Laptop Protector



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1. INTRODUCTION Laptop protector is a simple & compact electronic gadget which protects your valuable laptops against theft. This circuit works as a “miniature alarm generator”. It is sensitive to the position changes and effective one. This application is portable and comfortable for personal use. The circuit is fixed inside the laptop case, it will sound a loud alarm when someone tries to take the laptop. This highly sensitive circuit uses a homemade tilt switch to activate the alarm through tilting of the laptop case. In our project, we are introducing an electronic component called „mercury tilt twitch‟ which is handmade. It is a switch whose purpose is to allow or interrupt the flow electrical current in an electrical circuit in a manner that is dependent on the switch‟s physical position or alignment relative to the direction of the “pull” of earth‟s gravity, or other inertia. Generally we can use water tilt switch which is cheaper but due to low density and less conductivity the circuit efficiency may degrade .For that we are moved to mercury tilt switch. Mercury (Hg) is the only metal that is liquid at room temperatures. Because of its unreactivity, fluidity, high density, and high electrical conductivity, it finds many uses in scientific & electrical apparatus. As the electrical resistivity of mercury is very small ie., 961nΩ·m at 25 °C, it is fully supported the requirement of circuit. All the hardware components used in this project are passive and active components , so the excitation source of 12V DC makes them work. The excitation is employed by the battery source along with a 12V regulator to provide a constant DC voltage to the devices. As a whole the aim of our project is to develop a efficient circuit that can provide protection for your valuable laptops against theft. 1 2. CIRCUIT DESIGN 2.1. Block diagram: Figure.2.1.1: Block diagram of Laptop protector 2.1.1. DESCRIPTION: The block diagram consists of supply line. The monostable, five main blocks transistor switch, 5 main blocks along with the are tilt switch, comparator, is mercury switch piezo buzzer. The nothing but a tilt switch which works as a sensor switch. The change 2 in position causes switch to open (OFF) or to close(ON). This switch is sensitive to gravity. the movements given by the human and also earth‟s The IC TL071 acts as a comparator, which is LOW-NOISE JFETINPUT OPERATIONAL AMPLIFIER. The comparator circuit will compares the two inputs that is one from supply and other is the one of mercury switch terminal. If supply terminal is high than the other then the comparator drives the next block of the circuit. The IC CD4538 works as a monostable circuit, in which one of the states is stable, but the other state is unstable (transient). A trigger causes the circuit to enter the unstable state. After entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot. Here we are considering a large period pulse to run the buzzer. This monostable is triggered by the comparator circuit. The circuit the BC557 is a NPN transistor which acts as a switch in the supply and the buzzer. Whose base terminal is between and connected to the output of monostable circuit , emitter is connected to supply collector is given to the buzzer. As The small base and The current controls the larger collector current, the switch is closed When a small current flows into the base (B) of the transistor transistor amplifies this small current to through from its collector (C) to its emitter (E).Similarly collector current. allow a larger current to flow the switch is open When no base current flows, so the transistor switches off the 3 The last essential part of the circuit is the Piezo buzzer, acts as an “impact sensor”. A piezo buzzer is nothing more than a piezo crystal trapped between two metal plates. It works due to the piezoelectric effect. When an electric potential is applied to a piezo crystal, it will deform. The larger the potential, the larger the deformation.The reverse is also true, when a piezo crystal is deformed, it will generate electricity.The remaining blocks such as SPST switch, regulator, batteries are to maintain the proper DC voltage flow to the circuitry. 2.2 CIRCUIT DIAGRAM DESCRIPTION: Figure.2.2.1: Circuit diagram of Laptop protector 4 The circuit uses readily available components and can be assembled on a small piece of Vero board or a general- purpose PCB. It is powered by a 12V miniature battery used in remote control devices. IC TLO71 (IC1) is used as a voltage comparator with a potential divider comprising R2 and R3 providing half supply voltage at the non-inverting input (pin 2) of IC1. The inverting input receives a higher voltage through a mercury-activated tilt switch only when the probes in the tilt switch make contact with mercury. When the tilt switch is kept in the vertical position, the inverting input of IC1 gets a higher voltage than its non-inverting input and the output remains low. IC CD4538 (IC2) is used as a monostable with timing elements R5 and C1. With the shown values, the output of IC2 remains low for a period of three minutes. CD4538 is a precision monostable multivibrator free from false triggering and is more reliable than the popular timer IC 555. Its output becomes high when power is switched on and it becomes low when the trigger input (pin 5) gets a low-to high transition pulse. 2.3 OPERATION OF THE CIRCUIT: The unit is fixed inside the laptop case in horizontal position. In this position, mercury inside the tilt switch effectively shorts the contacts, so the output of IC1 remains low. The alarm generator remains silent in the standby mode as trigger pin 5 of IC2 is low. When someone tries to take the laptop case, the unit takes the vertical position and the tilt switch breaks the electrical contact between the probes. Immediately the output of IC1 becomes high and monostable IC2 is triggered. The low output from IC2 triggers the npn transistor (T1) and the buzzer starts beeping.Until we reset the S1 switch the buzzer will rings continuously. 5 2.3.1 DESIGN CONSIDERATIONS: Assemble the circuit as compactly as possible so as to make the unit matchbox size. Make the tilt switch using a small (2.5cm long and 1cm wide) plastic bottle with two stainless pins as contacts. Fill two-third of the bottle with water such that the contacts never make electrical path when the tilt switch is in vertical position. Make the bottle leak proof with adhesive or wax. Fix the tilt switch inside the enclosure of the circuit in horizontal position. Fit the unit inside the laptop case in horizontal position using adhesive. While using the mercury ,we should take care of because of its high fluidity. Once its falls down the mercury becomes impure and its conductivity will be reduced. Keep the fluid in non conducting containers such as wood, plastic, glass etc., The container‟s lid should be tight, to avoid the leakage of fluid out. Make sure that contact probes in the tilt switch are not in contact with each other. For the good results and reliability of the circuit, keep the complete circuit in a flat container which is not sound proof. 2.4 IC TL071: DESCRIPTION: The single JFET and TL071, TL071A and TL071B are high speed JFET input operational amplifiers incorporating well matched, high voltage bipolar transistors in a monolithic integrated circuit. The 6 device features high slew rates, low input bias and offset currents, and low offset voltage temperature coefficient. The low harmonic distortion and low noise make the TL07x series ideally suited for high- fidelity and audio preamplifier applications. Each amplifier features JFET inputs (for high input impedance) coupled with bipolar output stages integrated on a single monolithic chip. FEATURES:            Low Power Consumption Wide Common-Mode and Differential Voltage Ranges Low Input Bias and Offset Currents Output Short-Circuit Protection Low Total Harmonic Distortion 0.003% Typ Low Noise Vn = 18 nV/√Hz Typ at f = 1 kHz High Input Impedance . . . JFET Input Stage Internal Frequency Compensation Latch-Up-Free Operation High Slew Rate . . . 13 V/μs Typ Common-Mode Input Voltage Range Includes VCC+ SYMBOL: 7 PIN DIAGRAM: Figure 2.4.1: pin diagram of TL071 PIN FUNCTIONS: Pin 1 (Offset Null): Offset nulling, Since the op-amp is the differential type, input offset voltage must be controlled so as to minimize offset. Offset voltage is nulled by application of a voltage of opposite polarity to the offset. An offset null- adjustment potentiometer may be used to compensate for offset may voltage. The cause an null-offset potentiometer the also null compensates potentiometer for is irregularities in the operational amplifier manufacturing process which offset. Consequently, recommended. Pin 2 (Inverted Input): All input signals at this pin will be inverted at output pin 6. Pins 2 and 3 are very important (obviously) to get the correct input signals or the op amp cannot do its work. 8 Pin 3 (Non-Inverted Input): All input signals at this pin will be processed normally without invertion. The rest is the same as pin 2. Pin 4 (-V): The V- pin (also referred to as Vss) is the negative supply voltage terminal. Supply-voltage operating range for the 741 is -4.5 volts (minimum) to -18 volts (max), and it is specified for operation between -5 and -15 Vdc. The device will operate essentially the same over this range of voltages without change in timing period. Sensitivity of time interval to supply voltage change is low, typically 0.1% per volt. (Note: Do not confuse the -V with ground). Pin 5 (Offset Null): Same as pin 1 offset nulling mentioned above. Pin 6 (Output): Output signal´s polarity will be the opposite of the input´s when this signal is applied to the op-amp´s inverting input. For example, a sine-wave at the inverting input will output a square-wave in the case of an inverting comparator circuit. Pin 7 (+V): The V+ pin (also referred to as Vcc) is the positive supply voltage terminal of the 741 Op-Amp IC. Supply-voltage operating range for the 741 is +4.5 volts (minimum) to +18 volts (maximum), and it is specified for operation between +5 and +15 Vdc. The device will operate essentially the same over this range of voltages without change in timing period. Actually, the most significant operational difference is the output drive capability, which increases for both current and voltage range as the supply voltage is increased. Sensitivity of time interval to supply voltage change is low, typically 0.1% per volt. 9 Pin 8 (N/C): The ´N/C´ stands for ´Not Connected´. There is no other explanation. There is nothing connected to this pin, it is just there to make it a standard 8-pin package. ABSOLUTE MAXIMUM RATINGS: Table 2.4: absolute maximum ratings of TL071 ORDER CODES: NOTE: 10 1. All voltage values, except differential voltages, are with respect to the midpoint between VCC+ and VCC−. 2. Differential voltages are at IN+, with respect to IN−. 3. The magnitude of the input voltage must never exceed the magnitude of the supply voltage or 15 V, whichever is less. 4. The output may be shorted must to be ground limited or to to either supply. that the Temperature and / or supply Dissipation rating is not exceeded. 5. Maximum power dissipation is a function of TJ(max), θJA, and TA. The maximum allowable power dissipation at any allowable ambient temperature is PD = (TJ(max) − TA)/θJA. Operating at the absolute maximum TJ of 150°C can affect reliability. 6. The package thermal impedance is calculated in accordance with JESD51-7. 7. Maximum power dissipation is a function of TJ(max), θJC, and TC. The maximum allowable power dissipation at any allowable case temperature is PD = (TJ(max) − TC)/θJC. Operating at the absolute maximum TJ of 150°C can affect reliability 8. The package thermal impedance is calculated in accordance with MILSTD- 883. voltages ensure 11 2.4.1 SCHEMATIC DIAGRAM: Figure 2.4.2: Schematic diagram of ICTL071 Basically the internal structure of op-amp consists of three stages: 1. Differential amplifier – provides low noise amplification, high input impedance, usually a differential output. 12 2. Voltage amplifier – provides high voltage gain, a single-pole frequency roll-off, usually single-ended output. 3. Output amplifier – provides high current driving capability, low output impedance, current limiting and short circuit protection circuitry. The above schematic diagram refer to the terminal pin outs for the 8-pin IC package. The pin numbers are the same for both the 8pin mini-DIP package and the 8-pin round Type-T metal can. In both cases, pin 8 has no connection. There are a number of interesting points about this circuit. First, the input transistors are connected as npn emitter followers, feeding their outputs directly to a pair of pnp transistors isolates configured the as common-base amplifiers. This configuration inputs, preventing signal feedback that might otherwise have some harmful frequency-dependent effects. Note the two pairs of transistors shown in red. One transistor in each pair has its collector connected to its base, as well as to the base of the other transistor. In addition, the transistor emitters are connected together, in this case to the V+ power source. In some diagrams, the transistor with the collector and base shorted together is rendered as a diode, which shows bias for the other transistor, but doesn't show the full value of this configuration. This arrangement is known as a current mirror. The two transistors are manufactured side by side on the same silicon die, at the same time. Thus, they have essentially identical characteristics. The controlling transistor (on the left in each pair) will necessarily set its emitter-base voltage to exactly that value that will sustain the collector 13 current it is carrying, even down to fractions of a millivolt. In so doing, it also sets the emitter-base voltage of the second transistor to the same value. Since the transistors are essentially identical, the second transistor will carry exactly the same current as the first, even to an independent circuit. The use of a current mirror on the input circuit allows the inputs to accommodate large common-mode voltage swings without exceeding the active range of any transistor in the circuit. The second current mirror in red provides a constant-current active load for the output circuitry, again without regard for the actual output voltage. A third current mirror, shown in blue, is a bit different. That 5K resistor in series with the emitter of the mirrored transistor limits its collector current to virtually to the nothing. Thus, negative it serves as a higha impedance connection power supply, providing reference without loading the input circuit. This particular circuit is therefore able to transistors to provide the slight base bias current needed for the correctly over a wide common-mode input PNP transistors in the differential input circuit, while allowing those operate voltage range. APPLICATIONS: ICTL071 has involved in many applications such as:   audio and video pre-amplifiers and buffers voltage comparators 14                 differential amplifiers differentiators and integrators filters precision rectifiers voltage regulator and current regulator analog-to-digital converter digital-to-analog converter voltage clamps oscillators and waveform generators Schmitt trigger Gyrator Comparator Active filter Analog computer Capacitance multiplier Charge amplifier 2.4.2 ICTLO71 AS A COMPARATOR: 15 Figure 2.4.3: Comparator circuit The comparator is a device that is designed to be used without negative feedback (and often with positive feedback), so its output is always either at its maximum value, or its minimum value. In other words the output is digital, either logic 1 (high) or logic 0 (low). For this device we will use V+=5V(available on the top row of your breadboard) and V-=ground. The circuit compares the two inputs. If (input 2) is greater than IN- (input 3), the output is IN+ high, IF IN- is greater than IN+ the output is low (ground, GND). However, what makes this device a little hard to understand (but very useful), a high output is characterized by the output appearing as an open circuit (no current in or out), and a low output is characterized by zero voltage (a short circuit to pin 1, which is typically connected to ground). You can think of the output as a switch connected to ground -- for a low output the switch is closed (shorted to ground (i.e., pin 1)), for a high output the switch is open (the output voltage can float). What makes it useful is that we can use this to switch high voltages (for motors, lights...) on the output from low voltages on the input. 16 2.5 IC CD4538 Dual Precision Monostable: DESCRIPTION: The CD4538BC is a dual, precision monostable multivibrator with independent trigger and reset controls. The device is retriggerable and resettable, and the control inputs are internally latched. Two trigger inputs are provided to allow either rising or falling edge triggering. The reset inputs are active LOW and prevent triggering while active. Precise control of output pulse-width has been achieved using linear CMOS techniques. The pulse duration and accuracy are determined by external components RX and CX. The device does not allow the timing capacitor to discharge through the timing pin on power-down condition. For this reason, no external protection pins. resistor is required in series with the timing pin. Input protection from static discharge is provided on all FEATURES:           Wide supply voltage range: 3.0V to 15V High noise immunity: 0.45 VCC (typ.) Low power TTL compatibility: Fan out of 2 driving 74Lor 1 driving 74LS New formula: PWOUT = RC (PW in seconds, R in Ohms,C in Farads) ±1.0% pulse-width variation from part to part (typ.) Wide pulse-width range: 1 ms to ∞ Separate latched reset inputs Symmetrical output sink and source capability Low standby current: 5 nA (typ.) @ 5 VDC Pin compatible to CD4528BC 17 LOGIC SYMBOL: Figure 2.5.1: Logic diagram of IC CD4538 PIN DIAGRAM: 18 Figure 2.5.2: pin diagram of IC CD4538 TRUTH TABLE: 19 Table 2.5: Truth table of IC CD4538 PIN DESCRIPTION: INPUTS A1, A2 (Pins 4, 12): Positive−edge trigger inputs. A rising−edge signal on either of these pins triggers the corresponding multivibrator when there is a high level on the B1 or B2 input. B1, B2 (Pins 5, 11): 20 Negative−edge trigger inputs. A falling−edge signal on either of these pins triggers the corresponding multivibrator when there is a low level on the A1 or A2 input. Reset 1, Reset 2 (Pins 3, 13): Reset inputs (active low). When a low level is applied to one of these pins, the output of the corresponding multivibrator is reset to a low level and the Q output is set to a high level. CX1/RX1 and CX2/RX2 (Pins 2 and 14): External timing components. These pins are tied to the common points of the external timing resistors and capacitors (see the Block Diagram). Polystyrene capacitors are recommended for optimum pulse width control.Electrolytic capacitors are not recommended due to high leakages associated with these type capacitors. GND (Pins 1 and 15): External ground. The external timing capacitors discharge to ground through these pins. OUTPUTS Q1, Q2 (Pins 6, 10): Noninverted monostable outputs. These pins (normally low) pulse high when the multivibrator is triggered at either the A or the B input. The width of the pulse is determined by the external timing components, RX and CX. Q1, Q2 (Pins 7, 9): 21 Inverted monostable outputs. These pins (normally high) pulse low when the multivibrator is triggered at either the A or the B input. These outputs are the inverse of Q1 and Q2. ABSOLUTE MAXIMUM RATINGS: DC Supply Voltage (VDD) -0.5 to +18 VDC Input Voltage (VIN) -0.5V to VDD + 0.5 VDC Storage Temperature Range (TS) -65°C to +150°C Power Dissipation (PD) Dual-In-Line 700 mW Small Outline 500 mW Lead Temperature (TL) (Soldering, 10 seconds) 260°C         2.5.1THEORY OF OPERATION: 22 Figure 2.5.3: Waves forms of monostable Trigger Operation: As shown in Figure 2.5.2 and Figure 2.5.3, before an input trigger occurs, the monostable is in the quiescent state with the Q output low, and the timing capacitor CX completely charged to VDD. When the trigger input A goes from VSS to VDD (while inputs B and CD are held to VDD) a valid trigger is recognized, which turns on comparator C1 and N-Channel transistor N1(1). At the same time the output latch is set. With VSS transistor until N1 on, the is capacitor this CX rapidly the discharges output of toward VREF1 reached. At point comparator C1 changes state and transistor N1 turns off. Comparator C1 then turns off while at the same time comparator C2 turns on. With transistor N1 off, the capacitor CX begins to charge through the timing resistor, RX, toward VDD. When the voltage across CX equals VREF2, comparator C2 changes state causing the output latch to reset (Q goes low) while at the same time disabling comparator C2. This ends the timing cycle with the monostable in t he quiescent state, waiting for the next trigger. A valid trigger is also recognized when trigger input B goes from VDD to VSS (while input A is at VSS and input CD is at VDD)(2). It should be noted that in the quiescent state CX is fully charged to VDD, causing the current through resistor RX to be zero. Both comparators are “off” with the total device current due only to reverse junction leakages. An added feature of the CD4538BC is that the output latch is set via the input trigger without capacitor waveform. voltage. Thus, propagation delay from regard to the to Q is trigger independent of the value of CX, RX, or the duty cycle of the input 23 Retrigger Operation: The CD4538BC is retriggered if a valid trigger occurs(3) followed by another valid trigger(4) before the Q output has returned to the quiescent (zero) state. Any retrigger, after the timing node voltage at pin 2 or 14 has begun to rise from VREF1, but has not yet reached VREF2, will cause an increase in output pulse width T. When a valid retrigger is initiated (4), the voltage at T2 will again drop to VREF1 before progressing along the RC charging curve toward VDD. The Q output will remain high until time T, after the last valid retrigger. Reset Operation: The CD4538BC may be reset during the generation of the output pulse. In the reset mode of operation, an input pulse on CD sets the reset latch and causes the capacitor to be fast charged to VDD by turning on transistor Q1(5). When the voltage on the capacitor reaches VREF2, the reset latch will clear and then be ready to accept another pulse. If the CD input is held low, any trigger inputs that occur will be inhibited and the Q and Q outputs of the output latch will not change. Since the Q output is reset when an input low level is detected on the CD input, the output pulse T can be made significantly shorter than the minimum pulse widths specification. 2.5.2 PRECISION MONOSTABLE CIRCUIT DIAGRAM: 24 Figure 2.5.4: Circuit diagram of precision monostable CD 4538 is the Precision Monostable Multivibrator IC that is free from False triggering. It is more reliable than the popular timer IC 555. Here the IC is wired as a short duration monostable timer using R1 and C1 as timing components. With the given values, output of IC1 remains low for three minutes. By changing the value of C1 or R1 various time intervals can be obtained. Unlike 555 IC in the monostable mode, here in CD4530, output of IC becomes high at power on and becomes low when the trigger pin5 gets a low-to-high transition pulse. When S1 is pressed, the high going pulse triggers IC and its output goes low. This drives the load through the PNP transistor T1. Load can be an LED, Buzzer etc. 25 2.6 SPST SWITCH: In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. The most familiar form with one of or switch more sets is mly operated electromechanical device of electrical contacts. Each set of contacts can be in one of two states: either 'closed' meaning the contacts are touching and electricity can flow between them, or 'open', meaning the contacts are separated and the switch is nonconducting. The mechanism actuating the transition between these two states (open or closed) is normally of the toggle (flip switch for continuous "on" or "off") or momentary (push-for "on" or push-for "off") type. Through the use of logic gates, momentary switches can also activate timed-activation circuits. Selecting a Switch: There are three important features to consider when selecting a switch:    Contacts (e.g. single pole, double throw) Ratings (maximum voltage and current) Method of Operation (toggle, slide, key etc.) Switch contacts: Several terms are used to describe switch contacts:     Pole - number of switch contact sets. Throw - number of conducting positions, single or double. Way - number of conducting positions, three or more. Momentary - switch returns to its normal position when released. 26   Open - off position, contacts not conducting. Closed - on position, contacts conducting, there may be several on positions. For example: the simplest on-off switch has one set of contacts (single pole) and one switching position which conducts (single throw). The switch mechanism has two positions: open (off) and closed (on), but it is called 'single throw' because only one position conducts. Single Pole, Single Throw = SPST It is a simple on-off switch. This type can be used to switch the power supply to a circuit. When used with mains electricity this type of switch must be in the live wire, but it is better to use a DPST switch to isolate both live and neutral. CIRCUIT SYMBOL: FUNCTIONAL BLOCK DIAGRAM: 27 Figure 2.6.1: Block diagram of SPST APPLICATIONS:        Audio and Video Switching RF Switching Networking Applications Battery Powered Systems Communication Systems Relay Replacement Sample-and-Hold Systems 2.7 L7812CV POSITIVE VOLTAGE REGULATOR: DESCRIPTION: The L7800 series of three-terminal positive regulators is available in TO-220, TO-220FP, TO-220FM, TO-3 and D2 PAK packages and several fixed output voltages, making it useful in a wide range of applications. These regulators can provide local on-card regulation, eliminating the distribution problems associated with single point regulation. Each type employs internal current limiting, thermal shut-down and safe area protection, making it essentially indestructible. If adequate heat sinking is provided, they can deliver over 1A output current. Although designed 28 primarily as fixed voltage regulators, these devices can be used with external components to obtain adjustable voltage and currents. FEATURES:      Output current to 1.5A. Output voltages of 5; 5.2; 6; 8; 8.5; 9;10; 12; 15; 18; 24V. Thermal overload protection. Short circuit protection. Output transition SOA protection. CONNECTION DIAGRAM: Figure 2.7.1: connection diagram of L7812CV BLOCK DIAGRAM: 29 Figure 2.7.2: schematic diagram of IC L7812CV A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feedforward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages. Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. In automobile alternators and central power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line. 30 TILT SWITCHES Tilt switches are used to sense movement (tilt) of a device above and below a horizontal axis. A typical use is in a thermostat. A glass mercury switch is mounted to a bi-metallic spring which expands and contracts with temperature. As the spring moves, the switch contacts pass through the horizontal plane, opening or closing to operate the furnace or boiler. The angle through which the switch must move for proper operation – the differential angle – is measured from the point of just make to just break; it is specified as a maximum. When selecting a tilt switch, it is important to ensure that the operating mechanism can move the switch through an angle greater than the differential angle. Some additional applications include: level controls, appliances, security alarm systems, toys and games, float switches and water-treatment equipment (non-mercury) . CONDUCTIVITY OF MERCURY: Mercury (hg)is the only common temperatures. metal that is liquid at room The electrical conductivity of mercury has been measured at fourteen temperatures between 800 and 1700°C and at pressures between 1200 and 2100 bar. From the discontinuities of the conductance observed with increasing temperature at constant pressures, the vapor-pressure curve beyond 800°C has been derived; it has a critical point atTc=1490±15 °C and pc=1510±30 bars. The specific conductivity of liquid mercury in the lower temperature range increases only slightly with pressure (at 0°C from 1.06×104 to 1.14×104 Ω-1 cm-1 between 1 and 2100 bar; at 1200°C from 1.6×103 to 2.4×103 Ω-1 cm-1 between 610 and 2100 bar). Beyond 1200°C the conductivity becomes strongly pressure-dependent. At 1520°C the specific conductivity of supercritical mercury increases continuously by more than 4 31 orders of magnitude from 10-2 Ω-1 cm-1 to 5×102 Ω-1 cm-1 if it is compressed from 1580 to 2100 bar. At higher supercritical temperatures the behavior is similar, although the increase of the conductance with pressure becomes less steep. It follows that supercritical gaseous mercury exhibits metallic conductance if compressed to sufficiently high density. 3.1 MERCURY SWITCH: A mercury switch (also known as a mercury tilt switch) is a switch whose purpose is to allow or interrupt the flow of electric current in an electrical circuit in a manner that is dependent on the switch's physical position or alignment relative to the direction of the "pull" of earth's gravity, or other inertia. Mercury switches consist of one or more sets of electrical contacts in a sealed glass envelope which contains a bead of mercury. The envelope may also contain air, an inert gas, or a vacuum. Gravity is constantly pulling the drop of mercury to the lowest point in the envelope. When the switch is tilted in the appropriate direction, the mercury touches a set of contacts, thus completing the electrical circuit through those contacts. Tilting the switch the opposite direction causes the mercury to move away from that set of contacts, thus breaking that circuit. The switch may contain multiple sets of contacts, closing different sets at different angles allowing, for example, SinglePole, Double-Throw (SPDT) operation. 32 Figure 3.1: A Single-Pole, Single-Throw (SPST) mercury switch 3.2 TIP -OVER SWITCHES: These are also one type of tilt switches. Tip-over switches sense tilt over 360° of a vertical axis. A common use is in portable heaters used in the home to prevent electrical shock or fire. When the heater tilts more than a specified angle, the switch operates turning the heater off. This angle, called the operating angle, describes the angle from vertical to the point of contact operation, subject to a tolerance, ie. 30° ± 10° (35° to 55°). Both normally closed (tilt to open) and normally open (tilt to close) switches are available. Some additional applications include: portable lamps, PC anti-theft devices, vending-machine alarms and machinery security systems WORKING POSITION OF TIP-OVER SWITCH: 33 Tip-Over Switches operate when the switch is tilted from the vertical position. The angle through which the switch has to move before operating is called the operating angle. These switches are omni-directional ADVANTAGES: Advantages of the mercury switch over other types are that the contacts are enclosed, so oxidation of the contact points is unlikely; in hazardous locations, interrupting the circuit will not emit a spark that can ignite flammable gasses. Contacts stay clean, and even if an internal arc is produced, the contact surfaces are renewed on every operation, so the contacts don't wear out. Even a small drop of mercury has a low resistance, so switches can carry useful amounts of current in a small size. The sensitivity of the drop to gravity provides a unique sensing function, and lends itself to simple, low-force mechanisms for manual or automatic operation. The switches are quiet as there are no contacts that abruptly snap together. The mass of the moving mercury drop can provide an "over center" effect to avoid chattering as the switch is tilted. Multiple contacts can be included in the envelope for two or more circuits. DISADVANTAGES: Disadvantages when compared with other types include: Mercury switches have a relatively slow operating rate due to the inertia of the mercury drop, so they are not used when many operating cycles are required per second.[3] Mercury switches are sensitive to gravity so may be unsuitable in portable or mobile devices that can change orientation or that vibrate. Mercury compounds are highly toxic and accumulate in any food chain, so 34 mercury is not permitted in many new designs. Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damaging the envelope. The mercury drop forms a common electrode, so circuits are not reliably isolated from each other if a multipole switch is used. APPLICATIONS: Roll sensing: Tilt switches may be used for a rollover or tip over warning for construction equipment and lift vehicles operating in rugged off-highway terrain. There are several non-mercury types but few are implemented due to sensitivity to shock and vibration - causing false tripping. However devices resistant to this do exist. Automotive uses: Mercury switches were used in automobiles for lighting controls (for example, trunk lid lights), ride control, and Antilock braking control systems. Scrapped automobiles can leak mercury to the environment if these switches are not properly removed. These uses have been discontinued in new American-built cars since 2003. Fall alarms: Work performed in confined space (such as a welder inside a tank) has special labor safety requirements. Tilt switches are used to sound an alarm if a worker falls over. Thermostats: Mercury switches were commonly used in bimetal thermostats. The weight of the movable mercury drop provided some hysteresis by moving the bimetal spring slightly beyond the point it would normally assume, thereby holding the thermostat off slightly longer before flipping to the on state and then 35 holding the thermostat on slightly longer before flipping back to the off state. The mercury also provided a very positive on/off switching action and could withstand millions of cycles without degradation of the contacts. Vending: Mercury switches are still used in mechanical systems that are controlled electrically where the physical orientation of actuators or rotors is a factor. They are also commonly used in vending machines that have 'tilt alarms'. When the machine is rocked or tilted in an attempt to gain a product, the mercury switch activates, sounding an alarm. Bombs: A tilt switch can be used to trigger a bomb. Mercury tilt switches can be found in some bomb and landmine fuzes, typically in the form of antihandling devices, for example, a variant of the VS-50 mine. These sturdy and reliable tilt switches use a solid state precision tilt sensor at the heart of this tilt angle warning system. One of main applications is for the prevention of vehicle rollover. Vehicle rollover is a problem that is faced by many industrial, agricultural and commercial vehicles, such as ride on mowers, tractors, telescopic handlers, telescopic and scissor lift platforms and all types of cranes. 36 SOLDERING Soldering is “ a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the work piece”. In the soldering process, heat is applied to the parts to be joined, causing the solder to melt and to bond to the work pieces in an alloying process called „wetting‟. In stranded wire, the solder is drawn up into the wire by capillary action in a process called wicking. Capillary action also takes place when the workpieces are very close together or touching. The joint strength is dependent on the filler metal used, where soft solder is the weakest and the brass alloy used for brazing is the strongest. 4.1 TOOLS REQUIRED FOR SOLDERING: Soldering iron: For electronics work the best type is one powered by mains electricity (230V in the UK), it should have a heatproof cable for safety. The iron's power rating should be 15 to 25W and it should be fitted with a small bit of 2 to 3mm diameter. Soldering iron stand: It must a safe place to put the iron when you are not holding it. The stand should include a sponge which can be dampened for cleaning the tip of the iron. 37 Desoldering pump(solder sucker): A tool for removing solder when desoldering a joint to correct a mistake or replace a component. Real of solder: The best size for electronics is 22swg (swg = standard wire gauge). Side cutter: For trimming component leads close to the circuit board. Wire stripper: Most designs include a cutter as well, but they are not suitable for trimming component leads. Small plier: Usually called 'snipe nose' pliers, these are for bending component leads etc. If you put a strong rubber band across the handles the pliers make a convenient holder for parts such as switches while you solder the contacts. Heat sink: You can buy a special tool, but a standard crocodile clip works just as well and is cheaper. Track cutter: A 3mm drill bit can be used instead, in fact the tool is usually just a 3mm drill bit with a proper handle fitted. Small electric drill: Ideally this should be mounted in a drill stand. You will need a range of small drill bits, but for most holes a 1mm bit is suitable. Larger holes can be drilled with a hand drill but 1mm bits are too fragile to use reliably in a hand drill. 4.2 PREPARING THE SOLDERING IRON: 38  Place the soldering iron in its stand and plug in. The iron will take a few minutes to reach its operating temperature of about 400°C.  Dampen the sponge in the stand. The best way to do this is to lift it out the stand and hold it under a cold tap for a moment, then squeeze to remove excess water. It should be damp, not dripping wet.  Wait Wipe Melt a few the a minutes tip of for the the soldering on the iron the to damp warm up. You can check if it is ready by trying to melt a little solder on the tip.  iron on sponge. iron. This will clean the tip.  little solder tip of the This is called 'tinning' and it will help the heat to flow from the iron's tip to the joint. It only needs to be done when you plug in the iron, and occasionally while soldering if you need to wipe the tip clean on the sponge. 4.3 SOLDERING INSTRUCTIONS:  Soldering is a method of uniting two metallic surfaces by means of a fusible alloy, solder. The solder has of a lower melting point than the metals that are joined. A solder joint when completed will be as strong or stronger than the metals it connects. There are 5 sequences to accurate soldering. Each is important. The steps in sequence are: 1. Fitting- making a tight even joint between surfaces to be soldered. 2. Cleaning- removal of all surface films (grease, oil or oxides). 3. Fluxing- applying flux to all areas in sufficient amounts to prevent oxidation during heating.     39  4. Solder placement- placing the solder in the right places and in the right amounts. 5. Heating- in a manner which quickly and safely causes solder to melt and flow. Soldering alloys are applied to the joining surfaces with a flux. Flux is a substance which helps the fusing of the metals by keeping the surfaces clean and preventing any kind of oxide from forming during the heating process. One cannot solder without flux. It is important that it be well applied to the surfaces to be joined as well as to the solder itself. A liquid flux is the easiest form of flux to work with. Clean and brightly polished surfaces will permit better soldering results. The first rule in soldering is to clean the surfaces to be soldered. One cannot say this too often because when a soldering operation fails, the reason will most often be that the surfaces had a residue of grease from the fingers or oxidization tarnish was not totally removed. A clean surface is bright and allows the flux to spread evenly without forming drops and pools. Never try to solder without first rubbing the surface with a fine steel wool. If oxidation is evident (from dipping or brushing on an oxidizing bath) it must be cleaned off too. It is always worth spending an extra few moments doing this no matter how clean the surfaces appear to be. The same is true of the solder itself. Never cut pieces of solder from a strip without first cleaning the surfaces with emery paper or steel wool. To cut small pieces of solder, flatten the end of the strip with a hammer on a steel block. With a gate cutter cut into the end of the solder strip. Then cut at right angles and small pieces will fall away. The solder is cut into pieces in order to control the amount of solder applied. One can change the size of the pieces by varying the distances between the cuts. These pieces should be applied to the fluxed joints by means of a small paint brush which has been dipped into the flux. The flux from the brush will now transfer itself to the piece of solder. Always, use the brush for all fluxing operations. It is important that all surfaces to be soldered and      40 solder are completely covered with flux. Flux is applied to the metal parts to keep the metal clean and prevent oxides from forming. The melting point of the solder must be below that of the material which is being soldered. The flowing properties of the solder are in the solder and not in the flux.  The question of how much solder is needed for a specific joint is not an easy one to answer. There must be sufficient solder present for it to flow through the joint so that it can just be seen at the edges. Solder should not be used as a filler and should not form large blobs at the joints. Areas to be soldered are to be closely fitted by filing. Solder will not fill an irregular joint satisfactorily. The solder joint, if fitted properly,will be strongest where the least amount of solder flows through an area of contact between the two metals. Solder will flow freely into the smallest carefully fitted joint, alloy with the metal and freeze smooth. Small parts can be soldered together without clamps but practice doing this is required. Make sure both surfaces to be joined are clean and wellfluxed. Have your soldering iron tip well tinned (it should be shinny all over with no black blobs of oxides on it) with enough solder on it that it is just about to drip off. While holding the pieces together touch the corner of the chisel tip of the soldering iron to the joint. Excess solder can be trimmed off. You will find the use of the Helping Hands tool with the magnifier quite useful. The technique of soldering is like any art form, practice and more practice is the best teacher.   4.4 SOLDERING ADVICE TO COMPONENTS: It is very tempting to start soldering components onto the circuit board straight away, but please take time to identify all the parts first. 1. Stick all the components onto a sheet of paper using sticky tape. 2. Identify each component and write its name or value beside it. 41 3. Add the code (R1, R2, C1 etc.) if necessary. Many projects from books and magazines label the components with codes (R1, R2, C1, D1 etc.) and you should use the project's parts list to find these codes if they are given. 4. Resistor values can be found using the resistor colour code which is explained on our Resistors page. You can print out and make your own Resistor Colour Code Calculator to help you. 5. Capacitor values can be difficult to find because there are many types with different labelling systems collect them. 4.5SAFETY PRECAUTIONS:  Never touch the element or tip of the soldering iron. They are very hot (about 400°C) and will give you a nasty burn.  Take great care to avoid touching the mains flex with the tip of the iron. The iron should have a heatproof flex for extra protection. An ordinary plastic flex will melt immediately if touched by a hot iron and there is a serious risk of burns and electric shock. Always return the soldering iron to its stand when not in use. Never put it down on your workbench, even for a moment! Work in a well-ventilated area.   The smoke formed as you melt solder is mostly from the flux and quite irritating. Avoid breathing it by keeping you head to the side of, not above, your work.  Wash your hands after using solder. Solder contains lead which is a poisonous metal. 4.6 DESOLDERING: 42 At some stage you will probably need to desolder a joint to remove or reposition a wire or component. There are two ways to remove the solder: 1. With a desoldering pump (solder sucker)   Set the pump by pushing the spring-loaded plunger down until it locks. Apply both the pump nozzle and the tip of your soldering iron to the joint. Wait a second or two for the solder to melt. Then press the button on the pump to release the plunger and suck the molten solder into the tool. Repeat if necessary to remove as much solder as possible. The pump will need emptying occasionally by unscrewing the nozzle     2. With solder remover wick (copper braid)  Apply both the end of the wick and the tip of your soldering iron to the joint. As the solder melts most of it will flow onto the wick, away from the joint. Remove the wick first, then the soldering iron. Cut off and discard the end of the wick coated with solder.    4.7APPLICATIONS: Soldering was historically used to make jewelry items, cooking ware and tools. Currently, the two most common uses of soldering are in plumbing and in electronics where it is used to connect electrical wiring and to 43 connect electronic components to printed circuit boards(PCBs). It provides reasonably permanent but reversible connections between copper pipes in plumbing systems as well as joints in sheet metal objects such as food cans, roof flashing, rain machine tools gutters and and some automobile radiators. Jewelry components, refrigeration and plumbing components are often assembled and repaired by the higher temperature silver soldering process. Small mechanical parts are often soldered or brazed as well. Soldering is also used to join lead came and copper foil in stained glass work. It can also be used as a semi-permanent patch for a leak in a container or cooking vessel. 44 CONCLUSION With the available facilities and instructions provided, we are successful in completing the project Laptop protector. LAPTOP PRTECTOR is a compact electronic gadget which protects your valuable laptops against theft. This circuit works as a “miniature alarm generator”. Now, by keep the laptop case in horizontal position and on the unit. Your laptop is now protected. switch 45 BIBLIOGRAPHY Reference on Books: Electronic components -Delton T.Horn Electronics circuits and systems -Owen Bishop Electronic circuit analysis and design -Donald A,Neamen Reference on the Web:       46 47