555 Timer

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This is a simple and easy to use gadget which not only testes the IC 555 timer in its entire basic configuration but also tests the functionally of each pin of the timer. Once a timer is declared fit by this gadget, it will function satisfactorily in whatever mode or configuration you may try it.

The circuit thus can be used to check:

  1. The timer IC in astable configuration.


  1. ?the timer IC in monostable configuration??
  1. The capability of reset terminal to over ride all function and rest the output to low.


  1. The function of the control terminal to change the ?on? or ?high? time of output wave form in astable mode of operation and the output pulse width in monostable mode of ?operation.

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IC 555 TIMER ?

The 555 is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. The IC was designed and invented by Hans R. Camenzind. It was designed in 1970 and introduced in 1971 by Signetics (later acquired by Philips). The original name was the SE555/NE555 and was called "The IC Time Machine". It is still in wide use, thanks to its ease of use, low price and good stability. As of 2006, 1.5 billion units are manufactured every year.

The 555 timer is one of the most popular and versatile integrated circuits ever produced. It includes 23 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8). The 556 is a 14-pin DIP that combines two 555s on a single chip. The 558 is a 16-pin DIP that combines four, slightly modified, 555s on a single chip (DIS & THR are connected internally; TR is falling edge sensitive instead of level sensitive). Also available are ultra-low power versions of the 555 such as the 7555 and TLC555. The 7555 requires slightly different wiring using fewer external components and less power.

The 555 has three operating modes:

Monostable mode:

in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, Frequency Divider,Capacitance Measurement, Pulse Width Modulation (PWM) etc.

Astable mode :

Free Running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.

Bistable mode:

the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.


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A Resistor is a heat-dissipating element and in the electronic circuits it is mostly used for either controlling the current in the circuit or developing a voltage drop across it, which could be utilized for many applications. There are various types of resistors, which can be classified according to a number of factors depending upon:

  1. ?????Material used for fabrication
  2. Wattage and physical size
  3. Intended application
  4. Ambient temperature rating
  5. Cost

Basically the resistor can be split in to the following four parts from the construction viewpoint.
(1) Base
(2) Resistance element
(3) Terminals
(4) Protective means.
The following characteristics are inherent in all resistors and may be controlled by design considerations and choice of material i.e. Temperature co?efficient of resistance, Voltage co?efficient of resistance, high frequency characteristics, power rating, tolerance & voltage rating of resistors. Resistors may be classified as

  1. Fixed
  2. Semi variable
  3. Variable resistor.

In our project carbon resistors are being used.



The fundamental relation for the capacitance between two flat plates separated by a dielectric material is given by:-

Where: -

????????? C= capacitance in pf.
K= dielectric constant
A=Area per plate in square cm.
D=Distance between two plates in cm


????????? Design of capacitor depends on the proper dielectric material with particular type of application. The dielectric material used for capacitors may be grouped in various classes like Mica, Glass, air, ceramic, paper, Aluminum, electrolyte etc. The value of capacitance never remains constant. It changes with temperature, frequency and aging. The capacitance value marked on the capacitor strictly applies only at specified temperature and at low frequencies.?

 LED (Light Emitting Diodes):

As its name implies it is a diode, which emits light when forward biased. Charge carrier recombination takes place when electrons from the N-side cross the junction and recombine with the holes on the P side. Electrons are in the higher conduction band on the N side whereas holes are in the lower valence band on the P side. During recombination, some of the energy is given up in the form of heat and light. In the case of semiconductor materials like Gallium arsenide (GaAs), Gallium phoshide (Gap) and Gallium arsenide phoshide (GaAsP) a greater percentage of energy is released during recombination and is given out in the form of light. LED emits no light when junction is reverse biased.


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