CHAPTER 1
INTRODUCTION
Our project titled “Mobile Phone Triggered Combination Lock” is a simple electronic circuit which can be used for several applications. Several mistakes are happening in our daily life due to the carelessness such as locking and heavy electrical appliances, generators etc. So we decided to do something which can resolve this issue. Our mini project aims at developing a circuit which is capable of locking electrical circuits with the help of mobile phone. In the case of higher applications, this circuit with suitable control systems can perform desired applications.
1.1 Objective
The main objective of the project is to introduce wireless technology that provides the options of high working range, high frequency range and multiple controlling options.
This electronic combination lock is unlocked when the circuit recognizes a unique valid sequence of four tones from a mobile phone. This unique code is easily changed if needed be. Note that the phone is used ‘off-line’ so no phone expenses are involved.
One great benefit compared to a traditional code lock is that no door keypad is required. Installing is thus cheaper and simpler and there are no visible targets for vandalism.
The door to be locked needs only a small hole for a microphone, which can be very small. It is not essential to locate the microphone near to the lock. In addition to forming the basis of a combination lock, this circuit can be used to operate anything that runs off electricity.
Pressing a mobile phone button generates a so called dual-tone multiple- frequency, or DTMF code whose frequencies depend on which key is pressed. The circuit receives DTMF tones via its microphone and is set by the user to respond to a unique four-digit code using switches. After amplification, the DTMF signal received at the microphone is directed to a DTMF-receiver, which converts it to a 4-bit binary format.
Received digits are stored as four; 4-bit words in the 16-bit register. The digits are converted to decimal using BCD-to-decimal-decoders .When a new digit is keyed, the previous ones are shifted forward. In the diagram, the last entered digit is in the left.
If four digits in sequence match with the code set by switches, output from the AND gates goes high and triggers the monostable circuit, Output from the monostable circuit goes high for a moment, adjustable via resistor, and activates the lock relay via transistor.
CHAPTER 2
LITERATURE SURVEY
The aim behind the mini project is to improve the professional competency by selecting those areas which otherwise are not covered in the normal course. This is to enhance our knowledge into various fields, and thus to gain work experience, confidence, and logical thinking. Our aim was to select a topic which is simple enough to be done within the specified time. So we are planned to do a simple electronic project using basic electronics concept that we have studied yet. We interested to apply and modify the basic electronics concept than a new topic to be selected. While selecting a topic for our mini project, the first thing which came to our mind was that it should be a product that has got considerable importance in the modern era.
2.1. Selection:
Our concentration was to develop a system which can reduce the problems or difficulties due to carelessness. Also one more thing was in mind that to develop a system which can be applied for several applications associated with modern science and developments in technology. So the concept of a locking system triggered on mobile phone is selected which can be used as simple door locking applications to complicated electrical circuits and modern appliances such as motors, generators etc.
We obtained so many results for this. And by combining and reffering all these, we could able to design a new circuit by utilising basic electronic concepts only. Without much complexity, our device facilitates easy and safe locking. We came to know that both simple logic combination circuits and circuits containing programmable devices like microcontrollers may be used. We realized that combination circuits have a definite superiority than programmable and other circuits especially in terms of money and simplicity. Eventhough combination technologies are poor compard to the other technologies available, we could design the circuit fulfillng all the needs.
2.2. Design of the Circuit:
Main parts of mobile phone triggered combination lock system are DTMF receiver and decoder. This electronic combination lock is unlocked when the circuit recognizes a unique valid sequence of four tones from a mobile phone. Since receiver performs the major function in this circuit, we decided to select an efficient receiver IC. We came to know that 8870 IC can work with a wide operating range and are commonly available. Pressing a mobile phone button generates a so called dual-tone multiple- frequency, or DTMF code whose frequencies depend on which key is pressed.
The circuit receives DTMF tones via its microphone and is set by the user to respond to a unique four-digit code using switches. After amplification, the DTMF signal received at the microphone is directed to the DTMF-receiver, which converts it to a 4-bit binary format. Received digits are stored as four, 4-bit words in the 16-bit register formed by two 4015 ICs. The digits are converted to decimal using BCD-to-decimal decoders formed by four 4028 ICs.
When a new digit is keyed, the previous ones are shifted forward. In the diagram, the last entered digit is in the left. In the diagram, the last entered digit is in the left. If four digits in sequence match with the code set by switches, output from the AND gates 4081 IC goes high and triggers the mono stable circuit, 4047 IC. Output from the 4047 IC goes high for a moment, adjustable via resistor, and activates the lock relay via transistor.
Four nine-way DIL switches set the code. With the setting as shown, the relay is powered with digit combination 7398. Digit 0 is not used because the DTMF code for zero is not the same as the BCD code for zero.
Sensitivity of the microphone amplifier can be adjusted via resistor. Maximum operating distance is about 20 cm. If there is a lot of ambient noise, it is best to set the phone’s speaker sound level to maximum.
2.3. Assembling the Project:
Main components needed for the project are resistors, capacitors, LF357 IC, 8870 IC, 4015 ICs, 4028 ICs, 4047 IC, 4081 IC, 2N222 transistor, and diodes. The components were mounted on the bread board and were wired up. A 5V dc supply was generated.
A Nokia 1100 mobile phone was used. A microphone was connected at the start of the amplifier section. The main component in amplifier section is LF357 IC. DTMF receiver and decoder circuits were implemented using8870 IC. The output of this section was verified using four LEDs. Corresponding binary of each key was observed. We also designed and developed a DTMF keypad to proceed with the implementation of circuit more easily. The shift register and BCD to Decimal converters were implemented easily. Then we went for monostable circuit and relay connections. The output of the project was verified by connecting a LED at the output end. The supply to LED was controlled by the combination lock. The LED glowed when 7398 was pressed on the keypad of the mobile phone.
CHAPTER 3
DTMF FEATURES
DTMF or Dual Tone Multiple Frequency signaling is used for telephone signaling over the line in the voice frequency band to the call switching centre. The version of DTMF used for telephone tone dialing is known as ‘Touch-Tone’.
3.1. Keypad
The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key (such as ‘1’) will send a sinusoidal tone for each of the two frequencies (697 and 1209 hertz (Hz)). The original keypads had levers inside, so each button activated two contacts. The multiple tones are the reason for calling the system multi frequency. These tones are then decoded by the switching center to determine which key was pressed.
DTMF assigns a specific frequency (consisting of two separate tones) to each key so that it can easily be identified by the electronic circuit. The signal generated by the DTMF encoder is a direct algebraic summation, in real time, of the amplitudes of two sine (cosine) waves of different frequencies , i.e., pressing ‘5’ will send a tone made by adding 1336 Hz and 770 Hz to the other end of the line. The tones and assignment in a DTMF system are shown in Table 3.1.
The Touch-Tone system, using the telephone keypad, gradually replaced the use of rotary dial starting in 1963, and since then DTMF or Touch-Tone became the industry standard for both cell phones and landline service.
Table 3.1 Tones and Assignments in a DTMF system
| Frequencies | 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 |
Fig.3.1. 1209 Hz on 697 Hz to make the 1 tone
3.2. Multifrequency signaling
Prior to the development of DTMF, automated telephone systems employed pulse dialing (Dial Pulse or DP in the U.S.) or loop disconnect (LD) signaling to dial numbers. It functions by rapidly disconnecting and re-connecting the calling party's telephone line, similar to flicking a light switch on and off. The repeated interruptions of the line, as the dial spins, sounds like a series of clicks. The exchange equipment interprets these dial pulses to determine the dialed number. Loop disconnect range was restricted by telegraphic distortion and other technical problems, and placing calls over longer distances required either operator assistance (operators used an earlier kind of multi-frequency dial) or the provision of subscriber trunk dialing equipment.
Multi-frequency signaling (see also MF) is a group of signaling methods, that use a mixture of two pure tone (pure sine wave) sounds. Various MF signaling protocols were devised by the Bell System and CCITT. The earliest of these were for in-band signaling between switching centers, where long-distance telephone operators used a 16-digit keypad to input the next portion of the destination telephone number in order to contact the next downstream long-distance telephone operator. This semi-automated signaling and switching proved successful in both speed and cost effectiveness. Based on this prior success with using MF by specialists to establish long-distance telephone calls, Dual-tone multi-frequency (DTMF) signaling was developed for the consumer to signal their own telephone-call's destination telephone number instead of talking to a telephone operator.
DTMF tones are also used by some cable television networks and radio networks to signal the local cable company/network station to insert a local advertisement or station identification. These tones were often heard during a station ID preceding a local ad inserts. Previously, terrestrial television stations also used DTMF tones to shut off and turn on remote transmitters.
DTMF signaling tones can also be heard at the start or end of some VHS (Video Home System) cassette tapes. Information on the master version of the video tape is encoded in the DTMF tone. The encoded tone provides information to automatic duplication machines, such as format, duration and volume levels, in order to replicate the original video as closely as possible.
DTMF tones are sometimes used in caller ID systems to transfer the caller ID information, however in the USA only Bell 202 modulated FSK signaling is used to transfer the data.
3.3. Special tone frequencies
National telephone systems define additional tones to indicate the status of lines, equipment, or the result of calls with special tones. Such tones are standardized in each country and may consist of single or multiple frequencies. Most European countries use a single frequency, where the United States
| Event | Low frequency | High frequency |
| 480 Hz | 620 Hz | |
| 440 Hz | 480 Hz | |
| 350 Hz | 440 Hz |
Table 3.2 Special tone frequencies
The tone frequencies, as defined by the Precise Tone Plan, are selected such that harmonics and intermodulation products will not cause an unreliable signal. No frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies. The frequencies were initially designed with a ratio of 21/19, which is slightly less than a whole tone. The frequencies may not vary more than ±1.8% from their nominal frequency, or the switching center will ignore the signal. The high frequencies may be the same volume or louder as the low frequencies when sent across the line. The loudness difference between the high and low frequencies can be as large as 3 decibels (dB) and is referred to as "twist." The minimum duration of the tone should be at least 70 ms, although in some countries and applications DTMF receivers must be able to reliably detect DTMF tones as short as 45ms.
As with other multi-frequency receivers, DTMF was originally decoded by tuned filter banks. Late in the 20th century most were replaced with digital signal processors. DTMF can be decoded using the Goertzel algorithm.
CHAPTER 4
PERIPHERAL DESCRIPTION
In this project we are using five main IC’S. They are the LF357 IC, 8870 IC, 4015 ICs, 4028 ICs, 4047 IC and 4081 IC. The other components are microphone, electromagnetic type relay, 9-way DIL switch, crystal, transistor, diodes, resistors and capacitors. The hardware details of this are specified here.
4.1. LF 357 IC:
These are the first monolithic JFET input operational amplifiers to incorporate well matched, high voltage JFETs on the same chip with standard bipolar transistors (BI-FET™ Technology). These amplifiers feature low input bias and offset currents/low offset voltage and offset voltage drift, coupled with offset adjust which does not degrade drift or common-mode rejection.
Fig.4.1 Pin diagram of LF 357 IC
The devices are also designed for high slew rate, wide bandwidth, extremely fast settling time, and low voltage and current noise.
4.2. 8870 IC:
Today, most telephone equipment use a DTMF receiver IC. One common DTMF receiver IC is the IC 8870 that is widely used in electronic communications circuits. The 8870 is a 16-pin IC. It is used in telephones and a variety of other applications. When a proper output is not obtained in projects using this IC, engineers or technicians need to test this IC separately. A quick testing of this IC could save a lot of time in research labs and manufacturing industries of communication instruments.
Fig 4.2 Pin diagram of 8870
The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key (such as ‘1’) will send a sinusoidal tone for each of the two frequencies (697 and 1209 hertz (Hz)). The original keypads had levers inside, so each button activated two contacts. The multiple tones are the reason for calling the system multi frequency. These tones are then decoded by the switching center to determine which key was pressed.
Fig 4.3 DTMF Receiver Circuit
4.3. 4015 IC:
A shift register consists of a chain of bi stables connected together so that data can be transferred (shifted) along the chain from one end to the other.
The HCC4015B (extended temperature range) and HCF4015B (intermediate temperature range) are monolithic integrated circuits, available in 16-lead dual in-line plastic or ceramic package and plastic micro package.
The HCC/HCF4015B consists of two identical, independent; 4-stageserial-input/parallel output registers. Each register has independent CLOCK and RESET inputs as well as a single serial DATA input. “Q” outputs are available from each of the four stage son both registers. All register stages are D-type, master-slave flip-flops. The logic level present at the DATA input is transferred into the first register stage and shifted over one stage at each positive-going clock transition.
Fig 4.4 Pin diagram of 4015 IC
The 4015 contains two 4-bit shift registers which can be used independently, or linked to provide an 8-bit register. Each register has a serial data input (D), a clock input (CP), four parallel outputs (O0-O3) and a reset input (MR).
On the first rising edge, also called a LOW to HIGH transition, the logic state at the SERIAL INPUT is transferred to A, the output of the first D-type. This happens after a short delay, known as the propagation delay of the D-type. Before this change, the logic state at the D-input of the second D-type was LOW, logic 0. This 0 is transferred to B. In other words, no change in logic state is observed.
When the next CLOCK pulse arrives, The SERIAL INPUT and the D-input of the second D-type are both at logic 1. Output A remains at 1 and output B becomes 1. Each new pulse transfers the logic 1 signal to the next stage of the shift register. The diagram shows a 4-bit shift register:
Fig 4.5 Shift Register
You can follow these changes in logic levels from the V/t graphs given in the next diagram:
Fig 4.6 Waveform of a 4-bit shift registers
4.4. 4028 IC:
The HEF4028B is a 4-bit BCD to decimal decoder, a 4-bit BCO to octal decoder with active LOW enable or an 8-output (Y0 to Y7) inverting demultiplexer. The outputs are fully buffered for best performance. When used as a BCD to decimal decoder a 1-2-4-8 BCD code applied to inputs A0 to A3 causes the selected output to be HIGH. The other nine outputs will be LOW.
To use the HEF4028B as a BCO to octal decoder, input A3 is an active LOW enable pin and outputs Y8 and Y9 are not used. A 1-2-4 BCO code applied to inputs A0 to A2 causes the selected output (Y0 to Y7) to be HIGH. The other seven outputs will be LOW. When A3 is HIGH outputs (Y0 to Y7) will be forced LOW.
Fig 4.7 Logic diagram of 4028
When used as an 8-output (Y0 to Y7) inverting demultiplexer A0 to A2 are used as address inputs and A3 is the data input. Outputs Y8 and Y9 are not used.
It operates over a recommended VDD power supply range of 3 V to 15 V referenced to VSS (usually ground). Unused inputs must be connected to VDD, VSS, or another input. It is also suitable for use over the full industrial temperature range.
Fig 4.8 Pin diagram of 4028
4.5. 4047 IC:
The CD4047B is capable of operating in either the monostable or astable mode. It requires an external capacitor (between pins 1 and 3) and an external resistor (between pins 2 and 3) to determine the output pulse width in the monostable mode, and the output frequency in the astable mode.
Astable operation is enabled by a high level on the astable input or low level on the astable input. The output frequency (at 50% duty cycle) at Q and Q outputs is determined by the timing components. A frequency twice that of Q is available at the Oscillator Output; a 50% duty cycle is not guaranteed.
Fig 4.9 Pin diagram of 4047
Monostable operation is obtained when the device is triggered by LOW to HIGH transition at + triggers input or HIGH-to-LOW transition at − trigger input. The device can be retriggered by applying a simultaneous LOW-to-HIGH transition to both the + trigger and retrigger inputs. A high level on Reset input resets the outputs Q to LOW, Q to HIGH.
4.6. 1N4001:
A semiconductor diode is simply a P-N junction with connecting leads or terminals on the two sides of the P-N junction. A diode is a unidirectional device permitting the easy flow of current in direction but restraining the flow in other direction. Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).
When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.
Ordinary diodes can be split into two types: Signal diodes which pass small currents of 100mA or less and Rectifier diodes which can pass large currents. In addition there are LEDs and Zener diodes.
For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied.
Signal diodes are also used to protect transistors and ICs from the brief high voltage produced when a relay coil is switched off. The diagram shows how a protection diode is connected 'backwards' across the relay coil.
Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.
The arrow in the symbol of diode points in the direction of conventional current flow through the diode meaning that the diode will only conduct if a positive supply is connected to the Anode (A) terminal and a negative supply is connected to the Cathode (K) terminal thus only allowing current to flow through it in one direction only, acting more like a one way electrical valve, (Forward Biased Condition). However, we know that if we connect the external energy source in the other direction the diode will block any current flowing through it and instead will act like an open switch, in reverse biased mode as shown in Figure.7.
Fig 4.10 Diode in Forward and Reverse Biased Condition
The characteristics of a signal point contact diode are different for both germanium and silicon types and are given as: Germanium Signal Diodes - These have a low reverse resistance value giving a lower forward volt drop across the junction, typically only about 0.2-0.3v, but have a higher forward resistance value because of their small junction area. Silicon Signal Diodes - These have a very high value of reverse resistance and give a forward volt drop of about 0.6-0.7v across the junction. They have fairly low values of forward resistance giving them high peak values of forward current and reverse voltage. Signal Diodes are manufactured in a wide range of voltage and current ratings. There are bewildering arrays of static characteristics associated with the humble signal diode but the important ones are as follows; maximum forward current, peak inverse voltage and maximum operating temperature.
Fig 4.11 V/I Characteristics of diode
4.7. Resistors
A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm's law, V = IR. Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire. The primary characteristics of a resistor are the resistance, the tolerance, maximum working voltage and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance is determined by the design, materials and dimensions of the resistor.
4.7.1. Ohm's Law
The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law: V=IR. Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it where the constant of proportionality is the resistance (R). Equivalently, Ohm's law can be stated: V/R = I. This formulation of Ohm's law states that, when a voltage (V) is maintained across a resistance (R), a current (I) will flow through the resistance. For example, if V is 12 volts and R is 400 ohms, a current of 12 / 400 = 0.03 amperes will flow through the resistance R.
4.7.2. Power Dissipation
The power dissipated by a resistor (or the equivalent resistance of a resistor network) is calculated using the following: All three equations are equivalent. The first is derived from Joule's first law. Ohm’s Law derives the other two from that. The total amount of heat energy released is the integral of the power over time.
If the average power dissipated is more than the resistor can safely dissipate, the resistor may depart from its nominal resistance and may become damaged by overheating. Excessive power dissipation may raise the temperature of the resistor to a point where it burns out, which could cause a fire in adjacent components and materials. There are flameproof resistors that fail (open circuit) before they overheat dangerously. Note that the nominal power rating of a resistor is not the same as the power that it can safely dissipate in practical use.
4.7.3. Color Code
Four-band identification is the most commonly used color-coding scheme on resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The first three bands are equally spaced along the resistor; the spacing to the fourth band is wider. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits.Table 4.1 Colour code
4.8. Capacitors
A capacitor (formerly known as condenser) is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When a potential difference (voltage) exists across the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the conductors. The effect is greatest when there is a narrow separation between large areas of conductor; hence capacitor conductors are often called plates. An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce an equivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage.
A capacitor consists of two conductors separated by a non-conductive region. The non-conductive substance is called the dielectric medium, although this may also mean a vacuum or a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from an external electric field. The conductors thus contain equal and opposite charges on their facing surfaces and the dielectric contains an electric field. The capacitor is a reasonably general model for electric fields within electric circuits. An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of charge ±Q on each conductor to the voltage V between them: Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to vary. In this case, capacitance is defined in terms of incremental changes. The simplest capacitor consists of two parallel conductive plates separated by a dielectric with permittivity ε (such as air). The model may also be used to make qualitative predictions for other device geometries. The plates are considered to extend uniformly over an area A and a charge density ±ρ = ±Q/A exists on their surface. Assuming that the width of the plates is much greater than their separation d, the electric field near the centre of the device will be uniform with the magnitude E = ρ/ε. Capacitor is the fundamental component in any circuit.
4.9 Crystal
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits designed around them became known as crystal oscillators.
Quartz crystals are manufactured for frequencies from a few tens of kilohertz to tens of megahertz. More than two billion (2×109) crystals are manufactured annually. Most are small devices for consumer devices such as wristwatches, clocks, radios, computers, and cell phones. Quartz crystals are also found inside test and measurement equipment, such as counters, signal generators, and oscilloscopes.
Although crystal oscillators still most commonly use quartz crystals, devices using other materials are becoming more common, such as ceramic resonators. When a voltage source is applied to a small thin piece of crystal quartz, it begins to change shape producing a characteristic known as the Piezo-electric Effect. This piezo-electric effect is the property of a crystal by which an electrical charge produces a mechanical force by changing the shape of the crystal and vice versa, a mechanical force applied to the crystal produces an electrical charge. Then, piezo-electric devices can be classed as Transducers as they convert energy of one kind into energy of another. This piezo-electric effect produces mechanical vibrations or oscillations which are used to replace the LC tank circuit and can be seen in many different types of crystal substances with the most important of these for electronic circuits being the quartz minerals because of their greater mechanical strength.
4.10 Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. Microphones are used in many applications such as telephones, tape recorders, karaoke systems, hearing aids, motion picture production, live and recorded audio engineering, FRS radios, megaphones, in radio and television broadcasting and in computers for recording voice, speech recognition, and for non-acoustic purposes such as ultrasonic checking or knock sensors.
Fig 4.12 Microphone
Most microphones today use electromagnetic induction (dynamic microphone), capacitance change (condenser microphone), piezoelectric generation, or light modulation to produce an electrical voltage signal from mechanical vibration.
4.11 Relays
Here we use electromagnetic attraction type relays. Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by dc or ac quantities.
It consists of a laminated electromagnet M carrying a coil C and a pivoted laminated armature. The armature is balanced by a counter weight and carries a pair of spring at its free end. Under normal operating conditions, the current through the relay coil C is such that counter weight holds the armature in the position shown. However, when a short-circuit occurs, the current through relay coil increases sufficiently and the relay armature is attracted upwards.
Fig.4.13 Relay
CHAPTER 5
CIRCUIT WORKING
5.1 Block Diagram
 |
5.2 Circuit Diagram
Fig 5.1 Circuit Diagram
5.3 Working
Pressing a mobile phone button generates a so called dual-tone multiple- frequency, or DTMF code whose frequencies depend on which key is pressed. The circuit receives DTMF tones via its microphone and is set by the user to respond to a unique four-digit code using switches.
After amplification, the DTMF signal received at the microphone is directed to a DTMF-receiver, IC2, which converts it to a 4-bit binary format. Received digits are stored as four, 4-bit words in the 16-bit register formed by IC3, 4. The digits are converted to decimal using BCD-to-decimal-decoders IC‑5-8 .When a new digit is keyed, the previous ones are shifted forward. In the diagram, the last entered digit is in the left. If four digits in sequence match with the code set by switches, output from the AND gates IC10 goes high and triggers the monostable circuit, IC9.
Output from the IC9 goes high for a moment, adjustable via R10, and activates the lock relay via Tr1. Four nine-way DIL switches set the code. With the setting as shown, the relay is powered with digit combination 7398. Digit 0 is not used because the DTMF code for zero is not the same as the BCD code for zero.
Sensitivity of the microphone amplifier can be adjusted via R1. Maximum operating distance is about 20 cm. If there is a lot of ambient noise, it is best to set the phone’s speaker sound level to maximum.
CHAPTER 6
CONCLUSION
Hereby, we present a virtual model of the combination lock triggered by mobile phone. This land combination lock can be further improved to serve specific purposes. It requires only simple changes at the output end. There exist a grand scope for future expansion in increasing the operating range by developing and implementing high sensitivity microphones and by utilizing the already well established vast, wide and rigid mobile phone network, facilitating locking of electrical circuits and appliances from large distances in near future.
REFERENCES
1. ‘Digital Fundamentals’ by Thomas L Floyd, 3rd Edition, Universal Book Stall, New Delhi
- ‘Basic Electronics And Linear Circuits’ by N N Bhargava, S C Gupta, D C Kulshreshta, 46th reprint 2007, Tata McGraw-Hill Publishing Company Limited, New Delhi. Pages: 71-76.
3. http://www.wikipedia.org/wiki/dual-tone multi-frequency signaling
4. http://www.alldatasheet.com/4015
5. http://www.doctronics.co.uk/8870
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