A minor project report HOME AUTOMATION USING MOBILE PHONES
A Minor Project Report on HOME AUTOMATION USING MOBILE PHONES Submitted In partial fulfillment For the award of the degree of Bachelor of Technology in Department of Electrical and Electronics EngineeringSupervisor: Submitted by: Miss Himadri Singh Raghav Monika Sevda(090491) Neha Choudhary (090495) Pragya Rohatgi (090501) November, 2012 Mody Institute of Technology & Science(Deemed University u/s 3 of UGC Act 1956)Faculty of Engineering & Technology Lakshmangarh, Sikar – 332311 (Rajasthan)
Mody Institute of Technology and Science (A deemed University Under Section 3 of UGC Act 1956) Lakshmangarh-332311, Dist. Sikar (Rajasthan) Phones :( 01573)225001 to 225012 (12 lines) Fax :( 01573)225042___________________________________________________________________________________ EXAMINER CERTIFICATEThe minor project entitled ―HOME AUTOMATION USING MOBILEPHONES‖ by Ms. Neha Choudhary(090495), Ms. Monika Sevda(090491), Ms.Pragya Rohatgi (090501) is approved in partial fulfillment of the requirement ofthe Degree of Bachelor of Technology in Electrical and Electronics Engineering ofMody Institute of Technology and Science (a Deemed University), Lakshmangarh.Prof. B.P Singh Ms. Himadri Singh RaghavH.O.D (ECE) Supervisor Examiner
Mody Institute of Technology and Science (A deemed University Under Section3 of UGC Act 1956) Lakshmangarh-332311, Distt. Sikar (Rajasthan) Phones :( 01573)225001 to 225012 (12 lines) Fax :( 01573)225042___________________________________________________________________________________ CERTIFICATEThis is to certify that the minor project report entitled ―HOME AUTOMATIONUSING MOBILE PHONES‖ submitted by Ms. Neha Choudhary (090495), Ms.Pragya Rohatgi (090501), Ms. Monika Sevda (090491) for the partial fulfillment ofrequirements for the degree of Bachelor of Technology in Electrical andElectronics Engineering to be awarded by Mody Institute of Technology andScience (Deemed University), Lakshmangarh, is a record of their/her work undermy supervision and guidance.Date: Ms. Himadri Singh Raghav (Project Guide)
ACKNOWLEDGEMENT―Perseverance, inspiration and motivation have always played a key role in any venture. It is notjust the brain that matters most, but that which guides them. The character, the heart, generousqualities and progressive forces. What was conceived just as an idea materialized slowly intoconcrete facts. The metamorphosis took endless hours of toil, had its moments of frustration, butin the end everything seemed to have sense‖.At this level of understanding it is often difficult to understand the wide spectrum of knowledgewithout proper guidance and advice. Hence, we take this opportunity to express our heartfeltgratitude to our project guide Ms. Himadri Singh Raghav who had faith in us and allowed us towork on this project.We would like to thanks Prof. B.P. Singh (H.O.D Electrical) for his immense interest, valuableguidance, constant inspiration and kind co-operation throughout the period of word undertaken,which has been instrumented in the success of our project.We would like to pay our sincere gratitude to our respected Dean Prof DESAI for providing usopportunity to work in computer lab as a part of the major part.We also acknowledge our profound sense of gratitude to all the teachers who have beeninstrumental for providing us the technical knowledge and moral support to complete the projectwith full understanding.We thank our friends and family for their moral support to carve out this project and above allGOD for removing all hurdles in the way. Neha Choudhary Monika Sevda Pragya Rohatgi
CONTENTSAbstract 1CHAPTER-1 Dual-tone multi-frequency 2 1.1 Audio sample 1.2 Theory of operation 1.3 Construction 1.4 Construction highlightsCHAPTER-2 Various devices used 6 2.1 Resistors 2.2 Capacitors 2.3 Crystal oscillator 2.4 Voltage Regulator 2.5 SolderingCHAPTER-3 MT 8870 IC 10 3.1 Features 3.2 Applications 3.3 Description 3.4 Functional Description 3.5 Pin Description 3.6 Steering Circuit 3.7 Filter Section 3.8 Guard Time Adjustment
LIST OF FIGURES1.1 A DTMF telephone keypad 21.2 Tone frequency for "1" key 61.3 Tone Generator 71.4 Tone Decoder 82.1 Resistor 112.2 Resistor 112.3 Capacitor 122.4 Capacitor 122.5 Capacitor 132.6 Crystal Oscillator 132.7 Crystal Oscillator 142.8 Voltage Regulator 152.9 Voltage Regulator 152.10 Soldering 162.11 PCB 173.1 Functional Description of MT 8870 IC 213.2 18 PIN CERDIP/PLASTIC DIP/SOIC 223.3 20 PIN SSOP/TSSOP 223.4 Basic steering circuit 253.5 Filter response 26
3.6 Guard time adjustment 273.7 Oscillator connections 28
LIST OF TABLES1.1 DTMF keypad frequencies 41.2 DTMF event frequencies 41.3 DTMF row/column frequencies 61.4 Dip switch positions 103.1 Pin descriptions 233.2 Absolute maximum conditions 253.3 DC electrical characteristics 303.4 Operating characteristics 31
ABSTRACTHome automation refers to the use of computer and information technology to control homeappliances and features (such as windows or lighting). Systems can range from simple remotecontrol of lighting through to complex computer/micro-controller based networks with varyingdegrees of intelligence and automation. Home automation is adopted for reasons of ease, securityand energy efficiency.In modern construction in industrialized nations, most homes have been wired for electricalpower, telephones, TV outlets (cable or antenna), and a doorbell. Many household tasks wereautomated by the development of specialized appliances. For instance, automatic washingmachines were developed to reduce the manual labor of cleaning clothes, and waterheaters reduced the labor necessary for bathing.Other traditional household tasks, like food preservation and preparation have been automated inlarge extent by moving them into factory settings, with the development of pre-made, pre-packaged foods, and in some countries, such as the United States, increased reliance oncommercial food preparation services, such as fast food restaurants. Volume production and thefactory setting allows forms of automation that would be impractical or too costly in a homesetting. Standardized foods enable possible further automation of handling the food within thehome.The use of gaseous or liquid fuels, and later the use of electricity enabled increased automationin heating, reducing the labor necessary to manually refuel heaters and stoves. Developmentof thermostats allowed more automated control of heating, and later cooling.As the number of controllable devices in the home rises, interconnection and communicationbecomes a useful and desirable feature. For example, a furnace can send an alert message when itneeds cleaning, or a refrigerator when it needs service. Rooms will become "intelligent" and willsend signals to the controller when someone enters. If no one is supposed to be home and thealarm system is set, the system could call the owner, or the neighbors, or an emergency number.In simple installations, domotics may be as straightforward as turning on the lights when aperson enters the room. In advanced installations, rooms can sense not only the presence of aperson inside but know who that person is and perhaps set appropriate lighting, temperature,music levels or television channels, taking into account the day of the week, the time of day, andother factors.
IntroductionHere is a circuit that lets you operate your home appliances like lights and water pump from youroffice or any other remote place. So if you forgot to switch off the lights or other applianceswhile going out, it helps you to turn off the appliance with your cell phone. Your cell phoneworks as remote control to your home appliances. You can control the desired appliance bypresetting the corresponding key. The system also gives you voice acknowledgement of theappliance status.The Project ―Home Automation using mobile communication‖ has different sections such as:1.Microcontroller2.DTMF decoder3. Voice recording and playback device
Chapter-1 Dual-tone multi-frequencyDual-tone multi-frequency (DTMF) signaling is used for telephone signaling over the line inthe voice-frequency band to the call switching center. The version of DTMF used for telephonetone dialing is known by the trademarked term Touch-Tone and is standardized by ITU-TRecommendation Other multi-frequency systems are used for signaling internal to the telephonenetwork.As a method of in-band signaling, DTMF tones were also used by cable television broadcastersto indicate the start and stop times of local commercial insertion points during station breaks forthe benefit of cable companies. Until better out-of-band signaling equipment was developed inthe 1990s, fast, unacknowledged, and loud DTMF tone sequences could be heard during thecommercial breaks of cable channels in the United States and elsewhere. Fig 1.1 A DTMF telephone keypad1.1 AUDIO SAMPLE1.1.1 Dtmf push to talk id
Present-day uses of the A, B, C and D keys on telephone networks are few, and exclusive tonetwork control. For example, the A key is used on some networks to cycle through differentcarriers at will (thereby listening in on calls). Their use is probably prohibited by most carriers.The A, B, C and D tones are used in amateur radio phone patch and repeater operations to allow,among other uses, control of the repeater while connected to an active phone line.DTMF tonesare also sometimes used in caller ID systems to transfer the caller ID information, however in theUSA only Bell 202 modulated FSK signaling is used to transfer the data.1.1.2 KeypadThe DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, andeach column representing a high frequency. Pressing a single key (such as 1 ) will send asinusoidal tone of the two frequencies (697 and 1209 hertz (Hz)). The original keypads hadlevers inside, so each button activated two contacts. The multiple tones are the reason for callingthe system multifrequency. These tones are then decoded by the switching center to determinewhich key was pressed. DTMF keypad frequencies (with sound clips) 1209 Hz 1336 Hz 1477 Hz 1633 Hz 697 Hz 1 2 3 A 770 Hz 4 5 6 B 852 Hz 7 8 9 C 941 Hz * 0 # D
Table 1.1DTMF event frequencies Event Low frequency High frequency Busy signal 480 Hz 620 Hz Dial tone 350 Hz 440 Hz Ringback tone (US) 440 Hz 480 Hz Table 1.2The tone frequencies, as defined by the Precise Tone Plan, are selected such that harmonics andintermodulation products will not cause an unreliable signal. No frequency is a multiple ofanother, the difference between any two frequencies does not equal any of the frequencies, andthe sum of any two frequencies does not equal any of the frequencies. The frequencies wereinitially designed with a ratio of 21/19, which is slightly less than a whole tone. The frequenciesmay not vary more than ±1.8% from their nominal frequency, or the switching center will ignorethe signal. The high frequencies may be the same volume or louder as the low frequencies whensent across the line. The loudness difference between the high and low frequencies can be aslarge as 3 decibels (dB) and is referred to as "twist". The minimum duration of the tone shouldbe at least 70 msec, although in some countries and applications DTMF receivers must be able toreliably detect DTMF tones as short as 45ms.DTMF can be decoded using the Goertzelalgorithm.
1.2 Theory of Operation1.2.1 So what are these tones?In DTMF there are 16 distinct tones. Each tone is the sum of two frequencies: one from a lowand one from a high frequency group. There are four different frequencies in each group.Your phone only uses 12 of the possible 16 tones. If you look at your phone, there are only 4rows (R1, R2, R3 and R4) and 3 columns (C1, C2 and C3). The rows and columns selectfrequencies from the low and high frequency group respectively. The exact value of thefrequencies are listed in Table 3 below: TABLE 1.3: DTMF Row/Column Frequencies LOW-FREQUENCIES ROW # FREQUENCY (HZ) R1: ROW 0 697 R2: ROW 1 770 R3: ROW 2 852 R4: ROW 3 941 HIGH-FREQUENCIES COL # FREQUENCY (HZ) C1: COL 0 1209
C2: COL 1 1336 C3: COL 2 1477 C4: COL 3 1633 C4 not used in phonesThus to decipher what tone frequency is associated with a particular key, look at your phoneagain. Each key is specified by its row and column locations. For example the "2" key is row 0(R1) and column 1 (C2). Thus using the above table, "2" has a frequency of 770 + 1336 = 2106Hz The "9" is row 2 (R3) and column 2 (C3) and has a frequency of 852 + 1477 = 2329 Hz. The following graph is a captured screen from an oscilloscope. It is a plot of the tone frequency for the "1" key: Fig 1.2You can see that the DTMF generated signal is very distinct and clear. The horizontal axis is insamples. The frequency of the tone is about 1900 Hz - close to the 1906 Hz predicted by Table 3(697+1209).
1.3 ConstructionThis section is organized as follows: o Schematics o Construction Hightlights 1.3.1 SchematicsThe schematic in the figure below is relatively straightforward. I recommend that you use a combinationof soldering and wirewrapping using sockets for all IC component placement. 1.3.2 Tone Generator Fig. 1.3 1.3.3 Tone DecoderThe schematic for the DTMF decoder in the figure below. Again you can use a combination ofwirewrapping and soldering. Part placement is not critical.
Fig 1.41.4 Construction Highlights1.4.1 Tone GeneratorThe DTMF generator circuit is straight forward to construct. Only 3 of the 5089s 4 column pins(3,4,5) and all 4 row pins (11 to 14) were used. Thus it uses only 12 of the 16 touch tones (justlike your phone). In this schematic youll note the "/" in front of column and row pin labels (e.g./C1). This means that these pins are active low. In other words, a pin is enabled when it isgrounded. When the circuit is powered on, these pins normally high (+5V). C1-C3 and R1-R4are wired to an 8-position DIP switch. In a single-package this DIP contains 8 single-pole-single-throw (SPST) switches. It is much cheaper to use than 8 real SPST switches. You slide a DIPposition to open or close its switch. When closed that particular switch connects its associatedcolumn or row pin to ground and makes it active.You could use a 12-key keypad available from many surplus or electronics mail-ordercompanies. But you must be aware of what you buy. Not all keypads can be used with the 5089.I think the proper keypad will have 9 pins: 8 (for 4 rows plus 4 columns) plus 1 for a commonwhich youd connect to ground. Often surplus keypads do not come with techsheets, and you willhave to manually figure out which pin is associated with which row or column. I found to my
surprise that my particular surplus 12-key keypad (from Electronic Goldmine) did not have thiscommon pin and so I resorted to using a DIP.In this photo DIP positions 1 and 4 (C1 and R1 respectively) are in their ON positions. C1 andR1 is "1" on your phones keypad. The speaker will emit the touch-tone associated with the "1"key (see Table 4)The speaker is driven through the TIP31 transistor. Note: the labels 1, 2 and 3 that refer to thebase, collector and emitter pins respectively in the schematic are not standard. Be sure to checkyour spec sheet for your TIP31. TABLE 1.4: DIP SWITCH POSITIONS (1) DIP: 1+4 (2) DIP: 2+4 (3) DIP: 3+4 (4) DIP: 1+5 (5) DIP: 2+5 (6) DIP: 3+5 (7) DIP: 1+6 (8) DIP: 2+6 (9) DIP: 3+6 (*) DIP: 1+7 (0) DIP: 2+7 (#) DIP: 3+7Table 4 shows the DIP positions that will activate the tone associated with the key. The numbersin bold and parenthesis are your desired key tone (like your phone). Thus if you wanted to dial a"0", you would slide only positions 2 and 7 on the DIP switch.1.4.2 Tone DecoderThe decoder circuit is also easy to construct. You will have to physically wire (using alligatorclips for instance) the TONE OUT pinout from the generator to the TONE IN pinout of thedecoder.
Once physically wired together, the 7-segment display will light up the number associated withthe touch-tone you activate with the DIP switch.Note: A "0" tone lights up as "[" and not zero. This is because, "0" keys tone is actually a ten inbinary (1010). Because the 7-segment displays only a single digit, ten is displayed as a "[".Similarly, the "*" (binary 1011) and "#" (binary 1100) light up as "]" and "U" respectively.
CHAPTER 2 VARIOUS DEVICES USED2.1 RESISTOR :A resistor is a two-terminal electrical or electronic component that opposes an electric currentby producing a voltage drop between its terminals in accordance with Ohms law: The electricalresistance is equal to the voltage drop across the resistor divided by the current through theresistor while the temperature remains the same. Resistors are used as part of electrical networksand electronic circuits. Fig 2.1Axial-lead resistors on tape. The tape is removed during assembly before the leads are formed and thepart is inserted into the board. Fig 2.2 Three carbon composition resistors in a 1960s valve radio..
.2.2 CAPACITORA capacitor is an electrical/electronic device that can store energy in the electric field between apair of conductors (called "plates"). The process of storing energy in the capacitor is known as"charging", and involves electric charges of equal magnitude, but opposite polarity, building upon each plate.Capacitors are often used in electric and electronic circuits as energy-storage devices. They canalso be used to differentiate between high-frequency and low-frequency signals. This propertymakes them useful in electronic filters.2.2.1 Capacitor types Fig 2.3 Capacitors: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. Major scale divisions are cm. Fig 2.4
Various types of capacitors. From left: multilayer ceramic, ceramic disc, multilayer polyester film, tubularceramic, polystyrene, metallized polyester film, aluminium electrolytic. Major scale divisions are cm. Fig 2.52.3 CRYSTAL OSCILLATOR Fig 2.6A miniature 4 MHz quartz crystal enclosed in a hermetically sealed HC-49/US package, used as theresonator in a crystal oscillator.A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibratingcrystal of piezoelectric material to create an electrical signal with a very precise frequency. Thisfrequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stableclock signal for digital integrated circuits, and to stabilize frequencies for radiotransmitters/receivers.
Fig 2.7Inside construction of a modern high performance HC-49 package quartz crystal2.4 VOLTAGE REGULATORA voltage regulator is an electrical regulator designed to automatically maintain a constantvoltage level.It may use an electromechanical mechanism, or passive or active electronic components.Depending on the design, it may be used to regulate one or more AC or DC voltages.2.4.1 Voltage Regulator 7805The Digilab board can use any power supply that creates a DC voltage between 6 and 12 volts. A5V voltage regulator (7805) is used to ensure that no more than 5V is delivered to the Digilabboard regardless of the voltage present at the J12 connector (provided that voltage is less than12VDC). The regulator functions by using a diode to clamp the output voltage at 5VDCregardless of the input voltage - excess voltage is converted to heat and dissipated through thebody of the regulator. If a DC supply of greater than 12V is used, excessive heat will begenerated, and the board may be damaged. If a DC supply of less than 5V is used, insufficientvoltage will be present at the regulators output.
Fig 2.8If a power supply provides a voltage higher than 7 or 8 volts, the regulator must dissipatesignificant heat. The "fin" on the regulator body (the side that protrudes upward beyond the mainbody of the part) helps to dissipate excess heat more efficiently. If the board requires highercurrents (due to the use of peripheral devices or larger breadboard circuits), then the regulatormay need to dissipate more heat. In this case, the regulator can be secured to the circuit board byfastening it with a screw and nut (see below). By securing the regulator tightly to the circuitboard, excess heat can be passed to the board and then radiated away. Fig 2.9
2.5 Soldering Fig 2.10(De)soldering a contact from a wire.Soldering is a process in which two or more metal items are joined together by melting andflowing a filler metal into the joint, the filler metal having a relatively low melting point. Softsoldering is characterized by the melting point of the filler metal, which is below 400 °C. Thefiller metal used in the process is called solder.Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it isdistinguished from welding by the base metals not being melted during the joining process. In asoldering process, heat is applied to the parts to be joined, causing the solder to melt and bedrawn into the joint by capillary action and to bond to the materials to be joined by wettingaction. After the metal cools, the resulting joints are not as strong as the base metal, but haveadequate strength, electrical conductivity, and water-tightness for many uses. Soldering is anancient technique mentioned in the Bible and there is evidence that it was employed up to 5000years ago in Mesopotamia.2.5.1 ApplicationsThe most frequent application of soldering is assembling electronic components to printed circuitboards (PCBs). Another common application is making permanent but reversible connectionsbetween copper pipes in plumbing systems. Joints in sheetmetal objects such as food cans, roofflashing, rain gutters and automobile radiators have also historically been soldered, and
occasionally still are. Jewelry and small mechanical parts are often assembled by soldering.Soldering is also used to join lead came and copper foil in stained glass work. Soldering can alsobe used to effect a semi-permanent patch for a leak in a container cooking vessel.2.5.2 Desoldering and resolderingUsed solder contains some of the dissolved base metals and is unsuitable for reuse in makingnew joints. Once the solders capacity for the base metal has been achieved it will no longerproperly bond with the base metal, usually resulting in a brittle cold solder joint with acrystalline appearance.It is good practice to remove solder from a joint prior to resoldering—desoldering braids orvacuum desoldering equipment (solder suckers) can be used. Desoldering wicks contain plentyof flux that will lift the contamination from the copper trace and any device leads that arepresent. This will leave a bright, shiny, clean junction to be resoldered.The lower melting point of solder means it can be melted away from the base metal, leaving itmostly intact though the outer layer will be "tinned" with solder. Flux will remain which caneasily be removed by abrasive or chemical processes. This tinned layer will allow solder to flowinto a new joint, resulting in a new joint, as well as making the new solder flow very quickly andeasily.2.6 Printed circuit board Fig 2.11Part of a 1983 Sinclair ZX Spectrum computer board; a populated PCB, showing the conductive traces,vias (the through-hole paths to the other surface), and some mounted electrical components
Fig 2.11 PCB Layout ProgramA printed circuit board, or PCB, is used to mechanically support and electrically connectelectronic components using conductive pathways, or traces, etched from copper sheetslaminated onto a non-conductive substrate. Alternative names are printed wiring board(PWB),and etched wiring board. A PCB populated with electronic components is a printedcircuit assembly (PCA), also known as a printed circuit board assembly (PCBA).PCBs are rugged, inexpensive, and can be highly reliable. They require much more layout effortand higher initial cost than either wire-wrapped or point-to-point constructed circuits, but aremuch cheaper and faster for high-volume production. Much of the electronics industrys PCBdesign, assembly, and quality control needs are set by standards that are published by the IPCorganization.
Chapter 3 MT 8870 IC3.1 Features• Complete DTMF Receiver• Low power consumption• Internal gain setting amplifier• Adjustable guard time• Central office quality• Power-down mode• Inhibit mode• Backward compatible with MT8870C/MT8870C-13.2 Applications• Receiver system for British Telecom (BT) or CEPT Spec (MT8870D-1)• Paging systems• Repeater systems/mobile radio• Credit card systems• Remote control• Personal computers• Telephone answering machine
3.3 DescriptionThe MT8870D/MT8870D -1 is a complete DTMF integrating both the bandsplit filterand Digital decoder functions. The filter section uses witched capacitor techniquesfor high and lowgroup filters; the decoder uses digital counting techniques to detectand decode all 16 DTMF tonepairs into a 4 -bit code. External component count isminimized by on chip provision of a differential input amplifier, clock oscillator andlatched three-state bus interface3.4 Functional DescriptionThe MT8870D/MT8870D-1 monolithic DTMF receiver offers small size, low power consumption and highperformance. Its architecture consists of a bandsplit filter section, which separates the high and lowgroup tones, followed by a digital counting section which verifies the frequency and duration of thereceived tones before passing the corresponding code to the output bus.
3.7 Functional DescriptionThe MT8870D/MT8870D-1 monolithic DTMF receiver offers small size, low power consumption and highperformance. Its architecture consists of a bandsplit filter section, which separates the high and lowgroup tones, followed by a digital counting section which verifies the frequency and duration of thereceived tones before passing the corresponding code to the output bus.3.8 Steering CircuitBefore registration of a decoded tone pair, the receiver checks for a valid signal duration (referred to ascharacter recognition condition). This check is performed by an external RC time constant driven by ESt.A logic high on ESt causes vc to rise as the capacitor discharges. Provided signal condition is maintained(ESt remains high) for the validation period (tGTP), vc reaches the threshold (VTSt) of the steering logicto register the tone pair, latching its corresponding 4-bit code into the output latch. At this point the GToutput is activated and drives vc to VDD. GT continues to drive high as long as ESt remains high. Finally,after a short delay to allow the output latch to settle, the delayed steering output flag (StD) goes high,signalling that a received tone pair has been registered. The contents of the output latch are madeavailable on the 4-bit output bus by raising the three state control input (TOE) to a logic high. Thesteering circuit works in reverse to validate the interdigit pause between signals. Thus, as well asrejecting signals too short to be considered valid, the receiver will tolerate signal interruptions (dropout)too short to be considered a valid pause. This facility, together with the capability of selecting thesteering time constants externally, allows the designer to tailor performance to meet a wide variety ofsystem requirements.
Basic steering circuit Fig 3.43.9 Filter SectionSeparation of the low-group and high group tones is achieved by applying the DTMF signal to the inputsof two sixth-order switched capacitor bandpass filters, the bandwidths of which correspond to the lowand high group frequencies. The filter section also incorporates notches at 350 and 440 Hz forexceptional dial tone rejection (see Figure 3). Each filter output is followed by a single order switchedcapacitor filter section which smooths the signals prior to limiting. Limiting is performed by high-gaincomparators which are provided with hysteresis to prevent detection of unwanted low-level signals. The
outputs of the comparators provide full rail logic swings at the frequencies of the incoming DTMFsignals. Filter response Fig 3.53.10 Guard Time AdjustmentIn many situations not requiring selection of tone duration and interdigital pause, the simple steeringcircuit shown in Figure is applicable. Component values are chosen according to the formula:
tREC=tDP+tGTPtID=tDA+tGTAThe value of tDP is a device parameter (see Figure 11) and tREC is the minimum signal duration to berecognized by the receiver. A value for C of 0.1 μF is recommended for most applications, leaving R to beselected by the designer.Different steering arrangements may be used to select independently the guard times for tone present(tGTP) and tone absent (tGTA). This may be necessary to meet system specifications which place bothaccept and reject limits on both tone durationand interdigital pause. Guard time adjustment also allows the designer to tailor system parameters suchas talk off and noise immunity. Increasing tREC improves talk-off performance since it reduces theprobability that tones simulated by speech willmaintain signal condition long enough to be registered. Alternatively, a relatively short tREC with a longtDO would be appropriate for extremely noisy environments where fast acquisition time and immunityto tone drop-outs are required. Design information for guard time adjustment is shown in Figure .
Fig 3.63.11 Crystal OscillatorThe internal clock circuit is completed with the addition of an external 3.579545 MHz crystal and isnormally connected as shown in Figure (Single- Ended Input Configuration). However, it is possible toconfigure several MT8870D/MT8870D-1 devices employing only a single oscillator crystal. The oscillatoroutput of the first device in the chain is coupled through a 30 pF capacitor to the oscillator input (OSC1)of the next device. Subsequent devices are connected in a similar fashion. Refer to Figure for details. Theproblems associated with unbalanced loading are not a concern with the arrangement shown, i.e.,precision balancing capacitors are not required.
Fig 3.7 oscillator connections3.12 Absolute maximum condition
Table 3.43.19 ApplicationsThe purpose of this Application Note is to provide information on the operation and aplication of DTMFReceivers. The MT8870 Integrated DTMF Receiver will be discussed in detail and its use illustrated in theapplication examples which follow.More than 25 years ago the need for an improved method for transferring dialling information throughthe telephone network was recognized. The traditional method, Dial pulse signalling, was not only slow,suffering severe distortion over long wire loops,but required a DC path through the communicationschannel. A signalling scheme was developed utilizing voice frequency tones and implemented as a veryreliable alternative to pulse dialling. This scheme is known as DTMF (Dual Tone Multi- Frequency),
Touch-Tone™ or simply, tone dialling. As its acronym suggests, a valid DTMF signal is the sum of twotones, one from a low group (697-941Hz) and one from a high group (1209-1633Hz) with each groupcontaining four individual tones. The tone frequencies were carefully chosen such that they are notharmonically related and that their intermodulation products result in minimal signaling impairment(Fig. 1a). This scheme allows for 16 unique combinations. Ten of these codes represent the numeralszero through nine, the remaining six (*,#,A,B,C,D) being reserved for special signalling.Most telephone keypads contain ten numeric push buttons plus the asterisk (*) and octothorp (#). Thebuttons are arranged in a matrix, each selecting its low group tone from its respective row and its highgroup tone from its respective column .The DTMF coding scheme ensures that each signal contains one and only one component from each ofthe high and low groups. This significantly simplifies decoding because the composite DTMF signal maybe separated with bandpass filters, into its two single frequency components each of which may behandled individually. As a result DTMF coding has proven to provide a flexible signalling scheme ofexcellent reliability, hence motivating innovative and competitive decoder design.
CONCLUSIONI learned a lot in the process of writing this article, and I hope it will encourage many of you toconsider bringing HA into your own lives. Ill admit that wiring up light switches is not theeasiest of tasks for someone who hasnt done it before, but the plug-in devices are a snap to setup and make for an easy entry-point to working with the technology.Most people are very timid the first time they have to replace a component in their PC and, in thesame way, there is a learning curve to HA. And although the components youre dealing with arenot as sensitive as those inside a computer, there is a risk anytime you are working near electriccircuits (in this case more of a risk to yourself).Once I finished with the installation, the only signs of the work Id done was a set of nicer wallswitches and a handful of extra adapters plugged into the wall. The only way it could havelooked more professional was if I had finished the whole house with Insteon- and Decora-styleswitches to match the ones I changed. I could have also swapped the wall outlets to Insteonrather than using the lamp adapter, but then I couldnt use the adapter for my Christmas treewhen that time of year rolls around. I like having the choice of moving it.I wont try claiming that anyone can manage a home automation installation, so if youreuncomfortable around electronics, dont know what youre doing in a breaker box, or areparticularly accident-prone, dont even risk it. I would think that if youve soldered wires before,though, then you probably have enough knowledge and common sense to take on a task like this.