9.1 What You Need

I originally designed this project using a relay switch that we would have constructed part by part. This relay would have been used to turn on and off the power to the electric door latch. But after reviewing the potential safety hazards associated with improper wiring and handling of the circuit, I decided to take a safer, more conservative approach.

Instead of wrestling with the potentially jolting perils of accidental relay shocks, we will use a product specifically designed to address these concerns. Called the PowerSwitch Tail II, this simple switch houses a relay that can control standard 120V electrical devices. The relay can be energized via a 5V signal from a digital pin of a microcontroller board such as an Arduino or, in the case of this project, the PIC-based IOIO board. The PowerSwitch Tail’s prebuilt relay circuit is far easier and safer than building your own, and the cost is quite reasonable compared to the expense of procuring and assembling these parts on your own.

Rather than using an Arduino connected to a computer for data processing and control, we are going to use an Android phone connected to Sparkfun’s IOIO board. This hardware combination will serve the same function as an Arduino/PC coupling but without the size, bulk, and energy requirements that an always-on Arduino/PC combination would entail.

So what exactly is an IOIO board? It is a hardware bridge that allows Android phones to communicate with whatever sensors and motors are connected to the board. The IOIO board connects to the phone via Android’s USB debugging pathway. This pathway can be used to send and receive signals to and from the IOIO’s onboard PIC processor.

IOIO’s designer, Google software engineer Ytai Ben-Tsvi, designed the IOIO prior to Google’s official Open Accessory Protocol (ADK) initiative,[85] but he is working to make the board fully compatible with the ADK specification. The ADK is part of Google’s Android@Home home automation initiative. Investing in the board not only gives you the tools you need to make it work today, but it will also play nice with the future Android@Home APIs. And even more importantly, the IOIO works especially well today for custom projects like the one we will build.

Here is a list of all the parts we will need to construct the Android Door Lock (refer to the photo in Figure 37, ​Android Door Lock parts (some preassembled)​):

---

Figure 37. Android Door Lock parts (some preassembled)

1. A PowerSwitch Tail II (PN 80135)[86]

2. A 2.1mm female barrel jack cable to safely connect the 12V power supply to the electric door strike[87]

3. A 5VDC 1A power supply[88]

4. A 12V 5A switching power supply to electrify the electric door strike[89]

5. Three pieces of wire

6. An Android OS smartphone with a built-in camera, preferably the original Android G1 phone. This phone can be purchased from sites like http://craigslist.org or http://ebay.com for under $100 US. Note that not all Android phones are compatible with the IOIO Board. Check the IOIO Board discussion group for more details.[90]

7. A barrel jack to 2-pin JST cable that will connect to the IOIO board’s JST right angle connector[91]

8. A Sparkfun IOIO Board with JST right angle connector[92]

9. A Smarthome Electric 12VDC Door Strike[93]

10. A standard A to Mini-B USB cable to connect the G1 Android phone to the USB port on the IOIO board

You will also need a second Android device (phone, tablet, etc.) that can run the Door Lock client application along with the Eclipse IDE, Android SDK 1.5 or higher, and the Android Development Tools (ADK) plugin for Eclipse. Refer to the Chapter 7, ​Web-Enabled Light Switch​ project for more details about the Android development requirements.

While this project is one of the most expensive to build in this book, it is also one of the most flexible in terms of reusing and extending the hardware investment. Once you become familiar with how to leverage the IOIO board with an Android phone, you will understand why Google is so enthusiastic about their Android@Home effort. You will also learn how you can easily create a new category of home automation applications implementing your own ideas. But before you can attain those greater heights of actualization, you have to understand the basics. That’s what we’re going to do in the next section.