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device called “Saboten” meaning “cactus” in Japanese. It’s a device I designed originally for monitoring water levels in large water storage tanks in the Himalayas and also for monitoring mangrove forests in Costa Rica. It consists of a modified Arduino platform based on the ATMega1284P microcontroller. I chose this MCU because it has 16kB of SRAM as opposed to the 2kB used in the standard Arduino MCUs. The RAM goes very quickly since we’re using a FAT32 file system to store data locally on an SD card, as well as assembling long text strings to send to the aggregator. The SD card serves as a local backup for sensor data as well as maintaining a system log for debugging and analysis when the system has issues. I also wanted to stay with the Arduino environment since it’s widely used with a great developer community.
Sensing Water Level in Outdoor Paddies
The main sensor is an ultrasonic acoustic range sensor called MaxSonar by MaxBotix.Acoustic sensors are often used in non-contact liquid level sensing because they don’t have the drawbacks that float or submersion sensors often suffer and won’t risk leaching chemicals into the water. The MaxSonar is essentially an amplified acoustic speaker with a horn attached to focus the ultrasonic chirp in a single direction. We’re also experimenting with building our own ultrasonic level sensor using a waterproof piezo actuator and a 3-D printed horn to see what kind of range we can get. For the rice paddy monitoring, we only need about two meters of range so anything around there would be fine. The MaxSonar has a range of around 10m which is overkill for this particular application.
The front end RF interface consists of a 900 MHz 802.15.4 wireless transceiver with a 500 mW power amplifier on the transmitter and an 8 dB low noise amplifier on the receiver. These were implemented to optimize the transmission range since many applications have the nodes separated by a few km. With proper antennas, it’s possible to send data over ten kilometers so its overkill for the rice terraces which have a diameter of approximately 2 km.