Introduction to Electronics

L.I.T.

November 2009

Power (DC , AC)Resistors

CapacitorsDiodes

TransistorsLDR

TermistorsRelays

SwitchesMotor Control

Handling ComponentsTesting

Resistors come in different ranges such as E12 10% tolerance and E24 5% tolerance The E12 series has 12 values for each multiple of ten i.e.10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, .then it continues 100, 120, 150 etc.

The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their 'tolerance'). For example a resistor marked 390 could vary by ±10% × 390 = ±39, so it could be any value between 351 and 429.For most circuits the standard power ratings of 0.25W or 0.5W are suitable.

Capacitor types

Ceramic diskMonolithic ceramicDipped silvered-micaMylar or polyesterAluminum electrolytic (+/-)Tantalum (+/-)

Ceramic disk Monolithic ceramic Dipped siver-mica Mylar Mylar

Solid tantalum, polarized Radial aluminum electrolytic Axial aluminum electrolytic

Types of CapacitorPolarised (for large values, 1μF or more) andUnpolarised (for small values, up to 1μF).

Electrolytic capacitors are polarised and they must beconnected the correct way round - at least one of theirleads will be marked + or -.The value of electrolytic capacitors is usually printed, with

voltage rating, on the capacitor

Small value capacitors are unpolarised and maybe connected either way round.It can be difficult to find the values of these smallcapacitors because there are many types of themand several different labelling systems 4n7 means 4.7nF 0.1 means 0.1μF Labelling System:The 1st number is the 1st digit of the value. The 2nd number is the 2nd digit of the value. The 3rd number is the number of zeros to give thecapacitance in pF. 102 means 1000pF

Diodes

Diodes are semiconductor devices that allow current to flow in one direction.

Anode Cathode

Light-emitting diode (LED)

Rectifier diode

LEDS

An LED is a diode, so current will flow in one direction through it, but not in the other direction. When an LED is 'forward biased' it will light and there will be a voltage drop of around 0.7V across it. When an LED is 'reverse biased', current will not flow through it and it will not light. It is this property that enables a bi-colour LED to have only two leads. Most LEDs require a forward bias voltage of around 2V and consume a current of around 20mA, but you should check the data available for the type of LED you are using to determine its exact voltage and current consumption.

To supply an LED with 2V at 20mA, you simply need to place a current-limiting resistorin series with it as shown in. The resistor value can be calculated using Ohm's law...

...where R is the resistor value in Ohms (Ω), and Vs is the voltage of your power supplyin Volts (V). If you have different values for the forward bias voltage and current, then substitute them for the 2V and 0.02A values.

Which Way Around?

How can you tell which lead on an LED is which?If brought new, you will find that the longest lead is the positive lead, or'anode', and the shortest lead is the negative lead, or 'cathode'.

What if the leads have been cut?

The plastic package nearest the cathode will be flattened slightly.

When holding the LED up to a light, the largest section inside will benearest the cathode.

You can also connect up the LED both ways around (with resistor) tosee which is correct - reverse biasing the LED won't do it any harm.

Led used as a sensor

TransistorsThree terminal devices manufactured

in a variety of package styles.

2N2222 in a TO-92 package 2N2222A in a

TO-18 package2SC2078 in a TO-220 package

Transistors• 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.

• A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

Transistor types• 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).

Connections

• Please note that transistor lead diagrams show the view from below with the leads towards you.

Testing a transistor

Use a multimeter to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range. Test each pair of leads both ways (six tests in total): •The base-emitter (BE) junction should behave like a diode and conduct one way only. •The base-collector (BC) junction should behave like a diode and conduct one way only. •The collector-emitter (CE) should not conduct either way. The diagram shows how the junctions behave in an NPN transistor.

• 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 1 Mohm • Very bright light: minimum resistance, about 100 ohm • An LDR may be connected either way round and no

special precautions are required when soldering.

• Thermistor• A thermistor is an input transducer (sensor) which converts

temperature (heat) to resistance. Almost all thermistors have a negative temperature coefficient (NTC) which means their resistance decreases as their temperature increases. It is possible to make thermistors with a positive temperature coefficient (resistance increases as temperature increases) but these are rarely used. Always assume NTC if no information is given.

• A multimeter can be used to find the resistance at various temperatures, these are some typical readings for example:

• Icy water 0°C: high resistance, about 12k. • Room temperature 25°C: medium resistance, about 5k. • Boiling water 100°C: low resistance, about 400 ohm.• Thermistors take several seconds to respond to a sudden

temperature change, small thermistors respond more rapidly. • A thermistor may be connected either way round and no special

precautions are required when soldering.

• The following circuit diagram shows an LDR (light sensor) connected so that the LED lights when the LDR is in darkness. The variable resistor adjusts the brightness at which the transistor switches on and off.

• The 10k fixed resistor protects the transistor from excessive base current (which will destroy it) when the variable resistor is reduced to zero. To make this circuit switch at a suitable brightness you may need to experiment with different values for the fixed resistor, but it must not be less than 1k.

• The switching action can be inverted, so the LED lights when the LDR is brightly lit, by swapping the LDR and variable resistor. In this case the fixed resistor can be omitted because the LDR resistance cannot be reduced to zero.

Relays

• 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

For switching small DC currents (< 1A) at low voltage the transistor is usually a better choice than a relay. However transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed.

A low power transistor may still be needed to switch the current for the coil

Toggle Switch

Toggle switches are available with many contact configurations.

Those with three terminals are called SPDT (single pole, double throw).

If the switch is designated as "N.O.", then it is a normally open contact;

"N.C." indicates that it is a normally closed contact.

Dual ON-ONDouble Pole, Double Throw = DPDT A pair of on-on switches which operate together (shown by the dotted line in the circuit symbol).

ON-OFF-ONDPDT Centre OffA special version of the standard SPDT switch. It has a third switching position in the centre which is off. This can be very useful for motor control because you have forward, off and reverse positions.

Micro Switch /Limit Switch

The end of the lever can be plain or contain a small wheel(roller end) depending upon the intended use of the switch. The plain end is used where a rubbing or physical contact is required. The roller end is used when the end of the switch must roll on a surface.

Reed SwitchA reed switch consists of two or three springymetal reeds having plated, long-life contacts at thetips and encapsulated in a sealed glass tube.The two-reed type has normally open (NO)contacts which close when operated, and thethree-reed type is a changeover, i.e. it has a pairof normally open (NO) and a pair of normallyclosed (NC) contacts. When the switch isoperated, both these pairs change to the oppositestate.Reed switches are normally actuated by the field from anexternal permanent magnet or electromagnet placed inclose proximity.

Reed switches are rather fragile, particularly if you aresoldering them. It is easy to break the glass and seals. Ifyou need to bend the lead-out wires, make sure that yougrip them securely with pliers between the glass seal and the bend point.

Relays

Switch Types

Motor Control using DPDT

Limit Switches to Stop Motion

Direction Control with Relays and Limit Switches

Motor Speed Control(Not Good)

DC Motor Speed Control

When VR1 is at mid position, the op-amp output is near zero and both Q1 and Q2 are OFF. When VR1 is turned towards the positive supply side, the output will go positive and Q1 will supply the current to the motor and Q2 will be OFF As the potentiometer VR1 is moved toward either end, the speed increases in whichever direction it is turning (TIP3055 Q1 NPN MJE34 Q2 PNP )

A breadboard is used to make up temporary circuits for testing or to try out an idea.

No soldering is required so it is easy to changeconnections and replace components.

Parts will be available to re-use afterwards.

Breadboards have many tiny sockets (called 'holes') arranged on a grid.

The leads of most components can be pushed straight into the holes.

ICs are inserted across the central gap with their notch or dot to the left.

The top and bottom rows are linked horizontally all the way across as shown by the red and black lines on the diagram. The power supply is connected to these rows, + at the top and 0V (zero volts) at the bottom

The other holes are linked vertically in blocks of 5 with no link across the centre as shown by the blue lines on the diagram. Notice how there are separate blocks of connections to each pin of ICs.

Electrostatic Discharge (ESD) Protection

Ground your work surfaceUse an anti-static mat

Ground your tools (i.e., soldering iron)Many irons are constructed with a grounded tip

Ground yourselfUse a wrist or ankle strap, but always include a series

resistor of high value to avoid any shock hazard.Touch a grounded object before handling staticsensitive components.

Resistor handling and installation

Resistors are not polarized and may be installed in either direction.

Resistors are not generally susceptible to ESDdamage, so special precautions are notrequired.

Mechanical stress due to lead bending shouldbe minimized.

Capacitor handling and installation

Most capacitors are not polarized and may be installedin either direction.

Electrolytic capacitors ARE polarized and MUST beinstalled with proper polarity, else it may explode.

Capacitors are not generally susceptible to ESDdamage, so special precautions are not required.

Mechanical stress due to lead bending should be minimized.

Diode handling and installation

Diodes are polarized and must be installed with correct orientation.

Many diodes are modestly susceptible to ESDdamage, so normal ESD precautions should be taken.

Mechanical stress due to lead bending should beminimized.

Transistor handling and installation

Transistors are polarized and must be installed withcorrect orientation.

Most transistors are modestly susceptible to ESDdamage, so normal ESD precautions should be taken.

Mechanical stress due to lead bending should beminimized.

Electronic Sites on Net

Electronics Club http://www.kpsec.freeuk.com/

http://people.sinclair.edu/nickreeder/flashgames.htmNick Reeder has programs on meters, scopes etc