Optical memory storage devices are high density data storing devices. They differ from magnetic storage devices and S-RAM, RAM, EPROM, PROM in using optical pick up for reading and willing back data in disc. CD recording method makes use of optical recording using a beam of light from a miniature semiconductor larger to form pits and lands. The beam falls on the surface of CD through the pits and lands forms a difference inflected rays avoiding to the pits and lands, the intensity of the laser changes and thus we can recognize binary bits.

The first optical data storage device developed was CD-ROM in 1985. Low storage cost and faster access time, the first read only optical disc to become available was the optical video disc in 1978 followed by compact audio disc in 1980. They are highly cost effective high capacity mass storage medium besides being less loss due to mishandling and accidental demagnetization .They employ powerful error detection and correction mechanisms offering the highest reliability compared to any other storage technique used today .CD ROM is a 120mm diameter or 12cm diameter with 1.2mm thickness with motion of pick up from center towards rotational speed of CD-ROM varies tom 200 to 530rpm.

The triggering circuit makes use of 555 timer in the monostable mode. The 555 timer is highly stable device for generating accurate time delay or oscillation. Signetics Corporation first introduced this device as a SE555/NE555 and it is available in two-package style, 8-pin circular style, TO-99 can or 8-pin mini DIP or as 14-pin DIP. The 556 timer consist of two 555 timers and is a 14-pin DIP. There is also available counter timer such as Exam's XR-2240 which contains a 555 timer plus a programmable binary counter in a single 1pin package. A single 555 timer can provide time delay ranging from microseconds to hours whereas counter timer can have maximum timing range of days.

The 555 timer can be used with supply voltage in the range +5 V to +18V and can drive load up to 200mA.It is compatible With both TEL and CMGs logic circuits. Because of the widen range of supply voltage, the 555 timer is versatile and easy to various applications. Various applications include oscillator, pulse generator, ramp and square wave generator, monoshot, mutivibrator, burglar alarm, traffic light control and voltage monitor etc....

A counter is a sequential circuit consisting of a set of flip-flops which go through a sequence of states on the application cal clock pulses. Counters form important building blocks in digital systems. They are used for counting the occurrence of events, frequency division and generating timing sequence to control operations in a digital system. The counters can be constructed using flip-flops. All the flip flops may be changed the state at the trailing edge of the clock pulse.

555 Timer Introduction

The timer type. 555 is the most versatile linear IC. Sign Tie corporation as SE/ NE 555 first introduced it. The device is used in number of useful applications. Some typical applications monostable, and astable multivibrator, dc-dc converter, digital logic probes, waveform generator, analog frequency meter and tachometers, temperature measurement and control, infrared transmitters, burglar and toxic gas alarms, voltage regulators etc..

The device type 555 is monolithic timing circuit that can produce accurate and highly stable time delay or oscillations like general purpose operational amplifier, the 555 timer is reliable, easy to use and economical.The timer 555 is available as 8-pin metal can, as 8-pin mini DIP a 14-pin DIP.