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Electronics Letter Box Using IC 555
Post: #1

Electronics Letter Box Using IC 555

.doc  Electronics Letter Box Using IC 555.doc (Size: 256 KB / Downloads: 86)

The circuit illustrated below is used as an electronic letter box. When letter is kept in this box 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 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 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.
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 Diagram of Electronic Letter Box
Component List :

1) LED 3 pc's
2) LDR 1pc
3) Resistance 470 Ohms 3pcs
4) Variable resistance/preset 100 K
5) IC 555
6) Battery 9V with Snap
7) Switch
LED= Light Emitting Diode
LDR= Light Dependent Resistance
IC = Integrated Circuit

555 Features

- Direct replacement for SE555/NE555
- Timing from microseconds through hours
- Operates in both astable and monostable modes
- Adjustable duty cycle
- Output can source or sink 200 mA
- Output and supply TTL compatible
- Temperature stability better than 0.005% per °C
- Normally on and normally off output
- Available in 8-pin MSOP package
555 Applications
- Precision timing
- Pulse generation
- Sequential timing
- Time delay generation
- Pulse width modulation
- Pulse position modulation
- Linear ramp generator
Post: #2
Please can i get a full report on the literature review?
My supervisor asked me to cite the authors i used their material and i really need my defence is coming up 15th of November please i need an assistance.
Post: #3
The circuit described here can be used as an electronic mailbox. When someone releases a letter in their mailbox, the LED connected to the output stops and tells you that it has a letter. To detect the presence of a letter, we used an LDR and a light source. It has a resistance of approximately 1 megohm in total darkness and 2-5kohm in bright light. In this, the light source and LDR is so tight in the mailbox that the light will fall directly on LDR, so that when the letter falls into the box it will block the beam of light and LDR will be in darkness. The circuit gives you a visual alarm when someone throws a letter into your mailbox. This electronic mailbox circuit is based on the operational amplifier LM741, CD4001 and LDR. An operational amplifier produces an output voltage that is hundreds of thousands of times greater than the voltage difference between the input terminals.

Electronic Mailbox Circuit Diagram:

[Image: Circuit-Diagram-of-Electronic-Letter-Box.jpg]

Circuit Components:

• IC
• LM741-1
• CD4001-1
• Resistors
• R1 (2.2M) - 1
• R2 (560K) - 1
• R 3, R 4, R 5 (1 K) - 3
• R6 (220k) - 1
• R7 (220E) - 1
• C1, C2 (.1uF) - 2
• Various
• LED1 - 2
• Vr1 (10K) - 1
• LDR - 1

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