30-06-2012, 12:52 PM
SEMINAR ON 555 TIMER
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Introduction
The 555 timer is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. It was produced by Signetics Corporation in early 1970. The original name was the SE555/NE555 and was called "The IC Time Machine". The 555 gets its name from the three 5-KΩ resistors used in typical early implementations. It is widely used because of its ease to use, low price and reliability.
It is one of the most popular and versatile integrated circuits which can be used to build lots of different circuits. 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)(Refer to Figure 1).
The 555 Timer is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillations. The timer basically operates in one of the two modes—monostable (one-shot) multivibrator or as an astable (free-running) multivibrator. In the monostable mode, it can produce accurate time delays from microseconds to hours. In the astable mode, it can produce rectangular waves with a variable duty cycle. Frequently, the 555 is used in astable mode to generate a continuous series of pulses, but you can also use the 555 to make a one-shot or monostable circuit.
ASTABLE MULTIVIBRATOR
We now take up the application of 555 timer as an astable multivibrator. An astable multivibrator is a wave-generating circuit in which neither of the output levels is stable. The output keeps on switching between the two unstable states and is a periodic, rectangular waveform. The circuit is therefore known as an ‘astable multivibrator’. Also, no external trigger is required to change the state of the output, hence it is also called ‘free-running multivibrator’. The time for which the output remains in one particular state is determined by the two resistors and a capacitor externally connected to the 555 timer.
MONOSTABLE MULTIVIBRATOR
We now discuss another important application of 555 timer, that is, 555 timer as a monostable multivibrator. A monostable multivibrator is a pulse-generating circuit having one stable and one quasi-stable state. Since there is only one stable state, the circuit is known as ‘monostable multivibrator’. The duration of the output pulse is determined by the RC network connected externally to the 555 timer. The stable state output is approximately zero or at logic-low level. An external trigger pulse forces the output to become high or approximately. After a predetermined length of time, the output automatically switches back to the stable state and remains low until a trigger pulse is again applied. The cycle then repeats. That is, each time a trigger pulse is applied, the circuit produces a single pulse. Hence, it is also called ‘one-shot multivibrator’.