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Basics of designing a home automation system

10 Jul 2014  | Tushar Rastogi, Rahul Raj Sharma

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Different blocks found in a typical central control unit are:

Sensors: Sensors are the eyes of a home automation system. They "see" the environment and convert what they find into an electrical quantity that can be measured by a microcontroller or system processor. Basic home automation sensors include temperature sensors, humidity sensors, light sensors, and gas sensors. Data in the form of signals from these sensors can be used to control the various appliances directly without any human intervention. For example, lights can be automatically switched on upon sunset, an air conditioner can be switched off automatically when no movement is detected in the house for a half hour, or an alarm can be raised when the system detects a LPG leakage event.

Analogue Front End (AFE): Each sensor converts the change in a physical parameter such as temperature or light intensity to a similar change in electrical parameters such as resistance or capacitance. These physical quantities must be converted to a voltage equivalent so that the microcontroller can identify the variation in environment. For this purpose, an analogue front end (AFE) is interfaced with analogue sensors. The AFE preconditions output signals coming from the sensors by filtering out noise and providing required gain to the signals. AFEs are also required to calibrate the system for sensor readings, thus providing a base value for the system to identify any changes in the environment.

Remote Connectivity: Depending on need and various design considerations, users may need to be able to control the system and appliances remotely. The two most common ways of doing this are using GSM-based mobile telephony and the Internet. GSM, Ethernet, or both interfaces can be used to communicate with the system from a remote location. The system can also send or "push" useful information to users such as periodic updates, faults, intrusions, etc. These connectivity options generally require a serial communication protocol like SPI or I2C to communicate with the host processor.

Local Connectivity: The Central Control Unit and Room Control Units need to communicate with each other periodically as well as when events occur. There are multiple options available to establish communication between the CCU and RCUs that can be decided upon based on system cost and topology, including Bluetooth, RF transceivers, and XBEEs, among others. Each of these interfaces has its own pros and cons, hence system designers need to consider all of a system's requirements before selecting a particular interface.

Manual Control: In a typical home automation system, there are situations when the user needs to manually control one or more appliances. Keypads and/or infrared remotes are most commonly used to provide system control to users. Manual user control should be authorised by the system to prevent control of the system by an intruder and the shutting down of intrusion alerts.

Real Time Clock (RTC): Home automation systems must be able to control appliances based on time. An accurate time source is required to control appliances using time-based settings. An external RTC can be used to maintain time for the system and the central controller can access it to receive current time related information.

NFC Interface: Near Field Communication (NFC) is used for close proximity communication. This technology is quickly gaining traction in embedded applications for communications and information sharing. It can be used to at the main door to lock or unlock the door using the homeowner's NFC-enabled smartphone.

Power Monitor: In any house there are many electrical appliances that are sensitive to voltage fluctuations and need a reliable supply voltage within a specific range to work as expected. A power monitor block can be added to the Central Control Unit to check the instantaneous power supply voltage. This block brings down the voltage level of power supply to a level where the system can monitor it using an ADC. Using input from this block, the controller can detect low voltage, high voltage, and voltage fluctuation situations. In extreme cases, this block can instruct RCUs to switch off sensitive appliances to prevent damage. An example of a simple power monitor circuit is given in figure 5 and can be easily implemented into automation systems.

Figure 5: Power monitor circuit.

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