Living Observatory Sensor Network
Sensor Node Mark II
The second version of the sensor node was finalized in 2017 and integrated the lessons learned from the first version. The key improvements were an expanded set of built-in sensors, easy expansion to allow for external probes, and a built-in solar panel and rechargeable battery.
The second-generation sensor node uses the Hammond 1555F2F42GY enclosure, modified by the supplier to include holes for the sensors and connectors. Two small holes on the front serve as ports for the microphone and temperature/humidity sensor. A vent plug on the side equalizes pressure within the case and also allows the barometer to function. A hole on the bottom accommodates the main expansion connector, and a rectangular cutout on the top exposes the light sensors, solar panel, and motion sensor. This opening is potted with optically clear silicone to protect the electronics from moisture.
- Microcontroller: Atmel ATXmega128A4U
- Radio: Atmel AT86RF233
- Storage: AT45DB321E (4MB Flash memory)
- Real-Time Clock: AB0815
The core system uses the same microcontroller as the original design. The radio chip was updated to the AT86RF233.
To allow the sensor node to record data when the network is offline (either temporarily or when operating in a location without connectivity) this version added 4MB of flash memory and a real-time clock so that sensor readings can be timestamped and stored internally for later retrieval.
The mk2 sensor node is powered by a single lithium iron phosphate (LiFePO₄) 14550 (AA-size) cell. LiFePO₄ has a lower energy density than lithium ion, but is more robust in extreme conditions such as cold Massachusetts winters. The battery has a nominal voltage of 3.2V which is regulated to 3V to power the system.
The KXOB22-01X8F monocrystalline solar cell provides about 13μW of power in direct sunlight. This would take a very long time to recharge the battery from completely dead, but is significantly less than the average power consumption of the sensor node. So as long as the solar cell is receiving sunlight for part of the day for part of the year, the node has enough power to keep operating indefinitely, even with external probes attached.
- Temperature/Humidity: Sensirion SHT2x
- Microphone: Syntiant SPW0430HR5HB-B
- Atmospheric Pressure: Bosch BMP280
- Visible Light: Vishay VEML6030
- Ultraviolet: Vishay VEML6075
- Accelerometer: Analog Devices ADXL362
- Analog Expansion: Texas Instruments ADS7924 4-channel 12-bit ADC
Two of the main lessons learned from the mk1 sensor node deployments was that expansion is often desirable—many interesting things to sense lie outside the senor node and thus require external probes—and that wiring up external sensors in the field can be very time consuming. The mk2 sensor design aimed to have robust and streamlined expansion capabilities.
The main expansion connector is a 9-pin EN3P9FRAPCBN waterproof circular connector on the bottom of the case. It has four analog input pins that feed into a 12-bit analog to digital converter. Two of these pins can be alternatively used as either 1-wire buses (supporting peripherals such as DS18B20 temperature sensors) or together as a serial UART. The main I²C bus is also exposed on two pins, allowing additional digital sensors to be connected externally. The connector also provides two programmable power outputs that can be switched on and off and set to any voltage between 0.8 and 3.0V.
Each sensor node can be sent a unique configuration file as part of the deployment process, which tells the real-time operating system when and how to sample the external sensors. This allows many different types of sensors to be connected without having to modify the device's firmware.
There is also an internal 25-pin connector on the main board that allows a plug-in module to be installed. A plug-in module can intercept the analog signals before they reach the ADC, allowing additional analog signal conditioning to be installed, such as a preamplifier for a pH probe. Modules can also override any of the signals on the external connector, allowing the I/O to be completely customized.
There is also a dedicated high-speed SPI bus accessible on the internal connector for communicating with the main MCU. This permits upgrades such as adding a LoRa radio to communicate on standard LoRaWAN networks instead of the built-in 802.15.4 radio.