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                          Teach Yourself Electronics

                             By John Ilsley (27N).

                          --------------------------

 

         Welcome to the first in this short series of teaching yourself how to

do electronic work. This very first part will deal with the tools required and

how to use them, parts and components and how to evaluate them or read their

values. It will also deal with schematic component symbols and what they

mean... Ok, now that all seems a lot to start off with, but it is really the

very, very basics of what you need to know.

 

         First, the tools. The most important is a soldering iron. This

needs to be of between 15-30 watts for good performance. You will also need a

pair of cutters and a small screwdriver, a pair of pincers or tweezers and a

roll or tube of 22swg gauge 40/60 solder. The 40/60 just represents the mixture

of tin and alloy. However, make sure the solder you buy is fluxed as this will

help clean both the component and circuit board. You will also need to consider

buying a desoldering pump, as to start with I expect you to make a few

mistakes. All this will cost you about 10, but once you have all this, you

will never need to buy it again. One last item you will find very useful is a

test meter, these range from 5 to over 100. I have a 36 one, but a simple

analogue (needle) one will do. It should have voltage in DC and AC up to about

1000vDC and 500vAC. Resistance from 1 ohm to 2megaohms. These are the two

essential items on a simple test meter. I will be showing you how to make a

circuit tester later on.

 

         The components you will most likely use are resistors, capacitors,

transistors, LEDs, IC chips and diodes. Other components will be used, but

there are so many, to explain each would take up an entire disc and more.

So I will expalin these rather briefly.

 

                                   Resistors

                                   ---------

 

         Resistors. These are one of the easiest to recognise, and the easiest

to read. Most resistors have four coloured bands around them. To read a

resistor, you look at the colour of the bands and turn the resistor so that

the silver or gold band is to your right. You will then have three colours

from left to right.

                      ___________            Schematic for resistor

             ________:           :________

                     :___________:

 

Colour   Band1 : Band2 : Band3                  Example (see text below).

 

BLACK  =   0   :   0   :   none      : Brown/Blue/Black = 16 ohms

BROWN  =   1   :   1   :   0         : Red/Green/Brown = 250 ohms

RED    =   2   :   2   :   00        : Brown/Green/Red = 1500 ohms or 1.5k

ORANGE =   3   :   3   :   000       : Blue/Red/Orange = 62000 ohms or 62k

YELLOW =   4   :   4   :   0000      : Red/Red/Yellow = 220000 ohms or 220k

GREEN  =   5   :   5   :   00000     : Brown/Grey/Green = 1800000 ohms or 1.8m

BLUE   =   6   :   6   :   000000    :

VIOLET =   7   :   7   :   0000000   :

GREY   =   8   :   8   :   00000000  :

WHITE  =   9   :   9   :   000000000 :

 

         The first band gives the first value, the second the second value then

the third band tells you how many times the first two bands must be multiplied

by. Take the examples above. Starting with the first, I have put Brown Blue and

Black. This means :-

         (Brown) 1 (Blue) 6 (Black) 0 = 16*(no zeros) = 16 ohms

         (Red) 2 (Green) 5 (Brown) 1  = 25*(1 zero)   = 250 ohms

         (Brown) 1 (Green) 5 (Red) 2  = 15*(2 zeros)  = 1500 ohms or 1.5 k

         (Blue) 6 (Red) 2 (Orange) 3  = 62*(3 zeros)  = 62000 ohms or 62 k

         (Red) 2 (Red) 2 (Yellow) 4   = 22*(4 zeros)  = 220000 ohms or 220 k

         (Brown) 1 (Grey) 8 (Green)   = 18*(5 zeros)  = 1800000 ohms or 1.8 m

 

         The 'ohm' is the smallest value in resistance, after you reach 999

ohms you use the 'k' (kilohms) until after you reach 9999 where you then go

onto usually the highest value, 'm' (megaohms) which will take you upto 9,999.

After that you go onto 'g' (gigaohms)... But you will never use (or be likely)

to buy any value over 50mohms.

 

         The fourth band is the tolerance of the resistor. This is done in

percentages from 1% to 20%. These are:-

         BLACK  = 1%

         BROWN  = 2%

         GOLD   = 5%

         SILVER = 10%

Absence of band = 20%

 

         So lets say a resistor has the colours BROWN, BLACK, RED. The value

1000 ohms or 1k and the fourth band is gold. This means that the resistor can

really be between 995ohms and 1005ohms. Same resistor, but the fourth band is

now missing, this means it's real value could be between 980ohms and 1020ohms.

Neither of those differences really sounds much, but you can add or lose the

difference in the circuit, and while the 5% tolerance wouldn't make much

difference, the 10% or 20% is a lot, especially on the higher value

resistances. Almost all circuits will withstand 5%, but if you can get and

afford it, then use the 1% or 2% values.

 

         Resistors also have one more property. This is the wattage they can

handle. This can be determined most times by their size. You can get 1/4 watt

resistors which are used in personal hi-fis and are only a couple of

millimeters long and about one millimeter thick, to 1/2 watt which are found in

most appliances like computers and stereos and such. They are about 2/3rds of a

centimeter long and about 3-4 mm thick. Then you can go all the way up to 1

watt resistors which are about 2 centimeters long and 1 centimeter thick. The

value can be found on the pack that you buy the component in.

 

         Last thing you need to know about resistors is that the two leadout

wires have no requirements to positive or negative and so can be fitted either

way round in the circuit. They don't so much control voltage as much as they

control how much current they allow through. So if you're using a 9 volt

battery, and you want only 2 volts to light an LED, then you could use a 1k or

1.5k or 2.2k resistor of 1/4 or 1/2 a watt. You can work out voltage current

and watts using my program two issues ago, in case you have lost it, here is

the basic way of mathematically finding it out. NB: Current and Amps are the

same.

 

Volts   *  Amps       = Watts  (eg. 12 volts  *  3 amps = 36 Watts )

Amps    *  Resistance = Volts  (eg. 3  amps   *  4 ohms = 12 Volts )

Volts   /  Resistance = Amps   (eg. 12 volts  /  2 ohms = 6  Amps  )

 

         Resistors can be connected in series (one after the other) or in

parallel (one end all together the other ends separated). In series, the value

of each is added together giving a total resistance. In parallel, the value of

each is its total.

        

         That is basically all you need to know about resistors. There is quite

a lot of information there, but it's all very useful to the newcomer. If you

take time to learn the values then you will find it a lot easier to design your

own circuits later on.

 

                                  Capacitors

                                  ----------

 

         Capacitors come in a various assortment of shapes sizes and colours,

from round fat things to round thin things and round long things to round short

things and square things to rectangular things, green, yellow, grey, I expect

somewhere there is one that is pink and has purple spots on it! I haven't found

that one yet. There are also ceramic, carbonate, polystyrene, polyester and the

list goes on. As I won't be able to explain and describe every type, I will

stick with the two most common that I use.

 

         The first type is the Electrolytic type. Two forms of this are

avaliable. The first is a round upright type with two legs coming out of it's

base and the other is a flat type which has one leg coming from each end. It is

important that these legs are connected the correct way round to positive and

negative. You can find the negative leg on the upright type by looking at the

side of the capacitor for a (-) negative sign or a black strip. The leg nearest

this is negative. On the flat type, there is a black band around one end. This

is the negative end.

 

            (thin line) ::  ## (thick line)   Schematic for Electrolytic type

 

            ::  ##

   +  ______::  ##______  -

            ::  ##

            ::  ##

 

            :: ::

 +/- _______:: ::_________ +/-      Schematic for Ceramic and most other types

            :: ::

            :: ::

         

         The value of this is always shown on the side. First the voltage, it

can range from 1 volt to around 50 volts. They have another value which is

measured in Farads or (F) One farad is too large to be of any real use, so it

has been split into four values: F=Farad, uF=Microfarads, nF=Nanofarads and pF=

Picofarads. Working with these values is for the newcomer very daunting as some

very large numbers are involved, but please don't let it put you off.

 

         To translate between the four, here is how you do it. The highest

value to the lowest value avaliable.

 

1 F  = 1,000,000 uF or 1,000,000,000 nF or 1,000,000,000,000 pF

1 uF = 1,000 nF or 1,000,000 pF

1 nF = 1,000 pF

1 pF = 1 pF

 

         As I'm sure you will agree, having the same value twice in that

example is a little strange to say the least, so I've tried to make it simpler.

 

         To convert from uF to nF, move the decimal point three places right.

If there is no second number, then the second and third number is replaced by a

zero, (Example 0.1uF = 100nF). If there is a second number but no third, then

the third becomes a zero (example 0.22uF = 220nF)

 

         To convert from nF to pF, move the decimal point three places right.

If there is no second number then the second and third numbers become zeros,

(example 4.7nF = 4700pF). Otherwise you add on three zeros (example 10nF =

10000pF or 22nF = 22000, 220nF = 220000pF)

 

         To convert the other way round, simply move the decimal point the

appropriate number of places to the left instead.

 

        Now I'm sure you will take time to figure that out. Although it may not

seem to make sense, once you have studied it and a few capacitors and their

values, and done some pen and pencil work, it will 95% of the time come out

correct. I still make mistakes, and I've been doing electronics since I was 11

years old, I'm now 21, so ten years.

 

         The tolerance of the capacitor is not normally needed and so hardly

ever printed. However, the voltage that is written on it MUST be higher than

the power applied to it. So if your circuit requires 9 volts, then the

capacitor will have to be 10 volts (being the nearest value higher than 9

volts) to anything higher, even a 50 volt capacitor will be ok, but a 6 volt

capacitor would without a doubt start smouldering, give out a toxic gas, maybe

explode and expire with a hiss....

 

         If you asked for a 100uF 25v capacitor you may get a 100uF capacitor

with either 25volts or 35 volts or more written on it. If you asked for a

0.22uF 25volts, you would be more than likely to get a disc type capacitor as

the lowest value for the electrolytic type is .1uF. You must remember that the

electrolytic type must be placed in the circuit with the negative leadout wire

at the negative side of the circuit and the positive at the positive side.

 

         The other type of capacitor is the disc ceramic. This is as it states

a disc, most times coloured in a dark orangy brown colour. Their physical shape

is round, ranging from 2 millimeters in diameter to 1 centimeter. The thickness

of both being about 2 millimeters. Finding the value of these is hard as it is

not written on them. I have learnt how to tell the difference by looking at the

size. The smaller the value, say 0.001pF the smaller the size as opposed to

0.01uF which although similar in shape and colour, is slightly larger. If you

state ceramic when buying, then this is what you will get. Ask for them in a

serparate marked bag. You will find this helps to teach you.

 

         These disc ceramic capacitors have no preferance to positive and

negative and most times have a higher working voltage, some up to 150 volts.

This type can be inserted into the circuit either way round.

 

         The job of the capacitor is mainly a smoothing device. A good example

of this is in a transformer, where you might have a capacitor of 6800uF with a

working voltage of 50 volts, or in most cases a 2200uF capacitor of just 25

volts. A capacitor prevents DC (direct current) from passing through it. Yet

allows AC (alternating current) through it. Therefore a capacitor is ideal for

use in a sound amplifier since DC to a speaker doesn't do it any justice.

 

         In a transformer, a capacitor fills in the high peak in the DC, then

in the low peak, the capacitor discharges, filling in the peak, thereby making

an even output. While it is unimportant to have the correct voltage as long as

it's the same as or higher than the voltage specified. It is quite important

to use the same farad value as stated in a schematic. Using a different farad

can sometimes have wierd, bizzare and interesting effects. Other times it

may cause a serious problem, especially where timing circuits or ICs are used.

 

                                    Diodes

                                    ------

 

         These are the easiest to explain and the easiest to understand. Diodes

come in mainly four types. These are Power diodes, Silicon diodes, Zener diodes

and Light emitting diodes (or LED for short).

 

         Power diodes, Silicon diodes and Zener diodes come in an amazing

amount of numbers and sizes, they all do the same job in very slightly

different ways. A diode is easy to recognise, they only really come in

a few forms, these are a black plastic case with a silver or white band, a

glass case with a black band, these are either Silicon or Zener diodes and

finally in a metal case, these are usually fully made up bridge rectifiers

which are used to convert AC to DC.

 

         A diode can be recognised by its one band at one end. The end with the

band is called the Cathode. This end is the negative end and must go to the

negative end of the circuit. The numbers on a diode are a manafactures way of

stating what is the maximum voltage and amps allowed before the diode will

undergo a breakdown. For example, a IN4001 diode will withstand 50 volts

reverse voltage before it breaks down, or allow 1a to flow through it the

correct way.

 

                        :\            Schematic for Silicon and Power diode

             + _________: \__:_____ -

                        : /  :

                        :/

 

                        :\  _         Schematic for Zener diode

             + _________: \__:_____ -

                        : / _:

                        :/

 

         Power diodes and Silicon diodes have the property as in the example

just described, in that they will with stand a maximum 'Back voltage' before

they break down or a maximum forward amperage of so much. The only difference

is the Silicon diode has lower values and is cheaper to make and therefore

cheaper to buy. Power diodes are used to make bridge rectifiers or convert AC

to DC. They can be used to protect a circuit from incorrect power supply

connections like positive being connected to negative and they will also do

all the jobs a Silicon diode will do. Most power diodes cost about two pence

each.

 

         A Silicon diode is mainly used in low voltage low amperage stuff, it

is very versatile, simple and small. It provides a useful job in protecting

transistors and IC chips from back EMF from relays and such, (Back EMF being a

current relayed back to its source during a break in a relay or something being

switched off). They cost about about one and a half pence each.

 

         Testing the above two is very easy, using a tester. Put the (+) probe

to one wire and the (-) probe to the other, then reverse the (+ & -). Only one

way should give a reading, if both ways give a reading, chances are it is

blown.

 

         A Zener diode works in a similar way to Silicon diodes, except that

 

when a voltage is applied in reverse to it, it does not allow current to pass

through it until a minimum voltage is reached, then it will maintain a steady

voltage even if the current is increased. For example if you wanted a steady 9

volt supply from a 12 volt battery, you could use a Zener diode. This example

does work, but without a bit of additional circuitry, I don't advise you

attempting it.

 

         A Light emitting diode or LED for short works in a different way to

the others, and also comes in many different forms. If you look at the red

lights on your computer, the Caps Lock light for example, this is an LED. These

are not really used as voltage controls, though they can be used to indicate

reverse voltage. Depending on what voltage/current is applied, a 1k resistor is

used to cut the voltage from between 6 volts and 20 volts to the 2 volts

required by the LED. Or if only a small current is being used, say 50mAmps,

then a 500 ohm resistor will do. Most LEDs require about 10mAmps or between 1.8

and 2 volts.

 

         Leds come in a various amount of shapes and sizes and are mainly used

to indicate flow of current, say in a power supply to say it's on, or in a

stereo system to tell you if you have tuned into a stereo radio station, or on

the front of your disc drive. They come in many other forms, even 8 segment

displays, like numbers, CB radios or digital clocks. They can be used for

hundreds of items. The first two projects use them.

 

                                   IC Chips

                                   --------

 

         Unfortunately there are just so many intigrated circuit chips, I

wouldn't know where to begin. Some have three legs, some seem to have hundreds

 

of legs, (imagine, a computerised centipede!). There are two main types though.

These are LINEAR ICs and LOGIC ICs. Now I trust you all know what a computer

chip looks like: A black rectangular shape with metal legs at either side of

it with a notch or a tiny bit cut out at one end.

 

         Computer chips are now getting so advanced, it is hard to keep up with

them. There are now a few optical chips, unfortunately these require a fair

amount of technology to change an electrical signal to a light signal then back

again. Very effective over long distances, eg, Cable TV, but not to my

knowledge much use over short distances like inside your computer. Interesting

though.

 

         Some computer chips, do certain jobs, for example, analogue to digital

converters, Eproms (Erasible Programmable Read Only Memory), Timers,

Operational amplifiers, Audio amplifiers, Opto Isolators, Decade counters,

Darlingtons and a whole host of others for various purposes.

 

         A standard chip that you have in your computer will be black in colour

and be of a long, slim rectangular shape with legs protruding from either side

of it. The top or pin one of the IC is found by looking for a notch cut out

from the top center of the chip at either end or by a notch to the left of the

chip at one end, with the cut out notch in the center taking preferance to the

notch to the left, as in this example, the * being the notch to the left and

the :_: being the cut out notch. All other similar notches should be ignored.

                                     _   _

                             1     -:*:_: :-     8

                             2     -:     :-     7

                             3     -:     :-     6

                             4     -:_____:-     5

 

         You are looking at the chip from above as if it was a circuit with the

legs away from you. This type of chip is a DIL (Dual in line) type. Please note

how the pins are numbered. Pin one is to the top left where both notches are,

pin 2,3 and 4 run down that side of the chip, then pin 5 is opposite pin 4, pin

5,6,7 and 8 then run up that side of the chip. The pins run in a similar way on

all other chips for example pin 1 to pin 7 would be on the left hand side, pin

8 to 14 on the right and so on....

 

         There is no specific pin that is positive or negative, no specific

input or output. Unless you know the chip, you have to rely on schematic

diagrams. Like those you will be following over the next few issues.

 

         IC chips are made of silicon (sand) and different types of conductive

material. The chip may include different components (as described above) but

many many times smaller than their larger counterparts. The piece of the chip

that does all the work will only measure about 1.5mm by 0.2mm and contain

enough parts to control at least lets say part of your disc drive. Because

it is so small it has to have legs put at such distance that we can handle it.

For those legs it needs a case, hence the black plastic. The silicon chip is

connected to the outside legs by thin slivers of wire, finer than a human hair.

These are connected to the silicon chip and the legs not by soldering, but by

sound. A high frequency 'noise' bonds the wire to the silicon or the leg. It

melts it on to it to a tiny fraction of the wires thickness.

 

         There are other facts to a chip. These are their power comsumption,

and their current limitations and breakdown features. Again, all chips are

different, but there are two or three things that remain the same. There are

ICs that require a large amount of current or voltage for them to work. These

are the older type of TTL74, 74LS and 74ALS series chips, each requiring less

as you come down the line. The newer faster 74HC and 74HCT series, and another

type of chip. This is the CMOS (Complementary metal oxide semiconductor) range

of chips. They range from the 4000 range onwards. These are VERY VERY

sensitive to static electricity from our body or other sources. Our body can

hold over 1,000 volts! which won't really do any harm to things normally, but

if you touch the legs of any CMOS chip while it is unprotected, you will

destroy it.

 

         How to protect the chip is easy. It will come in a foil clad

conductive package, this will give it a fair amount of protection. To install

it, ensure that you first earth yourself. You can do this by touching say a

bare metal EARTH part of the OUTER case of your disc drive or a tap. If you

have to walk somewhere, it's best to take the entire circuit with you. Another

way is not to touch the pins or legs. You should have already installed a DIL

socket to hold the chip as no chip should really be soldered in, especially

these CMOS chips. Hold the two thin ends of the chip, the notch end and the end

opposite it. Hold the foil down with an insulator, say a piece of paper, then

lift the chip off the foil. Place it in the circuit and push it in carefully.

Ensure it is in, then you can power up your circuit.

 

         This CMOS chip is sensitive, it only requires about 4 volts, more

will blow it. It uses so little current that a 9 volt battery will last for

ages just powering the chip. I've had a circuit running on a CMOS for eleven

months, but I've heard of them running over a year. It will depend on what

other components are connected to the chip. The benefits outweigh the problems.

Some of these chips I've mentioned (74HC and CMOS4001 range) require only

2 volts, while others require a whole 12 volts! The 74HC and CMOS4001 series

chips are a lot faster than the old 74ALS (TTL) range, but unless you are

advanced in electronics this will not concern you for a while at least.

 

         LOGIC chips like computers work on only two voltages, 0 or LO and 1 or

HI. The other value of this being a voltage that you can find. This voltage

will be either almost 0 volts (LO) or almost the input voltage (HI). There is

no real inbetween. Computer chips have a range of zeros and ones, it is

commonly known as the 'Truth Table' and is pretty easy to understand. The way

it works is by deciding whether the chip in design will have one of the

following, a BUFFER gate, a NOT gate, an OR gate, a NOR gate, an AND, or a NAND

gate. In the next example remember, HI = 1, LO = 0, and each line represents a

different way that the gate can be used.

 

         INPUT OUTPUT      INPUT     OUTPUT

BUFFER    HI     HI          1         1          input ---->---- output

BUFFER    LO     LO          0         0

 

NOT       LO     HI          0         1          input ---->o--- output

NOT       HI     LO          1         0

 

         INPUT INPUT OUTPUT INPUT INPUT OUTPUT

OR        LO    LO     LO     0     0      1                ____

          HI    LO     HI     1     0      1      input ----\   \_____ output

          LO    HI     HI     0     1      1      input ----/___/

          HI    HI     HI     1     1      1

 

NOR       LO    LO     HI     0     0      1                ____

          HI    LO     LO     1     0      0      input ----\   \o____ output

          LO    HI     LO     0     1      0      input ----/___/

          HI    HI     LO     1     1      0

 

AND       LO    LO     LO     0     0      0                 ___

          HI    LO     LO     1     0      0      input ----:   \_____ output

          LO    HI     LO     0     1      0      input ----:___/

          HI    HI     HI     1     1      1

 

NAND      LO    LO     HI     0     0      1                 ___

          HI    LO     HI     1     0      1      input ----:   \o____ output

          LO    HI     HI     0     1      1      input ----:___/

          HI    HI     LO     1     1      0

 

         This is the basics of the inside of how a chip is designed to work. If

you examine it, you will see a pattern like this:-

 

BUFFER works if the input is HI then the output is HI

NOT works if the input is HI the output is LO

OR needs one input to be HI for the output to be HI otherwise it stays LO

NOR if any inputs are HI the output will stay LO

AND needs two HIs to go HI or two LOs to go LO otherwise it stays LO

NAND needs two HIs to go LO or two LOs to go HI otherwise it stays HI

 

         LINEAR chips contradict the HI / LO a bit in that LINEAR chips are

used mostly in circuits where amplification is needed since these chips mainly

deal with different voltages. A good example of this is the LM380 or the 741

IC. These two are amplifier chips. They both take in a minute current or

voltage then internally amplify it, this is called the 'gain'. The 741 has a

gain some 200,000 times greater than the input. This causes feedback which

must be supressed. Before I get loads of complaints about the HI / LO bit. It

really depends on how this chip is incorporated into a circuit. Sometimes the

HI / LO will apply, (as for NOR type).

 

         As you can see, the internal working of a chip is very complicated.

I have only explained the very basics of how HI and LO are high or low.

I haven't explained how it decides which to do or how it decides. I have no

intention of doing so unless I get thousands of requests!

 

                                  Transistors

                                  -----------

 

         Transistors compared to the IC chips, these are a doddle!. They come

in a few types. These are mostly plastic half circles with three legs

protruding from the bottom, or in a round metal case, again with three legs

protruding from the bottom.

 

         Each of these three legs has it's own name. One is the feed and is

called the Collector or 'C'. This holds a large current ready to go through the

transistor. Another is called the Base or 'B', this is the gate, a voltage of

0.6 volts will allow this gate to open thereby allowing current to pass from

the 'C' to the final leg which is the Emitter or 'E' which goes to the rest of

the circuit.

 

         There are two types, there are NPN and PNP. NPNs have a positive 'C'.

These NPNs are by far the most popular. The PNPs have a negative 'C'. The PNP

transistors are very seldom used, though are by no means obsolete. It is very

important that NPNs are never confused with PNPs. I personally only really

use the BC184L NPN and on only the one occasion, do I use the AC128 PNP.

 

         You can test a transistor very easily. You will need to know the

collector and emitter. Then connect a circuit tester (+) probe to one

and the (-) probe to the other. Only one way will give a reading.

 

         A transistor can be used for a number of things. Some are used as

amplifiers, as in the LED that lights up on your stereo when you are adjusting

the record level. Others amplify sound. Sometimes they are used as a switch

which will allow a very small current of just 0.6 volts to turn into a larger

voltage or current to switch a larger device on.

 

         Along with transistors, there is a device similar in name called a

thyristor. To stop these two names being mixed up, the thyristor is more often

referred to as a silicon controlled rectifier or SCR for short. This is

slightly different in design in that it is a square shape about 1cm x 1cm and

about .5cm thick. It has a metal plate or 'tab' at the rear, this is commonly

connected to the cathode. It has three legs.

 

         The SCR has one leg called the Anode or 'A'. One called the Cathode or

'C', and the last one called the Gate or 'G'.  The legs are in no fixed order.

So 'A' on one may be in a different place on another. This will mean that

the 'G' and 'C' are also in different places. This creates a lot of problems

for the newcomer. The SCR is normally in an off state unless a small current

is passed to the gate. This current only has to be in contact with the gate

for a fraction of a second. The SCR is then internally activated and current

that is waiting at it's anode is then allowed to pass to its cathode, thereby

being switched on. It works like a diode in that it will allow current to pass

through it only one way. It will then remain on until it is (1) switched off or

(2) the battery runs flat. So with this component it is quite important to have

a switching device. This doesn't have to be a manually operated switch. I have

a 555 timer circuit on my alarm system on my motorbike.

 

         Another one with a similar name is a TRIAC. This is a BI-DIRECTIONAL

SCR. This is exactly the same as the normal SCR except that this works like

two separate diodes. Allowing current to pass in both directions. Thereby

making this type of SCR very useful in mains applications. More expensive

BI-SCR's will quite easily handle 240 volts AC at quite a few amps. Though I

won't be dealing with this type as they are a little too expensive for me to

have had much experience with.

 

         An example of some simple circuitry that can do a very useful job and

which is priced in the shops at about 150 but can be made for about 10 in

total is by using a mercury tilt sensor connected to an SCR, which is in turn

connected to a touch sensitive transistor circuit. The touch sensor is

connected to a metal surface so that where I have it on my motorbike, if

anyone moves or touches it even, the alarm will sound. The alarm is switched on

with an SCR, which is controlled by a 555 timer IC chip. This is set to about 5

minutes, after that the timer runs down and turns off the current to the SCR

circuit. The 555 timer then resets, but turns on a flasing LED to tell me that

the alarm system has been activated.

 

         Now all that will I expect sound very complex to the newcomer, but

that entire system uses about 15 parts in all, and is extremely effective.

Though I would suggest that should you wish to do an alarm, that you use an

independent power source, and not the main battery, which could be cut.

Thinking about it now, with the addition of a couple of components, I could

also disable the entire power and ignition system quite easily.

 

                         Epilogue to the introduction.

 

                         -----------------------------

 

         Well thats a short introduction to the very basic components and how

they work, with an idea for a circuit which you should be able to do most of

at the end of the course in a few issues time.

 

         When you buy any component, state the relevant facts required, these

are:-

 

Resistors - Resistance, tolerance and wattage.

Diodes - I.D. number if avaliable, or reverse voltage and forward amps.

Capacitors - Farads, type where stated and minimum voltage where stated.

Transistors - I.D. number if avaliable, and type NPN or PNP

SCRs - I.D. number if avaliable or maximum amperage and voltage

 

         In case you are wondering what is coming in the next few issues, I

will be doing a couple of things for the BBCs User port and analogue port.

These include a very accurate light pen that costs about 2.50 to make. This

will also come with software, as will the input and output controllers for the

user port. There will be a joystick, a simple alarm system, a VERY sensitive

touch sensor, a timer circuit and a few more things.

 

         If you have any problems in either getting parts or have any questions

about what's been covered so far, then please feel free to leave me a message

or post me a letter. Include a 1st or second class stamp. You can send me a

disc if you think or require a drawing or lots of text for it, or if you just

hate losing letters, when you could have it on disc. I can handle 3.5" or 5.25"

DFS (31 file) 40/80 track or ADFS (S) (M) (L) format or even HADFS. Just post

them to:-        Mr J. Ilsley,

                   61 Kingsley Road,

                   Southsea,

                   Portsmouth,

                   Hampshire,

                   PO4 8HL.