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.

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As a whole the aim of our project is to develop a efficient

circuit that can provide protection for your valuable laptops against

theft.

2. CIRCUIT DESIGN

2.1. Block diagram:

Figure.2.1.1: Block diagram of

Laptop protector

2.1.1. DESCRIPTION:

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The block diagram consists of 5 main blocks along with the

supply line. The five main blocks are tilt switch, comparator,

monostable, transistor switch, piezo buzzer. The mercury switch is

nothing but a tilt switch which works as a sensor switch. The change

in position causes switch to open (OFF) or to close(ON). This switch is

sensitive to the movements given by the human and also earth’s

gravity.

The IC TL071 acts as a comparator, which is LOW-NOISE JFET-

INPUT 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 BC557 is a NPN transistor which acts as a switch in the

circuit between supply and the buzzer. Whose base terminal is

connected to the output of monostable circuit , emitter is connected

to the supply and collector is given to the buzzer. As The small base

current  controls the larger  collector current, the switch is

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closed When a small current flows into the base (B) of the transistor

and  The transistor amplifies this small current to allow a larger

current to flow through from its collector (C) to its emitter (E).Similarly

the switch is open When  no base current flows, so the transistor

switches off the collector current.

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:

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Figure.2.2.1: Circuit diagram of Laptop protector

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

5

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.

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.

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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 TL071, TL071A and TL071B are high speed JFET input single

operational amplifiers incorporating well matched, high voltage JFET

and bipolar transistors in a monolithic integrated circuit. The 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

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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:

PIN DIAGRAM:

8

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 voltage. The null-offset potentiometer also

compensates for irregularities in the operational amplifier

manufacturing process which may cause an offset. Consequently, the

null potentiometer is 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.

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):

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 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.

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.

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ABSOLUTE MAXIMUM RATINGS:

Table 2.4: absolute maximum ratings of TL071

ORDER CODES:

NOTE:

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

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the supply voltage or 15 V, whichever is less.

4. The output may be shorted to ground or to either supply.

Temperature

and / or supply voltages must be limited to ensure that the

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

MIL-

STD- 883.

2.4.1 SCHEMATIC DIAGRAM:

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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.

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.

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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 8-

pin 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 configured as

common-base amplifiers. This configuration isolates the 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 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.

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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 nothing. Thus, it serves as a high-

impedance connection to the negative power supply, providing a

reference without loading the input circuit. This particular circuit is

therefore able to provide the slight base bias current needed for the

PNP transistors in the differential input circuit, while allowing those

transistors to operate correctly over a wide common-mode input

voltage range.

APPLICATIONS:

ICTL071 has involved in many applications such as:

audio and video pre-amplifiers and buffers

voltage comparators

differential amplifiers

differentiators and integrators

filters

precision rectifiers

voltage regulator and current regulator

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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:

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

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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  IN+ (input 2)  is greater than IN- (input 3), the output is 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.

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 resistor is required in

series with the timing pin. Input protection from static discharge is

provided on all pins.

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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

LOGIC SYMBOL:

18

Figure 2.5.1: Logic diagram of IC CD4538

PIN DIAGRAM:

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Figure 2.5.2: pin diagram of IC CD4538

TRUTH TABLE:

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):

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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):

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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:

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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 transistor N1 on, the capacitor CX rapidly discharges

toward VSS until VREF1 is reached. At this point the output of

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 regard

to the capacitor voltage. Thus, propagation delay from trigger to Q is

independent of the value of CX, RX, or the duty cycle of the input

waveform.

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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:

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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.

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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 of switch is mly

operated electromechanical device with one or more sets 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.

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Momentary - switch returns to its normal position when released.

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

28

current. Although designed 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:

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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 "feed-forward" 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 metal that is liquid at room

temperatures.

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

31

the conductivity becomes strongly pressure-dependent. At 1520°C the

specific conductivity of supercritical mercury increases continuously by

more than 4 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, Single-Pole, Double-Throw (SPDT) operation.

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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:

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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.

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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 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

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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 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 anti-

handling 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

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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. 

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

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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.

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:

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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:

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 a few minutes for the soldering iron to warm up. 

You can check if it is ready by trying to melt a little solder on the tip.

Wipe the tip of the iron on the damp sponge. 

This will clean the tip.

Melt a little solder on the tip of the iron. 

This is called 'tinning' and it will help the heat to flow from the iron's tip

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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.

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

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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

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

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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 well-

fluxed. 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.

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.

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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:

At some stage you will probably need to desolder a joint to remove or re-

position 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.

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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 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 gutters and

automobile radiators. Jewelry components, machine tools and some

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.

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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”.

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Now, by keep the laptop case in horizontal position and switch

on the unit. Your laptop is now protected.

BIBLIOGRAPHY

Reference on Books:

Electronic components

-Delton T.Horn

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Electronics circuits and systems

-Owen Bishop

Electronic circuit analysis and design

-Donald A,Neamen

Reference on the Web:

www.kpsec.freeuk.com

www.electricandelectronicsworld.blogspot.com

www.circuiteasy.com

www.elektronika.ba

www.instructables.com

www.wikipedia.org

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