Jan 25
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<< Mini Project-12

Batch No2
TitleElectronic Letter Box
Name of Students
Ms. Angela Papachan
Ms. Dony J Muttath
Ms. Livya JAcob
Ms. Vidya K


If one wants to know whether there is a letter present or not inside the letter box, ELECTRONIC LETTER BOX PROJECT can be of help. The ELECTRONIC LETTER BOX is a very useful gadget for homes and offices. The circuit uses single Monolithic -555 in dual in line plastic package and a few external components. The main advantage of the letter box is we can find out the presence and absence of letter inside the box without opening the box. Here light depending resistor (LDR) is used as a sensor to detect the presence of letter. Thus this circuit solves the problem of going again and again to the gate of the house to check-up whether a letter is there in the box or not. In this circuit, two LEDs are used to indicate if any letter is present in the letter box or not. When there is no letter present in the letter box, the green LED will glow. As soon as a letter is put in the box, the red LED goes OFF, and the red LED will glow.



Figure 1.1 : Circuit diagram for Electronic Letter Box

This circuit uses I.C 555 which is connected as comparator to pin 6. Connected with positive supply, the output goes high-1 when the trigger pin 2 is lower than 1/3 level of the supply voltage. Conversely the output goes low, when it’s above 1/3 level. So a 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. 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. The L.D.R, most important part of this circuit, 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. The resistance of L.D.R. is minimum when the light falling on is maximum and vice versa when the light falling is minimum. The source of light and LDR is so adjusted in the letter box, that the light will directly fall on the LDR. When a letter is kept inside, 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 ohm variable resistance connected in series. Voltage is directly proportional to conductance; so more voltage is got by this divider when LDR gets light and low voltage in darkness. Divided voltage is given to pin 2 of 555. As soon as LDR is in darkness, the voltage of the pin 2 drops 1/3 of the supply voltage and pin 3 gets high, the LED glows.   LD.R is connected to pin no 6 of IC-555. When the letter is inside the letter box, the intensity of light falling on L.D.R. is min, so resistance is high; the voltage across the threshold pin goes above 2/3 VCC. The internal flip-flop resets and the output goes low and the red L.E.D is in the forward biased condition emitting red light. In the absence of letter in the box, light falling on L.D.R. is max, so resistance is low and the voltage across the threshold pin goes below 2/3 VCC. The internal flip-flop sets and its output go high which forward biases the green L.E.D. leading it to emit green light.



The electronic letter box is a simple circuit that can turn out to be very helpful for the people especially in this hectic world. People need not check the letter box every time for letters. Once a letter is present in this box, it’s indicated by the changing of the LED glow. The circuit can be improved by including in it a buzzer or some kind of sound producing circuit, which can indicate the presence of the letter.  Also some circuitry can be included to indicate the number of  letters present in the letter box .


written by JINESH

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