An economical Infra-Red based home automation module, for wireless control of electrical appliances using a very cheap TV/DVD player remote controller. The prototyped version uses arduino uno as microcontroller, three transistors and relays for switching three 230V/15W bulbs, IR TSOP receiver (1738), old VCR remote controller and some passive components.
Theory of Operation:
The cheapest way to remotely control a device within a visible range is via Infra-Red light. Almost all audio and video equipment can be controlled this way nowadays. Due to this wide spread use the required components are quite cheap, thus making it ideal for to use IR based control for this project.
Infra-Red actually is normal light with a particular colour. We humans can't see this colour because its wave length of 950nm is below the visible spectrum. That's one of the reasons why IR is chosen for remote control purposes, we want to use it but we're not interested in seeing it. Another reason is because IR LEDs are quite easy to make, and therefore can be very cheap.
Unfortunately, there are many more sources of Infra-Red light. The sun is the brightest source of all, but there are many others, like: light bulbs, candles, central heating system, and even our body radiate Infra-Red light. In fact everything that radiates heat also radiates Infra-Red light. Therefore we have to take some precautions to guarantee that our IR message gets across to the receiver without errors.
Modulation is the answer to make our signal stand out above the noise. With modulation we make the IR light source blink in a particular frequency. The IR receiver will be tuned to that frequency, so it can ignore everything else. The standard carrier frequency used is 38KHz. IR TSOP 1738 has been tuned to 38 KHz, and can ignore everything else.The method of modulating the signal depends upon the protocol used. Protocol defines the method of encryption, and is made by the remote controller manufacturing companies. Common protocols includes NEC (Japan based, common in India), RC-5(Europe based, common in India).
This project uses NEC protocol based remote controller, that transmits Pulse Position Modulated signal at a carrier frequency of 38KHz. Different protocols have different decoding algorithms. In serial communication we usually speak of 'marks' (or burst) and 'spaces'. The 'space' is the default signal, which is the off state in the transmitter case. No light is emitted during the 'space' state. During the 'mark' state of the signal the IR light is pulsed on and off at a particular frequency.
At the receiver side a 'space' is represented by a high level of the receiver's output. A 'mark' is then automatically represented by a low level. Please note that the 'marks' and 'spaces' are not the 1-s and 0-s we want to transmit. The real relationship between the 'marks' and 'spaces' and the 1-s and 0-s depends on the protocol that's being used. Protocol consists of several bits, grouped as address bits, command bits, start bit, toggle bit,flag bit etc.Some of the protocols will have more bits specifications where as some may not have.
NEC Protocol:
NEC protocol has the following bit specification:
· a 9ms leading pulse burst as start bit
· followed by a 4.5ms space
· 8-bit address ( differentiates different types of appliances of the same company)
· 8-bit inverse of address bits (for verification only)
· 8-bit command (distinguishes between different buttons on the remote controller)
· 8-bit inverse of command bits (for verification only)
The NEC protocol uses pulse distance encoding of the bits. Each pulse is a 560µs long 38kHz carrier burst (about 21 cycles). A logical "1" takes 2.25ms to transmit, while a logical "0" is only half of that, being 1.125ms. The recommended carrier duty-cycle is 1/4 or 1/3.
The picture above shows a typical pulse train of the NEC protocol. With this protocol the LSB is transmitted first. A message is started by a 9ms AGC burst, which was used to set the gain of the earlier IR receivers. This AGC burst is then followed by a 4.5ms space, which is then followed by the Address and Command. Address and Command are transmitted twice. The second time all bits are inverted and can be used for verification of the received message. The total transmission time is constant because every bit is repeated with its inverted length. If you're not interested in this reliability you can ignore the inverted values, or you can expand the Address and Command to 16 bits each!
A command is transmitted only once, even when the key on the remote control remains pressed. Every 110ms a repeat code is transmitted for as long as the key remains down. This repeat code is simply a 9ms AGC pulse followed by a 2.25ms space and a 560µs burst.
For fast prototyping and decoding the protocol, an arduino based library called IRremote.h developed and open sourced by Ken Shirriff
The IRremote library records the duration of each (modulated) pulse sent by the remote control. Each key on the remote corresponds to a particular code value, which is converted to a particular sequence of pulses. The library generates a unique value for each button press which is distinguishable from another button press.
It records the duration of successive pulses. If the pulse is shorter than the previous,0 is assigned. If the pulse is the same length, 1 is assigned. If the pulse is longer, 2 is assigned. (upon comparing on-durations with on-durations and off-durations with off-durations.) The result is a sequence of 0's, 1's, and 2's, then hashing these values into a 32-bit hash value.
Though the prototyped version is based upon this library, but has proved not very effective due to many limitations (repeated key press is not processed, platform non independence etc) of the library. To make this project more cost effective, low cost microcontrollers like ATmega8 is being used, and direct C programming is being implemented using standard protocol decoding algorithms and is under way to be completed soon as a very cost effective end product module. The overall production cost of a single module is expected to be less than Rs. 150/- .
Future Prospective:
Encrypted IR communication, resistant to ambient daylight can be utilised for robotic competions like Robocon where there will be instances for communication between manual and automatic bots;
The project idea can also be expanded with GSM based mobile communication for building energy conservation modules.
To read more click the link: home automation module
Credits: RAHUL MANOJ
