2018-08-20 - By Darwin Padoocattevilla, Bhavyasree Cherukat, Selbin Thelakkadan Xavier, Robert Elder
This article is a review of a capstone project completed by Darwin Padoocattevilla, Bhavyasree Cherukat, and Selbin Thelakkadan Xavier. The goal for their project was to calibrate and integrate a number of sensors together that could perform the following tasks that are important for monitoring the conditions of plant growth. This goal was achieved, and the resulting source code for integrating these sensors is included in their results. This includes monitoring of:
- Soil humidity levels.
- Ambient temperature.
- Lighting levels.
Here is an image of the completed circuit:
And here is an image of the corresponding circuit diagram:
The materials and costs for this project consist of the following:
- A cheap eBay Arduino with the listing title 'ATmega328P CH340G UNO R3 Board + USB Cable Compatible with Arduino LW' - Cost $4.98 CAD.
- A cheap eBay digital thermometer with the listing title 'Waterproof Digital Thermal Probe or Sensor DS18B20 Length：1M' - Cost $2.03 CAD.
- A cheap eBay photo-resistor with the listing title '10 pcs Photo Light Sensitive Resistor Photoresistor Optoresistor 12mm GL12528' - Cost $7.83 CAD for 10, $0.78 CAD unit cost.
- A cheap eBay soil humidity sensor with the listing title 'Soil Humidity Hygrometer Moisture Detection Sensor Module Arduino w/Dupont Wires' - Cost $1.34 CAD.
- Cheap eBay double-sided proto board with the listing title '10PCS Double Side Prototype PCB Bread board Tinned Universal FR4 2x8cm - 9x15cm' Unit cost $2.33 CAD.
- A bread-board prototyping kit from Addicore cost $11.50 USD + Shipping + import duties.
- The bluetooth module and ESP8266 were purchased separately by the students.
- The AR823 digital lux meter was borrowed for this project.
- All components purchased from eBay did not have shipping or import duties costs.
The team was able to successfully integrate all of the sensors together, and the associated source code for this can be found here. In order to get accurate readings, the photoresistor and humidity sensor required calibration. The photoresistor was calibrated using an AR823 digital lux meter:
The humidity sensor was also tested in a number of different soil conditions such as dry or wet sand:
All of the calibration tests are document in the team's GitHub repo.
DS18B20 Thermal Probe
This simple thermal probe required little set-up, and didn't require any external amplifier:
Soil Humidity Sensor
The students stated that this humidity sensor gave them inconsistent readings at first, but the readings eventually stabilized somewhat. In hindsight, resistive moisture sensors are prone to corroding and would not be dependable to function long-term:
The students developed a mathematical function (documented in code in their git repo) that maps values read from this photoresistor to those found on a standard lux meter. The function was found to be linear for one region, but non-linear in another:
The result of this project is an enlightening prototype of a setup that can monitor three important aspects of plant growth: soil humidity, light levels, and temperature. The fact that the components can be purchased and delivered for around $20 is particularly noteworthy.
One particularly meaningful insight gained from this project was that of the unreliability of resistive moisture sensors. Since these sensors have bare metal that makes direct physical contact with moist soil, corrosion to the metal surface is a certainty. This will result in physical damage to the sensor that can change readings as it ages until it eventually completely fails. Instead, capacitive moisture sensors are one potential alternative that can be explored in the future.
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