3. APPROACH
In today’s home appliances market, automation is becoming the norm and Smart Thermostat is a typical automation appliance able to be applied easily at home. With Smart Thermostat, the efficiency of Heating, Ventilation, and Air Conditioning (HVAC) systems will be even greater. In addition, it will allow for daily programs set by the user to coordinate the system to run on a convenient energy-saving schedule, as well as the convenience to override the schedules when deemed necessary to maintain home comfort from a smart device anywhere in the world. In this document we will discuss the different approaches used to accomplish our design.
A general system overview of the Smart Thermostat is shown in Figure 3.1. From an Android application, a user will have complete access to their HVAC system and the ability to send controls via web to the Arduino Yun that will be housed inside the Smart thermostat. Once the Arduino Yun receives the user’s commands it will communicate with the Smart Thermostat’s I/O daughter board, which will then carry out the
commands to the HVAC system. The user will essentially have complete control of their home or office HVAC system at their fingertips.
3.1 Hardware Subsystems
Smart Thermostat requires many hardware subsystems. In this section, the different Hardware components chosen to complete Smart Thermostat’s subsystems will be listed with a detailed description of why they were chosen and the role they will play in the operation of Smart Thermostat.
3.1.1 Microcontroller, Wireless Module, & EEPROM
After determining the initial operational characteristics and methods of the thermostat, specific features such as pin count, memory, communication, and libraries could be known. After much research the most promising microcontroller to emerge was the PIC24F16KL402. It offered all of the necessary features at an outstanding price. The second component that needed to be decided upon was the EEPROM. There were many suitable candidates, all very similar in cost and performance. The model 25AA080A was selected as the front runner, sporting a slightly lower cost than other EEPROM models. The last element required was the wireless module. As it turned out this was by far the most expensive part of the design. The wireless module WiFi Plus click was chosen and the resultant jump in overall expenditure from that singular
Cons Pros
Price Component
*Less inherent support for peripherals Familiar to team* Cheap* *Countless configuration $ 2.70 Microcontroller Microchip Technology PIC24F16KL402-I/SP *Surface mount *Static memory *Cheap *Small *Efficient $0.62 EEPROM Microchip Technology 25AA080A-I/ST-ND *Expensive *Works over most wifi
configurations *Includes companion controller *Low power consumption $45.00 Wireless Module MikroElektronica WiFi Plus Click
*Pre-configured (cannot change)
*Expensive *Pre-configured (quick
start)
*More libraries and support
*Expandable memory $71.95
Development Board Arduino Yun
Figure 3.2 Development Board Vs Individual Components
3.1.2 Temperature Sensor
Being a thermostat project, the temperature sensor became a very important
component in our design. The temperature sensor will determine the overall accuracy and efficiency of Smart Thermostat. With research, we found that there were several types of temperature sensors: The thermocouple (TC), the Thermistor, the Resistance Temperature Detector, and Infrared Sensors [1]. We first looked at the LM35 Integrated Circuit Temperature Sensor because it was accurate and cost efficient, but later
decided to go with the DS18B20 Digital temperature sensor module due to its added feature of mounting directly to our Arduino board. Both temperature sensors listed in Figure 3.3 are suitable for our design constraints, but the DS18B20 seems to be the best choice at this time.
Sensor Temperature
Range Accuracy Price
Arduino Compatibile LM35 +2°C to +100°C ± 0.4°C $1.58 No DS18B20 –10°C to +85°C ±0.5°C $2.87 Yes
Figure 3.3 Comparisons of Temperature Sensors 3.1.3 Relay
24VAC signal within operable current ranges. As seen in Figure 3.4. the G6L-1P-DC3 was a perfect fit for a low price and was deemed the winner.
Component Price Pros Cons
G6L-1P-DC3 $ 2.78 ● Cheap ● 2.25VDC turn on voltage ● Handle up to 2 amps ● Tighter operable temperature range 2274-05-021 $ 11.52 ● Faster switching speed ● Expensive
Figure 3.4 Comparison of Relay
3.1.4 LCD Display
Smart Thermostat is a user friendly device, so it must have a sufficient way to display data to the user. On the wall-mounted portion of Smart Thermostat, there must be a LCD display that will show all commands and current settings of the HVAC system. The data must be legible and visually appealing at the same time. The first option seen in Figure 3.5 of comparisons of LCD displays was the segmented LCD. The second option also in Figure 3.5 was the LCD character display. With the segmented display, the visual appeal and the amount of data that could be displayed was lacking. With the LCD character display the price was more, but the amount of data that could be
Display Pros Cons Price Seven Segment Display 160-1526-5-ND ● Less Expensive ● Ease of Implementation ● Limited Data Display ● Requires 10 I/O pins $0.42 LCD Character Display NHD-C0216CZ-NSW-BBW-3V3 ● Visually Appealing ● More Legible ● More Options ● Display 240 characteristics ● More Expensive $11.00
Figure 3.5 Comparisons of LCD Displays 3.2 Software Overview
The Smart Thermostat is going to make use of three software components for operation which include the smartphone application, python scripts, and microcontroller program. This section will detail how these three software components will work together to achieve control of the smart thermostat.
3.2.1 Mobile Operating System
In choosing the mobile operating system, three main factors were considered as seen in Figure 3.6; market accessibility, market penetration, and developer cost. A mobile
Operating System Market Share Market Accessibility Developer Cost
Android 68.8%[2] High $25
iOS 18.8%[2] High $99
Blackberry 4.5%[2] Medium None
Windows Phone 2.5%[2] Medium None
Others 5.4%[2] low ----
Figure3.6 Mobile operating system comparison 3.2.2 Mobile Application
The Mobile Application will be clean and simple so that it is easy and straightforward to use as illustrated in Figure 3.7. When the application is started for the very first time, the user will be directed to the setup wizard. The setup wizard is where the user will input data from his/her wireless module for the communication between the device and the actual thermostat. After the connection is setup through the setup wizard, the
application’s Home Screen will appear. This part of the application will show the status of the Smart Thermostat and provide remote access to the system. The application will also offer a schedule builder. The schedule builder allows the user to build a
personalized schedule of temperature settings. These personalized schedules will then be saved and can be modified in the schedule manipulation feature found on the
3.2.3 Microcontroller Software
Once Smart Thermostat is powered on, it will check the scheduling system first. If there is a schedule on present time, Smart Thermostat will read the temperature sensor to compare scheduled temperature and actual temperature. By checking the cool or hot condition, it will decide whether to operate the system or not. For example, smart thermostat will operate the HVAC system if the actual temperature is higher than the scheduled temperature in cool condition. If there is no schedule, it will operate the HVAC system from checking the current set temperature.
Usage Cases 3.2.4
References
[1] Temperature Sensors; the Basics Carolyn Mathas 10-27-2011
http://www.digikey.com/en-US/articles/techzone/2011/oct/temperature-sensors-the-basics
