TECHNOLOGIES TO FACILITATE RAINFALL OBSERVATIONS | AFFORDABLE, LOW-POWER LOGGERS FOR IOT OBSERVATIONS 

The continued decline in the number of surface-based automatic and manual weather stations, both locally and globally, is problematic. The low cost of off-the-shelf electronics to augment these observations is making this more viable. Even though these low-cost sensors do not currently reach the reliability, precision or repeatability of traditional equipment, they have seen large improvements in recent years. One of the objectives of this project was to explore and develop technologies to make use of new IoT opportunities, as has been reported in other fields (Coetzee and Kotze, 2018). A logger that met these requirements was developed in collaboration with Electronic Systems Development (ESD) and is discussed in the following sections. Firstly, an overview of the various IoT communication networks under consideration is given. The one limitation that IoT networks have is the trade-off between power and bandwidth. This technology implies a limited number of transmissions and a limited size of data packages. This design facilitates single observations or control endpoints with users. One of the fundamental ideas around IoT is small sensors that can operate independently for long periods of time. A communication network that can facilitate a large number of transmitters is needed. Three emerging networks are considered in this project: LoRa, Sigfox, and ThingStream. The logging device forms the heart of the observation system - a low-cost logger was developed that could acquire data from rain gauges, as well as other devices. The resulting hardware requires large batteries and solar panels to operate independently. The key to harnessing the potential of IoT is to develop hardware with a very small power footprint. The design specifications for loggers to use in an IoT environment, therefore, include the following factors: • Low power consumption • Interfaces to IoT communication networks like a long-range, low-power wireless protocol (LoRa), SigFox, or ThingStream • Interfaces to commonly used communication protocols used by meteorological equipment, including RS-232, SDI-12, analog, TTL UART and digital counters.

The result is the ESD L151M logger, which uses an STM32L151 low-power Arm Cortex M3 MCU 32 MHz. It boasts an onboard real-time clock with a backup battery. It is programmed using a three-wire ST-LINK2 pod or a TTY COM port (COM1). 

The logger boasts the following specifications: • RS232 port • Secondary RS232 or RS422/485 port • TTL UART • SDI-12 port • 12C port for connection to sensors, optional relay or GPIO expansion board • SPI port for connection to external devices such as an analog-to-digital board • 3-12 VDC power supply • 5-12 VDC at 3 Amp output supply to power probes • XBEE socket for compliant radio modems • Micro SD storage card • Size of 78 x 78 mm.

One of the main features of the board is its ability to go into sleep mode. This reduces power consumption to approximately 300 micro Amperes. An event, such as a trigger from a TBR or a timer, can then wake up the board. This takes about eight seconds. 

Specifications that allow for this kind of power management include the following: • Micro SD card • 3V supply • RS232 chips • RS485 and SDI-12 circuit • External power output • XBEE socket Figure 4.5: The ESD L151M logger design Quantifying rainfall – real-time technologies to facilitate capacity building 22 This allows the logger to optimize power usage based on the particular application. The current firmware implementation allows for the RTC chip-to-periodic-alarm function to wake the unit up in increments of 1 minute (from one per minute to one per 24 hours). 

Several configuration parameters determine what action the logger should perform and when it should write data to the SD card or send data to the external modem. Any external modem that supports a serial RS232 port can be used. The...