Project Report on Intelligent Bell

ABSTRACT

The circuit chosen for our mini project is INTELLIGENT BELL. In this bell, there is a 2

position switch marked IN and OUT which is under the control of the house owner.

When the house owner is at home, he keeps the switch at IN position and when a visitor

comes, without pressing any bell switch, the bell sound is produced, which is contrary

to the conventional bell systems in which the bell sound is produced after the visitor

presses the bell switch. When the house owner goes out, he keeps the switch at the OUT

position. When the switch is at OUT position and when a visitor comes, instead of a bell

sound, a voice message is played to visitor which says that the house owner has gone out

and asks the visitor to leave a voice message by pressing and holding the record button.

Once the visitor presses and holds the record button and says whatever information he

wants to convey to the house owner, it gets recorded onto the chip. When the house

owner returns, he can very well playback the recorded messages and know about the

visitors who had come to visit him while he was not at home. Hence the name intelligent

bell is justified.

CONTENTS

1. INTRODUCTION. 1

2. SCOPE OF THE PROJECT.

3. BLOCK DIAGRAM. 3.1 PROXIMITY SENSOR

3.2 IN/OUT SELECTOR

3.3PRERECORDED CHIP

3.4RECORDING CHIP

3.5 OUTPUT SELECTOR

4. STUDY OF ICs. 4.1 STUDY OF APR9600.

4.1.1 FEATURES

4.1.2PIN DIAGRAM

4.1.3MESSAGE MANAGEMENT

4.1.4 SIGNAL STORAGE

4.1.5 SAMPLING RATE & VOICE QUALITY

4.1.6 AUTOMATIC GAIN CONTROL(AGC)

4.2 STUDY OF 555 TIMER. 4.2.1 PIN DETAILS

4.2.2 ASTABLE OPEARTION

4.3 STUDY OF TSOP 1738.

5. CIRCUIT SCHEMATIC OF “INTELLIGENT BELL”.

6. CIRCUIT DESCRIPTION.

6.1 POWER SUPPLY UNIT

6.2 555 TIMER CIRCUIT

6.3 TSOP1738 CIRCUIT

6.4 CIRCUIT FOR PRERECORDED CHIP

6.5 CIRCUIT FOR RECORDING CHIP

7. PCB FABRICATION AND SOLDERING.

CONCLUSION.

BIBLIOGRAPHY.

LIST OF DIAGRAMS

No. Description Page No.

3.2.1 Pin diagram of 555 timer

3.2.2.1 Astable operation of 555 timer

3.2.2.2 Graph representing astable operation of 555 timer

4.3.1 TSOP pin out

4.3.2 Internal block diagram of TSOP 1738

5.1.1 Power supply for the entire circuit

5.2.1 555 timer circuit

5.3.1 TSOP 1738 circuit

5.4.1 Circuit for pre recorded chip

5.5.1 Circuit for recorded chip

INTRODUCTION

1. INTRODUCTION

The past 5 decades have seen the introduction of technologies that have radically changed

the way in which we analyze and control the world around us. Starting from the vacuum

tubes to the latest microcontrollers, Electronics has taken its place in almost all areas of

application. Here we take a very low ended application of electronics and we have named

it “INTELLIGENT BELL”.

INTELLIGENT BELL is project work fabricated around an IC called APR9600.This

chip is having inbuilt flash memory.

SCOPE OF THE PROJECT

2. SCOPE OF THE PROJECT.

In this project the IC used is APR 9600 which is having a recording capability of

maximum 60seconds.But this can be efficiently enhanced using microprocessor

controlled message management. Microprocessor control can be used to link several APR

9600 devices together in order to increase total available recording time.

A continuous message cannot be recorded in multiple devices, however because the

transition from one device to the next will incur a delay that is noticeable upon playback.

For this reason it is recommended that message boundaries and device boundaries always

coincide.

BLOCK DIAGRAM

3. BLOCK DIAGRAM

Block diagram of intelligent bell mainly consists of 6 blocks which are explained below:

3.1 PROXIMITY SENSOR

This block includes 555 timer and TSOP1738 ICs. The 555 timer works as an

astable multivibrator and the signal produced is received by the TSOP IC. A

continuous infrared beam is maintained between the two circuits and any

breakage to the infra red beam produces a logical low output.

3.2 IN/OUT SELECTOR

This block is under the control of the house owner. It is actually a SPDT

(single pole double terminal) switch. When the house owner goes out of the

house, he keeps this switch at OUT position and when he is inside the house,

he keeps this switch at IN position.

3.3PRERECORDED CHIP

This block represents the chip which contains pre-recorded messages. The

prerecorded messages are the bell sound and the play back message. When the

owner has gone out, the play back message is played instead of the bell sound.

This message asks the visitor to leave a message to the owner so that he can

understand who had visited him, when he wasn’t at home.

3.4RECORDING CHIP

This block represents the chip which is used for recording the messages of the

visitors.

2.5 OUTPUT SELECTOR

This block selects the output that is to be heard through the speaker. If the

selector switch is at IN position, the output produced is a bell sound. If the

switch is at OUT position, the prerecorded message is being played instead of

the bell sound. During the playback mode, the recorded messages by the

visitors are played.

STUDY OF ICs

3.1 STUDY OF APR9600

3.1.1 FEATURES

Single chip ,high quality voice recording and playback solution

- No external ICs required

- Minimum external components

Non-volatile Flash memory technology

- No battery backup required

User-selectable messaging options

- Random access of multiple fixed duration messages.

- Sequential access of multiple variable duration messages

User friendly ,easy-to-use operation

- Programming and development systems not required

- Level activated recording and edge activated playback switches

Low power consumption

- Operating current: 25mA typical

- Standby current: 1μA typical

- Automatic power down

Chip enable pin for simple message expansion

APR 9600 is a low cost, high performance sound record/replay IC,

incorporating flash analogue storage technique. The IC is non-volatile;

recorded sound is retained even after the power supply is removed from the

module. The device offers true single chip voice recording and play back

capability for 40 to 60 seconds. The replayed sound exhibits high quality

with the low noise level. Sample rates are user selectable which allows the

designers to customize their design for unique quality and storage time

needs. Sampling rate for a 60 second recording period is 4.2 kHz that gives a

sound record/replay bandwidth of 20 Hz to 2.1 KHz. However, by changing

the oscillation resistor a high sampling rate can be achieved. Higher

sampling rates improve the voice quality, but they also increase the

bandwidth requirement and thus reduce the duration. Sampling rates as high

as 8.0 KHz shortens the total length of sound recording to 32 seconds. Total

sound recording time can be varied from 32 seconds to 60 seconds by

changing the value of a single resistor.

APLUS integrated, achieves high levels of storage capability by using its

analog/multilevel storage technology implemented in an advanced flash non-

volatile memory process. The device offers both random and sequential

access of multiple messages.

The IC can be operated in two modes:

Serial mode

Parallel mode.

In serial access mode sound can be recorded in 256 sections; each memory

cell can store 256 voltage levels. This technology enables the APR9600

device to reproduce voice signals in their natural form. It eliminates the need

for encoding and compression which often introduces distortion.

In parallel access mode, sound can be recorded in 2, 4 or 8 sections. The IC

can be controlled simply using push button keys.

3.1.2PIN DIAGRAM

/M1 VCCD /M2 /RE /M3 Ext CLK /M4 MSEL2

/M5 MSEL1

/M6 CE OSCR /STROBE M7 Ana_Out

M8 Ana_In

BUSY AGC

BE MIC REF

VSSD MIC IN

VSSA VCCA SP+ SP-

Pin Name Functions

1 M1 Select 1st section of sound or serial mode recording and replaying

control(low active)

2 M2 Select 2nd section or fast forward control in serial mode (low active)

3 M3 Select 3rd section of the sound

4 M4 Select 4th section of the sound

1 28

2 27

3 26

4 25

5 24

6 23

7 22

8 21

9 20

10 19

11 18

12 17

13 16

14 15

5 M5 Select 5th section of the sound

6 M6 Select 6th section of the sound

7 OSCR Resistor to set clock frequency

8 M7 Select 7th section of sound or overflow indication

9 M8 Select 8th section of sound or select mode

10 BUSY Busy(low active)

11 BE =1,beep when a key is pressed

=0,do not beep

12 VSSD Digital circuit ground

13 VSSA Analogue circuit ground

14 SP+ Speaker, positive end

15 SP- Speaker, negative end

16 VCCA Analogue circuit power supply.

17 MIC IN Microphone input

18 MIC REF Microphone reference input

19 AGC Automatic gain control

20 ANA IN Audio input

21 ANA

OUT

Audio output from microphone amplifier

22 STROBE During recording and replaying, it produces a strobe signal

23 CE Reset sound track counter to zero/stop or start/stop

24 MSEL1 Mode selection 1

25 MSEL2 Mode selection 2

26 EXTCLK External clock input

27 RE =0 to record,

=1 to replay

28 VCCD Digital circuit power supply

3.1.3MESSAGE MANAGEMENT

Play back and record options are managed by on chip circuitry. There are several available

messaging modes depending upon desired operation. These message modes determine message

management style and message management length. The device supports five message

management modes:

Random access mode with 2,4 or 8 fixed duration messages

Tape mode with multiple variable duration messages, provides 2 options:

- Auto rewind

- Normal

Modes cannot be mixed. Switching of modes after the device has recorded an initial message is

not recommended. If modes are switched after an initial recording has been made some

unpredictable message fragments from the previous mode may remain present, and be audible on

playback, in the new mode. The table defines the necessary decoding required for choosing the

desired mode:

MODE MSEL1 MSEL2 /M8_OPTION

Random access-2 fixed duration messages 0 1 Pull this pin to VCC through 100k

Random access-4 fixed duration messages 1 0 Pull this pin to VCC through 100k

Random access-8 fixed duration messages 1 1 The /M8 message trigger becomes

input pin

Tape mode, Auto rewind option 0 0 0

Tape mode, Normal option 0 0 1

An important feature of the APR9600 Message management capabilities is the ability to audibly

prompt the user to change in the device’s status through the use of beeps superimposed on the

device’s output. This feature is enabled by asserting logic high on BE pin.

Random access mode

This mode supports 2, 4 or 8 message segments of fixed duration. Recording and play back can

be made randomly in any of the selected messages. The length of each message is the total

recording length available divided by the total number of segments enabled. Random access

mode provides easy indexing to message segments.

Tape mode

Tape mode manages messages sequentially much like traditional cassette tape recorders.

Within tape mode two options exist, auto rewind and normal. Auto rewind mode

configures the device to automatically rewind to the beginning of the message

immediately following recording or playback of the message. In tape mode, using either

option, messages must be recorded or played back sequentially, much like a traditional

cassette tape recorder.

Functional Description of Recording in Tape Mode using the Auto Rewind Option

On power up, the device is ready to record or playback, starting at the first address in the

memory array. To record, /CE must be set low to enable the device and /RE must be set

low to enable recording. A falling edge of the /M1_MESSAGE pin initiates voice

recording and is indicated by one beep. A subsequent rising edge of the /M1_MESSAGE

pin during recording stops the recording which is also indicated by a single beep. If the

M1_MESSAGE pin is held low beyond the end of the available memory, recording will

stop automatically which is indicated by two beeps. The device will then assert a logic

low on the /M7_END pin until the /M1 Message pin is released.

The device returns to standby mode when the /M1_MESSAGE pin goes high again. After

recording is finished the device will automatically rewind to the beginning of the most

recently recorded message and wait for the next user input. The auto rewind function is

convenient because it allows the user to immediately playback and review the message

without the need to rewind. However, caution must be practiced because a subsequent

record operation will overwrite the last recorded message unless the user remembers to

pulse the /M2_Next pin in order to increment the device past the current message.

A subsequent falling edge on the /M1_Message pin starts a new record operation,

overwriting the previously existing message. You can preserve the previously recorded

message by using the /M2_Next input to advance to the next available message segment.

.The auto rewind mode allows the user to record over the just recorded message simply

by initiating a record sequence without first toggling the /M2_NEXT pin.

To record over any other message however requires a different sequence. You must pulse

the /CE pin low once to rewind the device to the beginning of the voice memory. The

/M2_NEXT pin must then be pulsed low for the specified number of times to move to the

start of the message you wish to overwrite. Upon arriving at the desired message a record

sequence can be initiated to overwrite the previously recorded material. After you

overwrite the message it becomes the last available message and all previously recorded

messages following this message become inaccessible. If during a record operation all of

the available memory is used, the device will stop recording automatically, which is

indicated by a double beep and set the /M7_END pin low for a duration equal to 1600

cycles of the sample clock. Playback can be initiated on this last message, but pulsing

the /M2_Next pin will put the device into an "overflow state”. Once the device enters an

overflow state any subsequent pulsing of /M1_MESSAGE or /M2_NEXT will only result

in a double beep and setting of the /M7_END pin low for a duration equal to 400 cycles

of the sample clock. To proceed from this state the user must rewind the device to the

beginning of the memory array. This can be accomplished by toggling the /CE pin low or

cycling power. All inputs, except the /CE pin, are ignored during recording.

Functional Description of Playback in Tape Mode using Auto Rewind Option

On power-up, the device is ready to record or playback, starting at the first address in the

memory array. Before you can begin playback, the /CE input must be set to low to enable

the device and /RE must be set to high to disable recording and enable playback.

The first high to low going pulse of the /M1_MESSAGE pin initiates playback from the

beginning of the current message; on power up the first message is the current message.

When the /M1_MESSAGE pin pulses low the second time, playback of the current

Message stops immediately. When the /M1_MESSAGE pin pulses low a third time,

playback of the current message starts again from its beginning. If you hold the

/M1_MESSAGE pin low continuously the same message will play continuously in a

looping fashion. A 1,540ms period of silence is inserted during looping as an indicator to

the user of the transition between the beginning and end of the message. Note that in auto

rewind mode the device always rewinds to the beginning of the current message. To

listen to a subsequent message the device must be fast forwarded past the current

message to the next message. This function is accomplished by toggling the /M2_NEXT

pin from high to low. After the device is incremented to the desired message the user can

initiate playback of the message with the playback sequence described above. A special

case exists when the /M2_NEXT pin goes low during playback. Playback of the current

message will stop, the device will beep, advance to the next message and initiate

playback of the next message. If /M2 Next goes low when not in playback mode, the

device will prepare to play the next message, but will not actually initiate playback.

If the /CE pin goes high during playback, playback of the current message will stop, the

device will beep, reset to the beginning of the first message, and wait for a subsequent

playback command. When you reach the end of the memory array, any subsequent

pulsing of /M1_MESSAGE or /M2_NEXT will only result in a double beep. To proceed

from this state the user must rewind the device to the beginning of the memory array.

This can be accomplished by toggling the /CE pin low.

3.1.4 SIGNAL STORAGE

The APR9600 samples the incoming voice signals and stores the instantaneous voltage

samples in non-volatile flash memory cells. Each memory cell can support voltage ranges

from 0 to 256 levels. These 256 discrete levels are equivalent of 8 bit binary encoded

values. During playback stored signals are retrieved from memory, smoothed to form a

continuous signal, and then amplified before being fed to an external speaker.

3.1.5 SAMPLING RATE & VOICE QUALITY:

The APR9600 automatically filters its input, based on the selected sampling frequency.

Higher sampling rates increase the bandwidth and hence the voice quality, but they also

use more memory cells for the same length of recording time. Lower sampling rates use

fewer memory cells and effectively increase the duration capabilities of the device, but

they also reduce incoming signal bandwidth.

The APR9600 accommodates sampling rates as high as 8 kHz and as low a 4 kHz. The

quality/duration trade off can be controlled by controlling the sampling frequency.

An internal oscillator provides the APR9600 sampling clock. Oscillator frequency can be

Changed by changing the resistance from the OscR pin to GND.

Table summarizes resistance values and the corresponding sampling frequencies, as well

as the resulting input bandwidth and duration.

Resistance Sampling

Frequency

Input

Bandwidth

Duration

84k 4.2 kHz 2.1 kHz 60 sec

38k 6.4 kHz 3.2 kHz 40 sec

24k 8.0 kHz 4.0 kHz 32 sec

3.1.6 AUTOMATIC GAIN CONTROL(AGC):

The APR9600 device has an integrated AGC. The AGC affects the microphone input but

does not affect the ANA_IN input. The AGC circuit ensures that the input signal is

properly amplified. The AGC works by applying maximum gain to small input signals

and minimum gain to large input signals. This assures that inputs of varying amplitude

are recorded at the optimum signal level. The AGC amplifier is designed to have a fast

attack time and a slow decay time.

This timing is controlled by the RC network connected to pin 19. A value of 220K and

4.7uF has been found to work well for the English language. Be aware that different

languages, speakers from different countries, and music may all require modification of

the recommended values for the AGC RC network.

3.2 STUDY OF 555 TIMER

The 555 timer is a monolithic timing circuit. It is capable of producing accurate and

highly stable time delays. Basically the timer functions in one of the two modes either as

monostable multivibrator. The timer is available as an 8-pin metal can, an 8 bit mini DIP

or a 14 pin DIP. The figure shows the connection diagram and the block diagram of the

SE/NE 555 timer.

Fig:3.2.1

The important features of the 555 timer as follows:

1. The timer is designed to operate for the temperature range of -55˚C to +125˚C.

2. The timer operates on +5V to +18V supply voltage.

3. The timer has an adjustable duty cycle. That is the timing is from microseconds

through hours.

4. The timer can source or sink 200mA

5. The timer has a high current output.

6. The output of the timer is capable of driving TTL and has a temperature stability

of 50 parts per million (ppm) per degree Celsius change in temperature.

7. Highly reliable.

8. Easy to handle.

9. Economically feasible.

3.2.1 PIN DETAILS

Pin 1: Ground: All voltages are calculated with respect to this pin.

Pin 2: Trigger: External trigger pulse is applied at this pin which decides the amplitude

of the timer output. If the voltage at this pin is lesser than 1/3 vcc, the output will at the

comparator will be high and will reset the SR flip flop.

Pin 3: Output: To connect the load to the output terminal, there are two methods. Either

pin between pin 3 and ground or between pin 3 and supply voltage +Vcc.

When the output is low, the load current is flowing through the load connected between

pin 3 and +Vcc into the output terminal. This current is called sink current.

When the output is low, the current through the grounded load is zero. Usually, the load

connected between pin3 and Vcc is called normally on load.

The load connected between pin3 and ground is known as normally off load.

When the output is high, the current through the load connected between pin 3 and Vcc is

zero. But the output terminal supplies current to the normally off load. This current is

also known as the source current. The highest value of sink or source current is 200 mA.

Pin 4: Reset: By applying a negative pulse to this pin, the 555 timer can be reset. When

the reset is not in use, this terminal is connected to avoid the false triggering.

Pin 5: Control voltage: The external voltage given to this terminal changes the threshold

as well as the trigger voltage. That is, by imposing an external voltage on this pin or by

introducing a potentiometer between this pin and ground, the pulse width of the output

waveform can be changed. While not in use, the control pin should be by passed to

ground with 0.01 μF capacitor to avoid noise problems.

Pin 6: Threshold: This pin corresponds to the non-inverting input terminal of

comparator 1.It monitors the voltage across the external capacitor.

Pin 7: Discharge: Internally, this pin is connected to connected to the collector of

transistor Q1.If the output is low,Q1 is saturated. Therefore it acts as a short circuit. If the

output is high Q1 is off and it acts as an open circuit to the external capacitor connected

across it.

Pin 8: +Vcc: The supply voltage is in the range of +5V to +18 V is applied to this pin.

3.2.2 ASTABLE OPEARTION

Astable multivibrator is also known as free-running multivibrator. It is a rectangular

wave shaping circuit having non-stable states. This circuit does not require any external

trigger to change the state of its output and therefore is called as free running oscillator.

The time for either high or low is determined by a capacitor and two resistors which are

connected externally to the timer. The figure shows the 555 timer in the astable

multivibrator configuration.

Fig: 3.2.2.1

Initially, when the output is high, capacitor charges towards +Vs through R1 and R2. If

the capacitor voltage equals 2/3 Vs, then the comparator1 triggers the flip-flop and the

output goes low. Now capacitor discharges through R2 and Q1.If the capacitor voltage is

equal to 1/3 +Vs, then the comparator 2’s output triggers the flip-flop and output goes

high. Now the cycle repeats. The output voltage and the capacitor waveforms are shown

in the figure below.

Fig: 3.2.2.2

From the figure, we can observe that the capacitor is periodically charged and discharged

between 2/3 Vs and 1/3 Vs respectively. The time required for the capacitor to charge

from 1/3 Vs to 2/3 Vs is given by

t m =0.69(R1+R2)C

Where :

R1 and R2 are resistances in ohms.]

C is capacitance in farads.

The time during which the capacitor discharges from 2/3 Vs to 1/3 Vs is equal to

t s =0.69(R2)C

The total period of waveform is

T=t m + t s =0.69(R1 + 2 R2) C

3.3 STUDY OF TSOP1738

The TSOP17.. – Series are miniaturized receivers for infrared remote control systems.

TSOP1738 is the standard IR remote control receiver, supporting all major transmission

codes.

Features

Photo detector and preamplifier in one package

Internal filter for PCM frequency

Improved shielding against electrical field disturbance

TTL and CMOS compatibility

Output active low

Low power consumption

High immunity against ambient light

Continuous data transmission possible (up to 2400 bps)

Suitable burst length = 10 cycles/burst

The circuit of the TSOP1738 is designed in such a way that unexpected output pulses due

to noise or disturbance signals are avoided. A bandpassfilter, an integrator stage and an

automatic gain control are used to suppress such disturbances. The distinguishing mark

between data signal and disturbance signal are carrier frequency, burst length and duty

cycle.

The data signal should fulfill the following condition:

Carrier frequency should be close to center frequency of the band pass (e.g. 38

kHz).

Burst length should be 10 cycles/burst or

longer.

For each burst which is longer than 1.8 ms a corresponding gap time is necessary

at sometime in the DataStream. This gap time must have at least the same length

as the burst.

The figure shows the pin out of TSOP

Fig:4.3.1

When a disturbance signal is applied to TSOP1738, it can still receive the data signal. However sensitivity is reduced to that level that no unexpected pulse will occur.

After each burst which is between 10 cycles and 70 cycles a gap time of at

least 14 cycles is necessary.

.

Fig:4.3.2

The figure shows the internal block diagram of the IC.

5. CIRCUIT DESCRIPTION

The circuit schematic of the project consists of the following units:

5.1Power supply unit

5.2555 timer circuit

5.3TSOP1738 circuit

5.4Circuit for prerecorded chip

5.5Circuit for recording chip

5.1 POWER SUPPLY UNIT:

The power supply unit is the one which supplies power required by the whole circuit.

This power supply is derived from AC through a step-down transformer, regulator and

filter circuit as shown in the figure.

Fig: 5.1.1

When mains is available, the rectifier feeds the circuit. If a battery is connected, the

mains charges the battery and in case of failure of mains, the battery provides the

necessary voltage for the whole system to function.

The circuit mainly consists of a step-down transformer with secondary tapings. These

supplies are obtained by means of a centre tap rectifier with 2 diodes which gives an

unregulated output. This unregulated voltage is used to drive the outputs. Also a +5V

series regulator circuit converts this unregulated supply into +5V regulated supply.

The regulator used is a 3 terminal series regulator IC LM7805 to get +5V regulated

output.

5.2 555 TIMER CIRCUIT:

Fig: 5.2.1

The above figure shows the 555 timer circuit. The 555 timer is working as an astable

multivibrator.The IR LED emits Infrared radiations.

5.3 TSOP1738 CIRCUIT:

Fig: 5.3.1

The TSOP 1738 circuit receives the IR radiation from the 555 timer ciruit.Thus an IR

beam is established between timer circuit and the TSOP circuit. Whenever a person

breaks this IR beam, a logic low output is produced from this circuit and is fed into the

SPDT switch which is connected the pre-recorded chip.

5.4 CIRCUIT FOR PRERECORDED CHIP

The circuit consists of the pre recorded chip. This chip contains the bell sound and the

voice message that is to be played by the intelligent bell to the visitor when the owner has

gone out. When the owner is inside, the SPDT switch is at IN position and when the

visitor comes and causes the breakage of the IR beam, the bell sound is heard.

Fig: 5.4.1

When the owner goes outside and when the visitor comes and causes the breakage of the

IR beam, the voice message instructing the visitor to leave a voice message for the owner

is delivered by the intelligent bell. This portion of the job is the responsibility of the pre-

recorded chip.

5.5 CIRCUIT FOR RECORDING CHIP

Recording chip can otherwise be referred to as the visitor’s chip. When the house owner

goes out he keeps the SPDT switch at the OUT position. So when a visitor comes, due to

the breakage of IR beam, the voice message is played by the intelligent bell. The voice

message actually asks the visitor to leave a voice message by pressing and holding the

record button. Till the record button is released, whatever the visitor has told will be

recorded. The IC is having a maximum capacity of 60 seconds. If the visitor keeps on

holding the record button even after this 60 seconds, then beeps are produced which

indicate that its memory is full indicating no further recording can be possible.

Via a DPDT switch, both the chips, the recording as well as the pre recorded chips are

connected to the speaker.

Fig: 5.5.1

6. PCB MAKING

There are several ways of drawing PCB patterns and making the final

boards. The making of the PCB generally involves two steps (1) Preparing the PCB

drawing (2) Fabricating the PCB itself from the drawing.

The traditional method is PCB drawing with complete placement of parts,

taking a photographic negative of the drawing, developing the image of the negative

formed on the photosensitized copper plate, and dissolving the excess copper by etching.

This is the standard practice being followed in large scale operations. The cost saving

procedure presented here may be opted.

PCB DRAWING

The making of the PCB involves some preliminary considerations such as

placement of components (in the same order as in the circuit diagram) on a piece of

paper. Locating holes, deciding the diameters of various holes, the optimum area that

each component should occupy, the shape and location of islands for connecting two or

more components at a place, full space utilization and prevention of overcrowding of

components at a particular place. For anchoring levels of components, 1mm diameter

holes; and for fixing PCB holding screws to the chassis 3mm diameter holes can be

made. Following these hints a sketch of the PCB is made.

PCB FABRICATION

The copper clad PCB laminate is now prepared by rubbing away the

oxide, grease etc. with fine emery paper or sand paper. On this, final PCB drawing may

be traced using a carbon paper. Clips are used to prevent the carbon paper from slipping

while the PCB pattern is being traced on the laminate. Only the connecting lines in

PCB’s islands and holes should be traced. The marked holes in the PCB may be drilled

using 1mm or 3mm drill bits and the traced PCB pattern is coated with black, quick

drying-enamel paint, using a thin brush and a small metal case. In case if there is any

shorting of lines due to spilling of paint, these may be removed by scraping with a blade

or a knife, after the paint has dried.

After drying, 22-20 grams of ferric chloride in 75ml of water may be

heated to about 60 degree Celsius and poured over the PCB, placed with the copper side

upwards in a plastic tray. Stirring the solution helps speedy etching. The dissolution of

unwanted copper may take about 45 minutes. If etching takes longer time, the solution

may be heated again and the process may be repeated. The point on the PCB can be

removed by rubbing with a rag soaked in thinner, turpentine or acetone. The PCB is then

washed and dried.

Depending on the wiring diagram, the components are soldered on the PCB. Usually the

resistors are soldered first and then the IC’s are soldered.

SOLDERING

All components are first tested. The leads of all components are cleared

by rubbing with an abrasive. The PCB is also cleaned by scratching off the varnish layer

at the selected point. The lead of the component to be soldered is applied with some flux

to remove any remaining oxide coating.

7. SCOPE OF THE PROJECT.

In this project the IC used is APR 9600 which is having a recording capability of

maximum 60seconds.But this can be efficiently enhanced using microprocessor

controlled message management. Microprocessor control can be used to link several APR

9600 devices together in order to increase total available recording time.

A continuous message cannot be recorded in multiple devices, however because the

transition from one device to the next will incur a delay that is noticeable upon playback.

For this reason it is recommended that message boundaries and device boundaries always

coincide.

CONCLUSION

The intelligent bell has been successfully designed, assembled and tested for its

performance. Proper selection of the components and designing aesthetically shall

convert this into a commercially viable product.