Comprehensive Temperature Controller for Plant Growth Experiments

This tech spec was submitted by Yang Zhong as part of the University Technology Exposure Program.

Problem / Solution

In the field of plant experiments, commercially available temperature controllers are costly and unsuitable due to specific characteristics that restrict their use for plant experiments. Temperature controllers for plant growth experiments must comply with six main requirements, which are not all in conventional temperature controllers. First, temperature readings' accuracy must be below 0.5°C to avoid statistical errors. Second, the device must avoid temperature overshoot or a significant increase in temperature after reaching the threshold. Third, independent Pulse Width Modulation or PWMs regulates output channels' heating and cooling elements for rapid On-Off cycling of power. Fourth, simultaneous heating and cooling. Fifth, time-dependent control of temperature can be based on the season and location of a specific area on Earth. Lastly, the controller must store the sensor data in real-time for making actual reports of the experiment.

A simple temperature controller is specifically designed for plant growth experiments in enclosed spaces. Its low cost allows for the accessibility of conducting plant experiments and laboratory activities without needing to purchase expensive equipment. Furthermore, the temperature controller will provide accurate data readings and be capable of adapting to the specific scenarios encountered during plant experiments. The controller is also capable of real-time data display, which is essential in making reports.

Design

The designed temperature controller comprises six main components: the sensor, internal clock, microcontroller, Wi-Fi chip, LCD, and encoder.

A 3-D printed casing enclosed the entire temperature controller for electrical insulation with three female electrical sockets on the side and provision for the sensor probe. A Platinum Resistance Thermometer PT100 sensor is used for temperature reading with an accuracy of ± 0.3°C for statistical accuracy of data recording. 

The output channels feature independent 25A Solid-State-relays that allow high amperage heater, cooler, and grow lights to be connected without additional parts. These channels are placed inside the enclosed space for temperature adjustment and the sensor for measurement. The heating element activates when the air temperature is below the lower buffer temperature, while the cooling element activates when the air temperature is above the upper buffer temperature. The set point and measured temperature had an average difference of 0.3°C with a standard deviation of 0.6°C.

The microcontroller runs in Arduino IDE for its simplicity. Simultaneous cooling and heating are achieved via independent PWM to avoid temperature overshooting. This also ensures precise and accurate control within narrow ranges.

An internal clock allows any temperature threshold to be set and tracked at desired times of the day. The temperature controller maintains a repeatable dark/light temperature cycle within the set buffer, showing its long-term reliability.

An integrated Wi-Fi chip connects the temperature controller to a local internet network. It uploads collected data to a web application in real-time and sends out alerts when certain conditions are met. A liquid–crystal display (LCD) shows the set threshold, the current temperature, time, date, and phase period. In addition, a single rotary encoder allows the setting to be changed by looping over various variables and setting their values.