8-Bit Software
The BBC and Master Computer Public Domain Library
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-fi's 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
IC's 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 NPN's 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 NPN's are never confused with PNP's. 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 it's 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 issue's 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
SCR's - 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 BBC's 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.