ELECTRONIC LETTER BOX
Rs=150.00 (+ 80 Shipping)
India = Rs:230/ $5.11 USD
Other Country = $34.00 USD
Debit Card
ATM Card
Bank Transfer
then a LED glows which indicates that letter is inside the Box.
This circuit uses a popular timer I.C which is 555. I.C 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 can not 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 megaohms when in total darkness,but a resistance of only about 2-5 k ohms when brightly
illuminated. It responds to a large part of the light spectrum.
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 put 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 100 K 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.
LED= Light Emitting Diode LDR= Light Dependent Resistance IC= Integrated Circuit
deals with CBSE and
other School Physics
Project for student,
Hobbyist. Please also
contact for science
exhibition project
material,Components
and ready made
project as shown in
the fig.
For LED 2 and 3 you can use any other colour- green, red,
yellow or blue.
1. LED 3 pcs
2. LDR 1pc
3. Resistance 470 Ohms 3pcs
4. Variable resistance/preset 100 K
5. IC 555
6. Battery 9V with Snap
7 Switch