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Basics of designing a home automation system

10 Jul 2014  | Tushar Rastogi, Rahul Raj Sharma

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The process of controlling home appliances automatically for the convenience of users is referred to as home automation. This technology makes life easier for the user, and saves energy by utilising devices according to strict requirements. Controls can be as basic as dimming lights with a remote or as complex as setting up a network of items in the home that can be programmed using a main controller or even via cell phone from anywhere in the world.

A home automation system can involve switching off electrical appliances like air-conditioners or refrigerators when a desired temperature has been reached, then switching on again when the temperature has crossed a certain value.ÿA home automation system can also be used to secure a house from burglars by sending alerts to the nearest police station and the homeowner in case a trespasser is sensed.

Figure 1: A basic home automation system.

Apart from algorithmic automation, devices can be controlled by the user to suit personal requirements using direct buttons, cell phones, the internet, or infrared remotes. A network of appliances and sensors can interact with each other and make decisions for operation.

This article provides a framework for designing a cost-effective and functional home automation system, first discussing the general design considerations that should be evaluated before starting, followed by a review of the trade-offs amongst various architectural approaches, and then how to implement that design using system-on-chip technology.

Figure 2: Control unit examples.

Design considerations
There are several design challenges and considerations involved while developing a home automation system, many of which are determined by user needs. Once those have been determined, the designer can choose the appropriate processor, sensors, and communication protocol for the system, keeping the following parameters in mind:

Type of Interface: The most basic and crucial requirement in a home automation system, the interface is the basic communication protocol and hardware combination used for sending and receiving messages between devices and the user. Designers have many options for executing communication between devices, the user, and the overall system, depending upon the system, range, size of house, ease of use, etc. If a user wants to control the home appliances through the Internet, the designer needs to add an Ethernet/Wi-Fi interface to connect the system to the home network. If the user wants to control the system using Bluetooth from a cell phone, the designer needs to add a Bluetooth interface to communicate with the device.

The choice of communications interface also depends upon the topology used between the central control unit (CCU) and room control units (RCU). These units will be discussed in more detail later in this article.

Sensing Requirements: The designer needs to determine the sensing requirements of the user and decide upon the required sensor to perform the task. He or she also needs to assess the sensor specifications required for different needs and usability in different environments. The range of sensors that should be considered include:
 • Thermistors can be used to control air conditioners, refrigerators, geysers, heating system, or in case of fire.
 • Humidity sensors sense the moisture level in the environment.
 • Gas sensors can be used to detect gas leaks.
 • Light sensors can be used to detect the luminous intensity in the house.

The information provided by these sensors (after signal conditioning) is used by the processor to make several important decisions regarding the appliances and when to switch them ON or OFF.

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