ELECTRONIC LETTERBOX:
The circuit illustrated below is used as an electronicletter box. When letter is kept in this box then a LED glows which
indicates that letter is inside theBox.
This circuit uses a popular timer I.C which is 555.I.C 555is connected as comparator with pin 6 connected with
positiv e supply, theoutput goes high-1 when the trigger pin 2 is at lower than 1/3 levelof the supply voltage.
Conv ersely the output goes low-0 , when it is above 1/3 level.Sosmall change in thevoltage of pin 2 is enough to
change the output of pin 3 from 1 to0 and 0 to1 .Theoutput has only twostates high and low and can not remain in
any intermediate stage. It is power by 9V battery for portable use.The circuit is economicin power consumption. Pin
4,6&8 is connected tothe positive supply and pin 1 is grounded.
Todetect thepresent of letter we have used LDRand a source of light.LDRis a special type of resistancewhose
v alue depends on the brightness of the light which is falling on it. It has a resistanceof about 1 megaohms when in
total darkness, but a resistance of only about 2-5 k ohms when brightly illuminated. It responds toa large part of the
light spectrum.
The sourceof light and LDRis so adjusted in the letter box that light will directly fall on the LDRbut when letter is
kept inside then it will blockthe beam of light and LDRwill be under darkness.
We havemade a potentialdivider circuit with LDRand 100 Kv ariableresistance
connected in series.Voltage is directly proportional toconductance somore voltage we will get by this divider when
LDR is getting light and low voltage in darkness. Divided voltage is given topin 2nd of 555.As soon as LDR gets dark
the v oltage of the pin 2 drops 1/3 of the supply voltageand pin 3 gets high and LED glows.
We can alsouse twoLED 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 connectedas reverse bias toindicate both high and low conditions.Tolimit the
current of LED resistance is used in series.
For source light, use LED of colour red or white.For LED 2 and 3 you can use any other color-green, red,yellow or
blue.
Circuit Diagramof Electronic Letter Box

Component List :
1 ) LED 3 pc's
2) LDR 1 pc
3) Resistance 470 Ohms 3pcs
4) Variable resistance/preset 100K
5) IC 555
6) Battery 9V with Snap
7 ) Switch
LED= Light Emitting Diode
LDR= Light Dependent Resistance
IC = Integrated Circuit
Light dependentresistor(LDR) or Photo resistor
Posted Date: 25-Aug-2012 Category: Engineering Projects
Author: ANOOP Member Level: Silver Points: 25
Light dependent resistor is an electronic device which works on the principle of
photo conductivity(photo conductive effect). Resistance of Light Dependent
Resistor changes with respect to the light intensity fall on it.

Light dependent resistor and its working
This article explain about the Photo resistor, which have the capability of sensing light.We know that light is an
electromagnetic radiation.And when light falls on the LDR, its resistance will be change according to the light intensity.A
photo sensing material or a photo conductive material is used to build the Photo resistor.The operation of a Photo resistor
is based on photo conductivity or photo conductive effect.
Photo resistor have the same spectral characteristics of human eye that is, relative response is depend on the wave length
of the incident light.
Photo conductivity
Photo conductivity(Photo conductive effect) is the property of some materials to change the electrical conductivity with
intensity change of electromagnetic radiation or light.That is electrical conductivity will increase with increasing light
intensity and electrical conductivity will decrease with decreasing light inten sity.
Light dependent resistor
Light dependent resistor is a two terminal electronic component which is used as light sensing element in electronic
circuits.Principle of operation is based on photo conductive effect(also called photo conductivity). Photo resistor have
the same spectral characteristics of human eye that is, relative response is depend on the wave length of the incident light.
We can see in the figure above the upper surface of the light dependent resistor consists a photo conductive

material.When the light exposed to the upper area of the device its resistance changes with intensity of incident light.That
is, resistance increases when the intensity of light decreases.So the device give very high resistance if there is no light
incident on it.Resistance of the component decreases when the intensity of incident light increases.
If we apply electric current in one terminal of LDR and measure the current in other terminal,then we get maximum
current with maximum illumination of radiation and get minimum current with minimum illumination of radiation.
In the absence of light the resistance of the material will be maximum and a very small current will flow through the
resistor.This current is called dark current and the corresponding resistance is called dark resistance.
Commonly used photo conductive material for making Light Dependent Resistor is Cadmium sulphide(Cds) .
Applications
LDR has various applications in electronics.It works as the sensing element in many electronic equipments .Some of the
applications are listed below.
1)Light measurement.
2)Street light control.
3)Smoke detection.
4)Camera exposure control.
Read related articles: Photo resistor
Carbon composition
Three carbon composition resistors in a 1960s v alv e (v acuum tube) radio
Carbon composition resistors consist of a solid cylindricalresistive element w ith embedded w ire leads or metal end caps to w hich
the lead w ires are attached. The body of the resistor is protected w ith paint or plastic. Early 20th-century carbon composition
resistors had uninsulated bodies; the lead w ires were wrapped around the ends of the resistance element rod and soldered. The
completed resistor w aspainted for color-coding of its value.
The resistive element is made froma mixture of finely ground (pow dered) carbon and an insulating material (usually ceramic). A
resin holds the mixture together. The resistance is determined by the ratio of the fill material (the pow dered ceramic) to the carbon.
Higher concentrations of carbon, a good conductor, result in low er resistance. Carbon composition resistors were commonly used in
the 1960s and earlier, but are not so popular for generaluse now as other types have better specifications, such as toleranc e,
voltage dependence, and stress (carboncomposition resistors willchange value w hen stressed with over-voltages). Moreover, if
internal moisture content (fromexposure for some length of time to a humid environment) is significant, soldering heat w ill create a
non-reversible change in resistance value. Carbon composition resistors have poor stability w ith time and w ere consequently factory

sorted to, at best, only 5% tolerance.[5]
These resistors, however, if never subjected to overvoltage nor overheating were remarkably
reliable considering the component's size[6]
Carbon composition resistors are stillavailable, but comparatively quite costly. Values ranged fromfractionsof an ohm to 22
megohms. Due to their high price, these resistors are no longer used in most applications. How ever, they are used in pow er
supplies and w elding controls.[6]