Christian Brothers University 650 East Parkway South Memphis, TN

Transcription

1 Christian Brothers University 650 East Parkway South Memphis, TN WIRELESS SMART THERMOSTAT Martin Tribo Student IEEE Membership Number Submitted for consideration in Region 3, IEEE Student Paper Competition Endorsement of Branch Counselor: The author(s) of this paper are student members of this IEEE student branch and will still be undergraduate students at the time of SoutheastCon. This paper is the only entry of our student branch. Signature: Name (Typed): John Ventura Telephone Number of Counselor: (901) Wireless

2 Smart Thermostat Abstract Today more devices are becoming remotely accessible over the network. Lighting, security systems, garage doors and TVs are just some examples of devices that are wirelessly connected. The purpose of this project is to design and implement a wireless smart thermostat. The thermostat should be a fully functional prototype of a device that could be used in industry and homes. The device would perform the regular functions of a thermostat, but also would have networked access. The prototype performs all critical functions, with the exception of automatic configuration on the network. 1

3 I. Introduction This project involves the implementation of a thermostat offering basic temperature control and accessibility over a wireless network. The main benefit of the network component is the ease of access to the thermostat controls. This convenience also facilitates the efficient use of the thermostat, allowing users to save energy by optimizing their usage. This project also includes the implementation of web and mobile applications for interfacing with the thermostat. These applications provide intuitive software interfaces to the thermostat, giving advanced control and instant feedback. In addition to the features offered in this thermostat, external applications could be used to extend functionality. External applications can monitor the thermostat over time and provide control that is more advanced. Some possible external applications would be neural network controllers, data loggers or other interfaces. These external applications could be used to improve the thermostat in the future. II. User Needs The Wireless Smart Thermostat needs to meet the following criteria: Perform all functions of a regular thermostat o Controls temperature using heating, ventilation and air conditioning (HVAC) systems o Have the ability to configure device manually without using a wireless network Can be configured from any network attached device o Web interface o Mobile Application interface 2

4 III. Product Design Specifications Control standard heating and air conditioning systems via relays Connect to standard home wireless networks Provide HTTP interface for interfacing with device Manual configuration interface via switches and displays Figure 1 is an example of a webpage that is used to interface wirelessly with the thermostat. The wireless system interfaces with Windows, Linux and Mac operating systems. Figure 1 Sample Web App 3

5 IV. Concept Generation Several different approaches were considered when designing this project. All approaches involved the following components: Microcontroller General electronic components Temperature Sensor Relays Wi-Fi Adapter A microcontroller will be necessary in any solution to control all of the components. The possible Wi-Fi adapters also depend on the microcontroller. The electronic components and the temperature sensor should remain the same no matter which microcontroller is used. Two microcontrollers were considered: Arduino and Raspberry Pi. Arduino is an opensource prototyping platform that has been around for several years and has many available components that could assist in creating the hardware interfaces for the thermostat. The Raspberry Pi on the other hand has only recently become available. While the Raspberry Pi does not have the massive support of Arduino, it is a functional computer that can run Linux and many devices through its USB interface. The software is easy to write and there is a large range of available hardware to interface with it; and the Wi-Fi adapter is useful. 4

6 V. Concept Selection / Screening Matrix The Raspberry Pi might appear to be the better option at first glance; but because it has just recently been released, there is the possibility of running into problems that could cause significant delays in the implementation of the thermostat. The designer is also more experienced with the Arduino as opposed to the Raspberry Pi, which makes working with the Arduino easier. Design Criteria Productivity Requirements that: Cost Factors Health and - Safety - Performance - Mobility - Size - Familiarity - Support - Cost to Buy - Reliability - Cost to - Develop - Environmental - Requirements - Totals Weighting Factor (WF) Arduino Rating Factor (RF) WF X RF W Raspberry Pi Rating Factor (RF) WF X RF W Table 1 Concept Selection / Screening Matrix The result of the Concept Selection / Screening matrix of Table 1 is that the Arduino is the best selection. This is due to the fact that support is an important factor. 5

7 VI. System Design Block Diagram Figure 2 is a block diagram of the thermostat. The different components of the thermostat are shown and how they interact. The user interacts with the thermostat through the software or hardware interface, and the thermostat controls the HVAC system through relays. HVAC Interface WiFi Adapter Software Interface Microcontroller HVAC System Temperature Sensor Hardware Interface Figure 2 Block Diagram of Thermostat HVAC Interface The HVAC interface consists of the relays that control the HVAC systems. There are three relays being used, each relay is for one of the HVAC systems. Digital output pins from the Arduino control these relays. The Arduino itself does not supply enough current to control a 6

8 relay, so a transistor is used to provide the necessary current. Figure 3 ( is the circuit used for the relay. JZC-11F 1 kω 2N2222 Figure 3 Relay Circuit One consideration behind the design of this system was to prevent the HVAC systems from rapidly being toggled. The software controlling the relays is designed to prevent this. The software uses only one method to control the HVAC systems. This method keeps track of how long since the last relay state changed and will only allow the relay states to be changed after a minimum threshold of time has been reached. For testing purposes, this time was set to one minute. If the software attempts to change the state before the threshold, the new state is stored and will be applied once the threshold is reached. If the software attempts to change the state multiple times before the threshold, only the most recent change will be implemented. 7

9 Microcontroller The Arduino Uno is used for the thermostat s microcontroller. It takes 7-12V DC for power, which can be provided from a transformer using a 110 Volt wall receptacle or HVAC wiring. The main functionality of the thermostat is implemented in the Arduino programming language. This includes controlling all of the hardware components and the temperature control logic. Wi-Fi Adapter The thermostat uses the Weburban Maple as its Wi-Fi adapter. The Maple provides b/g/n capability to the thermostat. A library for the Arduino called WiShield is used to interface with the Maple. The library provides support for DHCP and the various wireless security protocols. In addition, it also provides the web server implementation that the thermostat uses. It allows a single method to be used that accepts a URL, and returns a message. Long messages are not supported, so only temperature related data is sent directly from the thermostat, not the whole HTML page. Temperature Sensor The TMP-36 temperature sensor is used to read temperature values. The sensor has a precision of +/- 1 C and functions in the 0 C to 100 C range. It requires a ground and a 3.3V connection and outputs a voltage that can be read by the Arduino s analog input. The datasheet of Figure 4 provides a graph that shows how the output voltage translates into Celsius. 8

10 Figure 4 Output Voltage vs. Temperature There are several different steps in calculating the temperature using this graph. First, the Arduino represent a voltage with an integer value from 0 to In order to make use of the datasheet s graph, that integer must be converted to a voltage. This can be done by a simple unit conversion: multiplying the integer value by 5/1024. With the voltage calculated, the equation for Celsius can be derived considering the following: when the voltage is 0.5V, the temperature is 0 C, and 1 V is about equivalent to 100 C. The equation for the temperature in Celsius is then: C = (V 0.5) 100 Finally, once the temperature is calculated in Celsius, it needs to be converted to Fahrenheit using: F = ( C 1.8) + 32 Once the temperature is calculated in Fahrenheit, it is saved and will be used in the calculations for controlling the HVAC systems. 9

11 Hardware Interface The thermostat has a simple hardware interface consisting of a button, potentiometer and LEDs for feedback. Since the mobile and web interfaces will be the primary ways to control the thermostat, the hardware interface just provides a way to manually set the temperature without any advanced control. The button is used to toggle between the different HVAC systems with one RGB LED indicating the system that is chosen and another LED indicating whether it is currently activated. The potentiometer is used so that the user can set the temperature through a turn knob. In order for the software and hardware interfaces to coexist, only one can be responsible for the temperature setting at a time. This is because if the temperature is changed through the software interface, it will not match up with the potentiometer. Therefore, the last used interface is what determines the temperature. Software Interface The web and mobile applications use Hypertext Transfer Protocol (HTTP) to communicate with the thermostat. The thermostat responds to GET requests directed at its root URL. It takes up to two parameters: system and temperature. The system parameter can have a value of heating, cooling, fan or none. The temperature value can be any integer ranging from 40 to 90. The response to any HTTP request is the current temperature, the activated system and current set temperature, all separated by newline characters. The web application is an external page that uses XMLHttpRequests to communicate with the thermostat. XMLHttpRequests are used in many modern day websites, they allow a page to create its own HTTP requests and fetch content from servers without notifying the user. With these requests, the page is able to send and receive data from the thermostat. This keeps 10

12 data between the device and thermostat to a minimum. The thermostat would otherwise have to transmit whole webpages, which is an action the Arduino is unable to perform. The mobile application is developed on the Android operating system. It functions similarly to the web application, but uses methods provided by the Android API. The application was designed to be simple and offer quick access to the controls. The temperature slider is towards the bottom offering quick access to the thumb when holding the phone with one hand. The mobile app is displayed in Figure 4 below. Figure 4 Mobile App 11

13 VII. Summary The prototype has several different systems that were tested: Temperature Sensing Proper control of HVAC system Manual controls function properly HTTP Interface Web App Mobile App At first glance, the temperature sensor would appear to have plug and go usage. It actually required a significant amount of testing in order to verify the accuracy and precision of its readings. An average of multiple samples ended up being used to combat erratic measurements. Testing with a HVAC system is not an option because if something goes wrong, it would be expensive to replace the damaged system. Instead, an array of lights was used to demonstrate the prototype s use of its relays. Because these lights provide instant visible feedback, they are good to use for observing how the HVAC systems were controlled. The manual controls do not offer as much functionality, so they were easy to test. The manual switches simply toggle the systems between heating, ventilation and air conditioning and the potentiometer is used to set the temperature. For each of the activated systems, a high or low temperature was set and the prototype s response was examined. The HTTP Interface was examined with a web browser by connecting directly to the thermostat instead of using the Web App. The actual interface does not have buttons, so 12

14 commands had to be manually typed in. Since the Web App and Mobile App work off the HTTP interface, they provided a secondary form of testing. Testing for the Web and Mobile Apps consisted of verifying each of the HVAC systems were properly being used. The Apps were used to verify that the whole prototype was working correctly since they rely on all other parts of the system to be functioning properly, except for the manual controls. VIII. Conclusion This project makes practical use of different components to create a thermostat. The implementation of the project was successful and all basic features of the thermostat were made wirelessly accessible over the network. In the future, this project could be extended to include programmable temperatures profiles and better hysteresis control. These features could be directly implemented into the thermostat, or implemented through an external application that constantly controls the thermostat through the HTTP interface. In addition, as the Raspberry Pi continues to become more widely used, it would be well worth implementing a Raspberry Pi in a wireless thermostat. The built in components such as the clock and SD card allow extended functionality such as keeping track of the time of day and storage of data. The higher power of the CPU would also allow more data to be sent over the wireless, allowing a sophisticated Web application to be included on the thermostat itself. The future of wirelessly connected devices such as this thermostat is exciting and this thermostat is a step towards a connected future. 13

15 IX. Prototype Estimated Cost and Budget The cost of materials is given in Table 2 below. Commercial programmable thermostats can cost approximately $ X. References Arduino Transistors $3.50 Resistors $3.00 Diodes $3.00 LEDs $2.50 Potentiometer $1.00 Switch $0.50 Breadboard $8.00 Temperature Sensor $1.50 Arduino $35 Web Urban Maple (Wi-Fi Adapter) $70 Total: $ Table 2 Prototype Cost Weburban Maple Hayt, William, Kemmerly, Jack, and Durbin, Steven (2012). Engineering Circuit Analysis, 8 th Ed., McGraw Hill. Hambley, Allan (2000). Electronics, 2 nd Ed., Prentice Hall Thermostat 14