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Mobile Controlled Electrical Appliances
Project Introduction:-
This project talks about the basic definitions needed to understand the project better and further defines the technical criteria to be implemented as a part of this project. This project is a teleremote circuit that enables switching ‘ON’ and ‘OFF’ of appliances through telephone lines. It can be used to switch appliances from any distance, overcoming the limited range of infrared and radio remote controls. The circuit described in the project can be used to switch up to ten appliances (corresponding to the digits 0 to 9 of the telephone keypad). This circuit is based on the DTMF controller circuit. DTMF means dual tone multiple frequency. The DTMF signals on telephone instrument are used as control signals.
Why automation?
Earlier, we are looking into the face of future when we talked about automated devices, which could do anything on instigation of a controller, but today it has become a reality.
1. An automated device can replace good amount of human working force, moreover humans are more prone to errors and in intensive conditions the probability of error increases. Whereas an automated device can work with diligence, versatility and with almost zero error.
2. This is why this project looks into construction and implementation of a system involving hardware to control a variety of electrical and electronics instruments.
What is home/office automation?
Home/office automation is the control of any or all electrical devices in our home or office.
Whether we are there or away. Home /office automation is one of the most exciting developments in technology for the home that has come along in decades. There are hundreds of products available today that allow us control over the devices automatically, either by remote control; or even by voice command.
What can be automated?
Virtually anything in the home/office that is powered by electricity can be automated and/or controlled. We can control our electrical devices with our cordless phone from our easy chair.
We can turn our porch lights on automatically at dark or when someone approaches and can see who is at the front door from any nearby television, and talk to them or unlock the door from any nearby telephone. Have the security system turn off lights, close drapes and setback the temperature when we leave and turn on the alarm system. The possibilities are only limited by our imagination.
How is the system controlled in this project?
This circuit enables one to utilize a telephone for remote switching ‘on’ and ‘off’ of any electrically operated device. It does not require any physical connection to the telephone lines or lifting of the handset. It does not require any physical connection to the telephone department. The switchinig operation is performed by making use of doded telephone ring signal.
While designing it has been ensured that the circuit is free from any false triggering by normal telephone ring signals. Since the instrument has no hardwired connection to the telephone the line it does not affect the normal operation of the telephone set. To avoid false triggering, suitable reset feature is incorporated in this circuit.
Why not any other device?
In this age of automation many other device like microprocessor or micro-controller, infrared remote, voice controlled devices etc. Are used for the automation purposed, but the have certain limitation which are described below :-
1. The use of microprocessor or micro-controller involves complexities like microprocessor operating voltages; interrupt servicing, poling, memory access mechanism and extensive soldering. Moreover, if we use micro-controller or a microprocessor we can not change the working as and when desired. The problem being, while using them we have to hardwire the code into ROM chips and in case we need to amend we have to burn a new ROM chip to replace the earlier one. The ear ROM becomes useless and has to be scraped. And this has to be done on every single time we need add something new.
2. An infrared remote control can work for a device up to a specified range to distance after that it can not be used for controlling the device.
3. A voice controlled device works on a single voice and cannot be used by any other person and have a certain range of working.
What is DTMF?
When you press a button in the telephone set keypad, a connection is made that generates a resultant signal of two tones at the same time. These two tones are taken from a row frequency and a column frequency. The resultant frequency signal is called "Dual Tone Multiple Frequency". These tones are identical and unique.
A DTMF signal is the algebraic sum of two different audio frequencies, and can be expressed as follows:
f(t) = A0sin(2*П*fa*t) + B0sin(2*П*fb*t) + ........... ------->(1)
Where fa and fb are two different audio frequencies with A and B as their peak amplitudes and f as the resultant DTMF signal. fa belongs to the low frequency group and fb belongs to the high frequency group.
Each of the low and high frequency groups comprise four frequencies from the various keys present on the telephone keypad; two different frequencies, one from the high frequency group and another from the low frequency group are used to produce a DTMF signal to represent the pressed key.
The amplitudes of the two sine waves should be such that
(0.7 < (A/B) < 0.9)V -------->(2)
The frequencies are chosen such that they are not the harmonics of each other. The frequencies associated with various keys on the keypad are shown in figure (A).
When you send these DTMF signals to the telephone exchange through cables, the servers in the telephone exchange identifies these signals and makes the connection to the person you are calling.
The row and column frequencies are given below:
When you press the digit 5 in the keypad it generates a resultant tone signal which is made up of frequencies 770Hz and 1336Hz. Pressing digit 8 will produce the tone taken from tones 852Hz and 1336Hz. In both the cases, the column frequency 1336 Hz is the same. These signals are digital signals which are symmetrical with the sinusoidal wave.
A Typical frequency is shown in the figure below:
Along with these DTMF generator in our telephone set provides a set of special purpose groups of tones, which is normally not used in our keypad. These tones are identified as 'A', 'B', 'C', 'D'. These frequencies have the same column frequency but uses row frequencies given in the table in figure (A). These tones are used for communication signalling.
Due to its accuracy and uniqueness, these DTMF signals are used in controlling systems using telephones. By using some DTMF generating IC’s (UM91214, UM91214, etc) we can generate DTMF tones without depending on the telephone set.