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What is LEDs? Short note

LEDs are special diodes that emit light when connected in a circuit. Light emitting diodes (LEDs) are semiconductor p-n junction operating  under proper forward biased conditions and are capable of emitting external spontaneous radiations in the visible range (370nm to 770 nm ) or they nearly ultraviolet and infrared regions of the electromagnetic spectrum.
Circuit Symbol

Advantages of LEDIt has a simple fabrication since there are no mirror facets and in some structure no striped geometry.The simple construction of LED leds to much reduced cost.It is reliable as it does not exhibit catastrophic degradation.It has simple drive circuitry due to generally lower drive currents and reduced temperature dependence. The LED has a linear light output against current characteristics. Disadvantages of LEDLower optical power coupled into a fiber.Usually lower modulation bandwidth.Harmonic distortion.

What is RAM (Random access memory)?

RAM is a type of data storage used in computers that is generally located on the motherborard. RAM is one of the most important components of not only desktop PCs, but laptops, tablets, smartphones, and gaming consoles.
A memory unit is collection of storage cells together with associate circuits needed to transfer information in and out of the memory device. The time it takes to transfer information to or from any desired location is always same hence it is called random access memory abbreviated RAM.
Types of Memory
Memory is two types of their modes of access of a memory system.
1       Random access memory:-The word of data reading or writing from or to the memory requires same time. We can access the data randomly.
2.       Sequential access memory-:- The information stored in some medium is not immediately accessible but is available at certain intervals of time. The time it takes to access a word depends on the position of the word with respect the reading head position: therefore the access time variable.
a.       Static RAM (SRAM)
It consist of internal latches that store the binary information. The stored information remains valid a long as power is applied to the unit.
b.      Dynamic RAM (DRAM)
The dynamic RAM (DRAM) stores the binary information in the form of electric charges on capacitors. The capacitors are provide inside the chip by MOS transistor. The stored charge on the capacitors tends to discharge with time and the capacitors must be periodically recharged by refreshing the dynamic memory.
Read only Memory (ROM) it is a non-volatile memory, which retains the data even when power is removed from this memory. Programs and data cannot be altered are stored in ROM. The required paths in a ROM may be programmed in four different ways.
a.       Mask programming: it is done by the company during the fabrication process of the unit. The procedure for fabricating a ROM requires that the customer fills out the truth table be wishes the ROM to satisfy.
b.      Programmable Read Only Memory (PROM): PROM contain all the fuses intact giving all 1’s in the bits of the stored words. The fuses in the PROM are blown by application of high voltage pulse to the device through a special pin. A blown fuse defines binary 0 state and an intact fuse give a binary 1 state. This allows the user to program the PROM by using a special instruments called PROM programmer.
c.       Erasable PROM (EPROM):  In a PROM or ROMs once fixed pattern is permanent and can-not be altered. Once bit pattern has been established, the unit must be discarded of the bit pattern is to be changed.
d.      Electrically Erasable PROM (EEPROM)  it is similar to EPROM except that the previously programmed connections can be erased with an electrical signal instead of ultraviolet light. The advantage is that the device can be erased without removing it from its socket.


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