours while the processor is in sleep mode. The battery only lasts about three hours if the processor runs and transmits continuously. Recharge time depends upon the amount of light available. In full sunlight, the charge time is only 2 hours. On a cloudy day that turns to 6 hours. A 50 to 200 Lux light source will charge the battery in a day. Even partial lighting per day is enough to keep the processor cycling through sleep mode and uploading information on a regular basis.
The kit includes a wireless USB adapter that plugs into a PC where the graphical application provides status from any number of sensor nodes (Fig. 2). The USB adapter features a Silicon Labs’ Si4431 EZRadioPRO transceiver paired with another Silicon Labs micro running USB-HID class software. The micros utilize Silicon Lab's EZMac wireless software stack. The wireless communications operates between 240MHz and 960MHz.
The Si1012 is based on an 8051 core with 512 bytes of XRAM, 256 bytes of SRAM and 16 Kbytes of flash. It has as 12-bit, 75 Ksamples/s ADC with multiple inputs including on-chip temperature sensor. There are dual comparators and a 6 bit voltage reference. It has timers, serial port plus SPI and I2C interfaces.
The wireless sensor node reports on temperature, light (via solar cell) and the charge level of thin film battery. The Si1012 uses only 20nA in deep sleep mode and takes just 2 µsec to wake up. The dc-dc voltage regulator runs on inputs of 0.9V to 3.6V allowing low voltage sources like the thin film battery. The preprogrammed demo application has a 1 minute cycle time that uses only 11 µA per cycle.
The energy harvesting support uses discretes. It can handle 3.8V to 4.1V input and use as little as 1 µA to charge the battery. The solar cell delivers from 4V to 6V.
The reference design includes a complete circuit design with RF layout. The sensor board has a board-based antenna. Schematics and Gerber files are also available at no charge.
Overall, a designer needs to balance cycle times versus charge time. The advanages to energy scavenging are significant but a design needs to make sure that the system will operate over the long run. Silicon Labs reference design is a good starting point.