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ABSTRACT
This paper reports a design and development of electronic letter box which indicates the presence and
absence of letter designed using timer IC 555 and LDR. The setup also consists of LED’S, when the letter is put
inside, LED glows which indicate the presence of letter.
Here we are introducing electronic letter
box, which is designed using Timer IC 555 and
LDR basically indicate the presence of letter. IC 555
timer is a one of the most widely used IC in
electronics and is used in various electronic
circuits.The IC555 timersis called so, the timer got
its name from the three 5 kilo-ohm resistor in series
employed in the internal circuit of the IC. As shown
in fig 1 and 2, the 555 timer comes as 8 pin DIP
(Dual In-line Package) device. There is also a 556
dual version of 555 timer which consists of two
complete 555 timers in 14 DIP and a 558 quadruple
timer which is consisting of four 555 timer in one IC
and is available as a 16 pin DIP in the market.
components like transistors, resistors etc. the biasing
voltage (Vcc) is divided in three parts through
voltage divider using same value of resistors R.
From these, 1/3Vcc is given to non-inverting
terminal of trigger comparator and 2/3 Vcc is given
to inverting terminal of threshold comparator. The
outputs of both comparators are given to R and S
inputs of flip-flop. The Q’ output is actual output of
IC and Q output drives discharging transistor that
provides discharging path to external capacitor
whenever it is high.When negative trigger <1/3 Vcc
is applied at trigger input pin, the trigger comparator
gives high output that resets the flip flop and Q’
output that is the output of chip goes high.When
positive trigger >2/3 Vcc is applied at threshold
input pin, the threshold comparator gives high
output that sets the flip flop. The Q output will
become high and the output of chip goes low. At
that time discharging transistor that provides
discharging path to external capacitor.The high reset
input keeps flip-flop enable. If it is low, flip-flop
disables and output will be low. No any effect of
threshold and trigger comparator outputs[Available
at 1 and 3]
MATERIALS AND METHODS
Comparator
The Comparator is the basic electronic
component which compares the two input voltages
i.e. between the inverting (-) and the non-inverting
(+) input and if the non-inverting input is more than
the inverting input then the output of the comparator
is high. Also the input resistance of an ideal
comparator is infinite.
Voltage Divider
As we know that the input resistance of the
comparators is infinite hence the input voltage is
divided equally between the three resistors. The
value being Vin/3 across each resistor.
Flip/Flop
Flip/Flop is a memory element of Digitalelectronics.
The output (Q) of the flip/flop is ‘high’
if the input at ‘S’ terminal is ‘high’ and ‘R’ is at
‘Low’ and the output (Q) is ‘low’ when the input at
‘S’ is ‘low’ and at ‘R’ is high.
FUNCTION OF DIFFERENT PINS
a. Ground: This pin is used to provide a zero
voltage rail to the Integrated circuit to divide the
supply potential between the three resistors shown
in the fig.2.b. Trigger: the voltage at the noninverting
end of the comparator is Vin/3, so if the
trigger input is used to set the output of the F/F to
‘high’ state by applying a voltage equal to or less
than Vin/3 or any negative pulse, as the voltage at
the non-inverting end of the comparator is
Vin/3.c. Output: It is the output pin f the IC,
connected to the Q’ (Q-bar) of the F/F with an
inverter in between as show in the figure 2.d. Reset:
This pin is used to reset the output of the F/F
regardless of the initial condition of the F/F and also
it is an active low Pin so it connected to ‘high’ state
to avoid any noise interference, unless a reset
operation is required. So most of the time it is
connected to the Supply voltage as shown in the
figure 2.e. Control Voltage: As we can see that the
pin 5 is connected to the inverting input having a
voltage level of (2/3) Vin. It is used to override the
inverting voltage to change the width of the output
signalirrespectiveoftheRCtiming
network.f. Threshold: The pin is connected to the
non-inverting input of the first comparator. The
output of the comparator will be high when the
threshold voltage will be more than (2/3) Vin thus
resetting the output (Q) of the F/F from ‘high’ to
‘low’.g. Discharge: This pin is used to discharge the
timing capacitors (capacitors involved in the
external circuit to make the IC behave as a square
wave generator) to ground when the output of Pin 3
is switched to ‘low’.h. Supply: This pin is used to
provide the IC with the supply voltage for the
functioning and carrying of the different operations
to be fulfilled with the 555 timer.
The IC 555 can be used as following 3 types
a. Astablemultivibrator – it has no stable state. It
has two quasi stable states that automatically
changes from one to another and back. So actually it
changes from high to low state and low to high state
without any trigger input after pre determine time.
b. Monostablemultivibrator – it has one stable state
and one quasi stable state. It jumps into quasi stable
state from stable state when trigger input is applied.
It comes into stable state from quasi stable state
after pre determine time automatically.
c. Bistablemultivibrator – it has both stable states.
Two different trigger inputs are applied to change
the state from high to low and low to high.
APPLICATIONS
The IC 555 timer is used in many circuits,
for example One-shot pulse generator in Monostable
mode as an Oscillator in Astable mode or in Bistable
mode to produce a flip/flop type action. It is also
used in many types of other circuit for achievement
of various purposes for instance Pulse Amplitude
Modulation (PAM), Pulse Width Modulation
(PWM) etc.
LDR
A LDR as shown in figure 3 (light
dependent resistor) is a light controlled variable
resistor made of a high resistance semiconductor.
An LDR is also called as photoresistor or photocell.
In the dark, a photoresistor can have a resistance as
high as several mega ohms (M ), while in the light,
a photoresistor can have a resistance as low as a few
hundred ohms. If incident light on a photo
resistorexceeds a certain frequency, photons
absorbedbythesemiconductorgive
bound electrons enough energy to jump into
the conduction band. The resulting free electrons
(and theirhole partners) conduct electricity, thereby
lowering resistance. An LDR can be applied in light
and dark activated switching circuits.LDR used in
some dynamiccompressors togetherwitha
small incandescent or neon lamp, or light-emitting
diode to control gain reduction. A common usage of
this application can be found in many guitar
amplifiers thatincorporatean
onboard tremolo effect, as the oscillating light
patterns control the level of signal running through
the amp circuit [Available at 3 and 5]
RESULTS AND DISCUSSION
This circuit as shown n figure 4 uses a
timer IC which is 555. IC 555 is connected as
comparator with pin 6 connected with positive
supply, the output goes high-1 when the trigger pin
2 is at lower than 1/3 level of the supply voltage.
Conversely the output goes low-0, when it is above
1/3 level. So small change in the voltage of pin 2 is
enough to change the output of pin 3 from 1 to 0 and
0 to 1. The output has only two states high and low
and cannot remain in any intermediate stage. It is
power by 9V battery for portable use. The circuit is
economic in power consumption. Pin 4, 6&8 is
connected to the positive supply and pin 1 is
grounded.
To detect the present of letter we have used
LDR and a source of light. LDR is a special type of
resistance whose value depends on the brightness of
the light which is falling on it. It has a resistance of
about 1 mega ohms when in total darkness, but a
resistance of only about 2-5 k ohms when brightly
illuminated. It responds to a large number of light
spectrums. 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. We have made a potential divider circuit
with LDR and 100k variable resistance connected in
series. Voltage is directly proportional to
conductance so more voltage we will get by this
divider when LDR is getting light and low voltage
in darkness. Divided voltage is given to pin 2nd 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. We can also use two LED 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 resistance is used in series. For source light, use Red or White LED. For LED 2 and
3 you can use any other color- green, red, yellow or
blue[1,6].
As in fig. 5 & 6 when there is no letter
present red light will glow, red is the indication that
no letter is present. As letter is put inside the letter
box, shown in fig 7 and fig 8, there is no light
falling on the LDR then its resistance increases
which results in decrease of the voltage at pin 2 of
the IC 555. IC 555 has got comparator inbuilt,
which compares between the input voltage from
pin2 and 1/3rd of the power supply voltage. When
input falls below 1/3rd then output is set high
otherwise it is set low. So when letter is put inside it,
the result will be decrease in input voltage then
output set to high, which indicates the presents of
letter by glowing LED1. If there is no letter inside,
then LED2 will be on[Available at 2 and 7].
CONCLUSIONS
The progress in science & technology is a
non-stop process. New things and new technology
are being invented. As the technology grows day by
day, we can imagine about the feature in which
thing we may occupy every place. The presentation
of “ELECTRONICS LETTER BOX “is used to
indicate the presence of letter in a letterbox. In our
daily life it is much useful because there is no need
of checking empty letter box .Here by using this
setup; it will make LED to glow which indicate
presence of letter. Hence we conclude that the
principle of the development of science is that
“nothing is imppossible”. So we shall look to a
growth of science in bright world.