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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
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
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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
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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
Breadboard
Battery
I.C
LED
LDR
Resistor
Connecting wires
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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
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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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
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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
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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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
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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
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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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
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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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
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
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
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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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
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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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
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