A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays".

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2.  Led dot matrix
 Relays
 Interfacing with atmega16
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5.  LEDs are arranged in rows and columns in a matrix format.
 we can address each LED by row and column.
 Led matrices are of types. Common-row anode and common-row cathode.
With common-row anode current sources (positive voltage) are attached to
rows and currents sink (negative voltage, ground) to columns.
 With common-row cathode current sinks are attached to rows and currents
source to columns

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6.  A relay is an electromechanical device that uses small electrical currents and
voltages to control larger electrical currents and voltages.
 The heart of a relay is an electromagnet (a coil of wire that becomes a
temporary magnet when electricity flows through it).
 Relays bridge the gap, making it possible for small currents to activate
larger ones.
 That means relays can work either as switches (turning things on and off) or
as amplifiers (converting small currents into larger ones).
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8.  When power flows between points V1 and V2,it activates
magnetic field in the coil.
 The magnetized coil attracts the contact which then
activates the second circuit.
 When the power is switched off, a spring pulls the contact
back up to its original position, switching the second circuit
off again.
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