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                               Continuous Alarm


                            By: John Ilsley (27N).



         Welcome  to  the  third  part of this series of electrical projects, I

hope all of you who attempted the last project managed to achieve the  required

results.  If  you  didn't, then please inform me, and I will attempt to explain

what has gone wrong, be it my own fault or yours.


         Anyway, back to the matter in hand. This months electronic project  as

outlined  at  the end of last months project is an alarm that will remain quiet

until the two contact wires are touched, then as soon as those  two  wires  are

touched,  the  alarm will sound. Now, thats quite normal, because that is how a

simple door bell works, you push the button, this makes  two  pieces  of  metal

make  contact  which  completes the circuit, this then makes the bell ring  and

when you release the button, the two pieces of metal break contact and the bell

stops  ringing.  This  project  has  an  additional  advantage in that the bell

continues to sound even after the button is released. It will basically  remain

on until the unit is switched off then on, or until the battery runs out.


         Please  note,  whilst  I  have  made  every  effort  to  make  this an

interesting and informative article neither 8BS,  the  editor  or  myself  will

accept  responsibility  for  any  damage  caused  to  yourself,  other persons,

components, belongings  or  other  equipment  used  in  conjunction  with  this



         The  components  you will need for the alarm are as follows.


R1  = 1 * 1k (1000ohm) 5% resistor

R2  = 1 * 1k (1000ohm) 5% resistor

R3  = 1 * 1k (1000ohm) 5% resistor

Th1 = 1 * C106D thyristor

Bz1 = 1 * 6 volt buzzer

BT  = 1 * 9 volt radio battery or power supply unit (PSU) and leads

Veroboard = 8 holes across * 5 tracks

22swg multicore sheathed wire, solder, soldering iron etc....


         First the veroboard. Place it so the tracks are face down running left

to right, similar to this:- NB( the 'Os are HOLES, '----' are tracks).


                                      Hole number

              1    2    3    4    5    6    7    8


Track A   ----O----O----O----O----O----O----O----O


Track B   ----O----O----O----O----O----O----O----O


Track C   ----O----O----O----O----O----O----O----O


Track D   ----O----O----O----O----O----O----O----O


Track E   ----O----O----O----O----O----O----O----O


         Ok, you've either been to the shop, or you've found the stuff lying in

the workshop if your lucky, please don't take the parts from your family stereo

system,  otherwise  you  may find  yourself in trouble because someone won't be

able to listen to their "Barry Manilow" or "Des O'Connor" records!...  And  you

wouldn't  want  that  now  would  you?!... I'm 21, and don't get much money, so

owing to the extremely high cost of this project, I suggest  you  'borrow'  the

parts from a BROKEN piece of equipment or ask a friend if they have what you're



                             Building the project.



Now comes the fairly easy bit, This project is as easy to build as last months

project as it has a similar amount of components. First we need to put in the

resistors. These are very easy to put in  and are not affected by heat from the

soldering iron too much, take one of the three resistors and hold the body with

one leg pointing up, and the other leg pointing down, I suggest you only try

this if you are a contortionist, as in fig 1. Using your thumb, bend the top

leg over so it faces down, as in fig 2. Example... 


         Fig 1      :           Fig 2

                    :                  :----:

                   ***                 :   ***

                   ***                 :   ***

                   ***                 :   ***

                    :                  :    :

                    :                  :    :

                    :                  :    :

         Now do exactly the same with the remaining two resistors. Now you have

done that, we must place each of the resistors onto the veroboard  so  that  we

start  our  circuit. Hold the veroboard with the copper strips facing down, and

running left to right. Take one of the resistors, R1, and place one of the legs

into hole A2 and the other leg into hole C2, now solder it into place. Take the

second R2, resistor and place one of the legs into hole B3 and  the  other  leg

into  hole  D3,  and solder that one in. Now take the third and final resistor,

R3, and place one of it's legs into hole D2 and the other leg into hole E2  and

finally  solder  that  one  in place. That is now all the resistors fitted. You

should have all the resistors fitted nicely, with no problems.


         Now take the thyristor, Th1. This is not exactly  sensitive  to  heat,

but  it  is  best  when you come to soldering it in  not to leave the soldering

iron on the legs for too long. Hold  the thyristor with the  legs  facing  down

and  the metal tab to your right, (facing hole 8). Place it in the veroboard so

that the metal tab faces the right, and the leg furthest away from  you  is  in

hole  B5.  Thereby  the  other  two  legs  will fit snugly into holes C5 and D5

without any problems, now solder it into place. The leg in B5 is  the  CATHODE,

the leg in C5 is the ANODE, and the leg in D5 is the GATE.


         That is all the components soldered in by now, all you have to  do  is

add the power cables, and a few other wires, so lets carry on. Find two lengths

of wire of about six inches in length, strip and flux both ends of both lengths

of  cable,  and  name  one length 'A', and the other 'B'. Place length 'A' into

hole A1, and solder it in, and place length 'B' into hole E1, and solder it in.

These  two wires when touched together make up the trigger. Now take the buzzer

Bz1, I don't call this a true component as it isn't fixed into the veroboard as

such. However, take the red or positive (+) wire of the buzzer and if necessary

strip and flux the end and place it into hole A7, and solder it in place.  Take

the  black  or  negative (-) wire of the buzzer and if necessary strip and flux

the end and place it into hole C7, and solder it in place. All that is left  to

do  involving  the construction of the circuit is to add the power. So take the

power leads, these can be the normal 9volt battery connecting terminals,  or  a

separate  power  supply,  which  ever you choose. Place the red or positive (+)

wire into hole A8 and solder it in, and now place the  black  or  negative  (-)

wire into hole B8 and solder it in place.


         You  should  only  have  components or holes in the same places as the

diagram below. If you don't, then check you haven't done anything wrong. If you

haven't,  then it means I have most likely made a mistake in my text, for which

I am sorry. But I have read it all back, and it seems all ok too me. Now  check

that  all  your solder joints are clean and tidy and that no solder is bridging

or going onto any of the other tracks and that all the components  are  in  the

right  places.  Below should give you an idea of where leads should go. The ':'

are components leads, the 'W' are wires. Now you can plug your battery onto the

battery lead, or turn on your PSU.


Key -     : = where a component is fitted.

          W = where there is a wire fitted.


                          Hole number

              1    2    3    4    5    6    7    8


Track A   ----W----:----O----O----O----O----W----W


Track B   ----O----O----:----O----:----O----O----W


Track C   ----O----:----O----O----:----O----W----O


Track D   ----O----:----:----O----:----O----O----O


Track E   ----W----:----O----O----O----O----O----O






         The buzzer should be quiet, if it starts buzzing, then check (1), that

the wires in holes A1 and E1 are not touching each other, or (2)  that  Th1  is

fitted the correct way around.


         If the buzzer is silent, then take the leads from holes A1 and E1, and

touch them together, the buzzer should sound. If it doesn't,  then  check  (1)

the  buzzer  is  fitted the correct way around. (2) the thyristor is fitted the

correct way around. (3) the power is turned on.


         If the buzzer was silent until you touched the wires from  A1  and  E1

together then so far everything is ok. Now disconnect the two wires, the buzzer

should continue to sound. If it does, then all is well, and you  can  stop  the

buzzer  only  by  turning  the power first off, then on. However, if the buzzer

stops buzzing as soon as you break the contact of the wires  from  A1  and  E1,

then  check  (1)  that R1, R2  and R3 are in the correct place. (2)  That there

is no bridge of solder between the legs of Th1.


                         OTHER THINGS TO TRY WITH THIS



         You  can lengthen the wires from A1 and E1 and instead of the two bare

wires making contact, you can place a touch  sensor  or  pressure  mat  on  the

connections  to  say tell you when someone enters the room or house, or to tell

you if your pet dog is lying in his box. You could, however ( I  don't  suggest

this unless  you have the necessary skills use a similar pressure idea) use  it

to tell you if a pressure boiler is about to surpass it's limits.


         You can also have it so it will automatically reset itself if you use

a 555 timer chip. These will be explained and used in a later project.


                                 HOW IT WORKS



         So how does this work? Well, if I knew I would  tell  you...  Sorry  I

mean  it's  so  simple,  even a baby can understand it... I'm a big baby, and I

understand it...


         Well, first of all the thyristor works in a similar way  to  a  normal

transistor,  in that a small current flow to the GATE causes a much larger flow

from the ANODE to CATHODE. Those last two work in the  same  way  as  a  normal

diode in that they allow current to flow only one way. But once a small current

is fed to the GATE, and the ANODE and CATHODE allow current to pass from one to

another, they are basically glued open until the power is cut to them.


         R1 is there to prevent the thyristor being blown to bits should aliens

strike. I mean R1 is there to help prevent the thyristor being blown up if  too

much  current is passed to  it. R2 is there to prevent accidental trigger happy

thugs activating the buzzer, for example, your static electricity and  airborne

signals.  R3  is  there so that when the trigger is finally activated, there is

power for the thyristor when the trigger wires are disconnected.


                                  Next Month.



                 Now, what shall I do for next month? How about a Thermonuclear

Tactical Global Warfare Nuclear warhead... No, too  complicated.  How  about  a

timer for a time bomb... Hay!, that's a easy one. I'll tell you what, how about

a simple low voltage alarm. If your'e lucky enough to  own  a  unit  that  uses

battery  power  of  more  than  five  volts,  then  this will work fine. So for

example, if you own a Camper or Caravan, then you will be able to tell when the

battery power is running down.