20-11-2012, 04:41 PM
ELECTRONIC LETTER BOX
[attachment=40405]
ABSTRACT:
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. 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.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 green LED goes OFF, and the red LED will glow.
WORKING:
The L.D.R. is the most important part of this circuit. The Resistance of L.D.R.is minimum. When the light falling on is maximum and its resistance is maximum when the light falling on is minimum. LD.R is connected to pin no2 of C-555.When the letter is inside the letter box, the intensity of light falling on L.D.R.is minimum, so resistance is high , the voltage across the threshold pin no 6 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 letter box, light falling on L.D.R. is maximum so resistance is low and the voltage across the threshold pin goes below 2/3 VCC. The internal flip-flop sets and its output goes high which forward bias the green L.E.D. emitting green light.
[attachment=40405]
ABSTRACT:
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. 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.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 green LED goes OFF, and the red LED will glow.
WORKING:
The L.D.R. is the most important part of this circuit. The Resistance of L.D.R.is minimum. When the light falling on is maximum and its resistance is maximum when the light falling on is minimum. LD.R is connected to pin no2 of C-555.When the letter is inside the letter box, the intensity of light falling on L.D.R.is minimum, so resistance is high , the voltage across the threshold pin no 6 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 letter box, light falling on L.D.R. is maximum so resistance is low and the voltage across the threshold pin goes below 2/3 VCC. The internal flip-flop sets and its output goes high which forward bias the green L.E.D. emitting green light.