Relays are electromechanical devices that use an electromagnet to operate a pair of movable contacts from an open position to a closed position. The advantage of relays is that it takes a relatively small amount of power to operate the relay coil, but the relay itself can be used to control motors, heaters, lamps or AC circuits which themselves can draw a lot more electrical power.
Resistor:
Resistors resist the flow of electrical current. A voltage flowing through a resistor will limit the current proportional to the value of the resistor (see Ohm’s law). The bands on a resistor indicate the resistor’s value. Symbol and components shown in fig.A
Inductor:
An inductor, also called a color reactor, is a passive two-terminal electrical component which resists changes in electric current passing through it. It consists of a conductor such as a wire, usually wound into a coil. When a current flows through it,energy is stored temporarily in a magnetic field in the coil. When the current flowing through an inductor changes, the time-varying magnetic field induces a voltage in the conductor, according to Faraday's law of electromagnetic induction, which opposes the change in current that created it. Symbol and components shown in fig.B
Capacitor:
Capacitors store an electrical charge for a short time and are used in digital circuits to filter (smooth out) dips and spikes in electrical signals. The most commonly used capacitor is the non polarized ceramic capacitor; for example, a 100 nF disc capacitor used for decoupling (reducing noise spikes). Electrolytic capacitors can generally store more charge than ceramic caps and are used for higher current circuits, such as power supplies and motor circuits. Electrolytic capacitors are usually polarized, and the negative leg (marked with a minus sign) must be connected to ground (or to a point with lower voltage than the positive leg). Chapter 8 contains examples showing how capacitors are used in motor circuits. Symbol and components shown in fig.C
Diode:
Diodes permit current to flow in one direction and block it in the other direction. Most diodes have a band to indicate the cathode (negative) end. Diodes such as the 1N4148 can be used for low-current applications such as the levels used on Arduino digital pins. The 1N4001 diode is a good choice for higher currents (up to 1 amp). Symbol and components shown in fig.D
LED:
An LED (light-emitting diode) is a diode that emits light when current flows through the device. As they are diodes, LEDs only conduct electricity in one direction. Symbol and components shown in fig.F
Transistor:
Transistors are used to switch on high currents or high voltages in digital circuits. In analog circuits, transistors are used to amplify signals. A small current through the transistor base results in a larger current flowing through the collector and emitter. For currents up to .5 amperes (500 mA) or so, the 2N2222 transistor is a widely available choice. For currents up to 5 amperes, you can use the TIP120 transistor. Symbol and components shown in fig.E
