Electronic letter box

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L E T T E R B O X B Y M U H A M M A D A H T S H A M

Muhammad Ahtsham

Electronic Letter box2


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Breadboard

Battery

I.C

LED

LDR

Resistor

Connecting wires

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As the holes of breadboard are

connected internally, we construct

a circuit consisting mainly of:

1. An I.C

2. 3 Resistors

3. LDR

4. 3 LEDs

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An (IC) is a semiconductor device on

which thousands of tiny

resistors, transistors are fabricated.

It reduces the size and simplifes the

circuits.

Usage in project:In project the IC has 2 diodes and 2

transistors.

.

LED produces narrow-spectrum light at

the time it is electrically biased in the p-n

junction at the forward bias state.

These are used as indicator lamps in many

device we have used it for the detection of

presence of a letter.

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LDR is a special type of resistance

whose value depends on the brightness

of light falling on it.

Usage in project:

The source of light and LDR is so

adjusted in the letter box that light will

directly fall on the LDR

But when letter is kept inside then it

will block the beam of light and LDR

will be under darkness

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3 current limiting resistors are

used in our project in series with

the led.

Their function is to protect the

leds from excessive flow of

current.

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Connecting wires connect

different elements of a circuit.

They also connect the source

with the circuit.

To detect the presence of a letter we have used LDR and

a source of light.

LDR has a resistance of about 1 mega ohms in total

darkness and a resistance of only 2-5 k ohms in the

presence of light.

The source of light and LDR is so adjusted in the letter

box that light directly falls on the LDR but when letter is

kept inside then it will block the beam of light and LDR

will be under darkness.12


We have made a potential divider circuit with LDR and 100

K variable resistance connected in series.

Voltage is directly proportional to conductance so we will

get more voltage when LDR experiences light and low

voltage in darkness.

Divided voltage is given to pin 2 of 555. As soon as LDR

gets dark the voltage of the pin 2 drops 1/3 of the supply

voltage and pin 3 gets high and LED glows.

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We can also use two LEDs at output pin 3, for present LED

1 and for absent LED 2. For this method one LED is

connected as forward bias and other is connected as reverse

bias to indicate both high and low conditions

To limit the current of LED a 470 ohm resistor is used in

series.

For source light, we used Red or White LED.

For LED 2 and 3 any other color- green, red, yellow or blue

can also be used.

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Engineering

Electronic letter box