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Diodes
Function
Diode characteristic Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.

Forward Voltage Drop
Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon.

Light Dependent Resistor (LDR)

         An LDR is an input transducer (sensor) which converts brightness (light) to resistance. It is made from cadmium sulphide (CdS) and the resistance decreases as the brightness of light falling on the LDR increases.

A multimeter can be used to find the resistance in darkness and bright light, these are the typical results for a standard LDR:

    Darkness: maximum resistance, about 1Mohm.
    Very bright light: minimum resistance, about 100ohm.

Transistors

Function
1.Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.
2.A transistor may be used as a switch

Types of transistor
  There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors.

  The leads are labelled base (B), collector (C) and emitter (E).

Transistor codes
   There are three main series of transistor codes used in the UK:
   Codes beginning with B (or A)
   The first letter B is for silicon, A is for germanium

Transistor Pin

Relay

Relay

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.
The relay's switch connections are usually labelled COM, NC and NO:

COM = Common, always connect to this, it is the moving part of the switch.
NC = Normally Closed, COM is connected to this when the relay coil is off.
NO = Normally Open, COM is connected to this when the relay coil is on

Choosing a relay
You need to consider several features when choosing a relay:

    1.Physical size and pin arrangement
    If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue.
    2.Coil voltage
    The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.
    3.Coil resistance
    The circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current:
    Relay coil current   =        supply voltage / coil resistance
     4. Switch ratings (voltage and current)
The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".
    5.Switch contact arrangement (SPDT, DPDT etc)
Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO).

Protection diodes for relays

Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected 'backwards' across the relay coil to provide this protection.

Relay Pin

 

Resistors

Resistors

Function
     Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.

Connecting and soldering
     Resistors may be connected either way round.
     
    
Colour Code
  Colour    Number
  Black          0
  Brown        1
  Red            2
  Orange       3
  Yellow        4
  Green         5
  Blue           6
  Violet         7
  Grey          8
  White        9

Transformer

The input coil is called the primary and the output coil is called the secondary. There is no electrical connection between the two coils, instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core.

Transformers waste very little power so the power out is (almost) equal to the power in. Note that as voltage is stepped down current is stepped up.

The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.

  turns ratio =     Vp/Vs      =     Np /Ns       and       
  power out = power in Vs × Is = Vp × Ip
Vp = primary (input) voltage
Np = number of turns on primary coil
Ip  = primary (input) current             Vs = secondary (output) voltage
Ns = number of turns on secondary coil
Is  = secondary (output) current