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                              Moisture Detector.


                            By: John Ilsley (27N).


         Welcome to the first hands on project. I have  chosen  a  very  simple

project to start with which should not cost you much more than about one pound

fifty pence to make.


         This moisture detector will detect moisture in anything,  I've  tested

it on everything from a jug of tap water down to damp walls, the human skin and

even in a plant pot!  So you could use it to see if your plants need  watering.

I  have  taken  a simple version, but by changing component 'L1' to a sensitive

needle meter and changing component 'R1' a resistance value to suit the  needle

meter you would get a much more accurate reading.


       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 project.


         The components you will need for the simple version  are  as  follows.


T1 = 1 * BC184L Transistor

R1 = 1 * 1k (1000ohm) 5% resistor (see text for info on needle meters).

R2 = 1 * 1k (1000ohm) 5% resistor

L1 = 1 * 10 mA LED

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

Veroboard = 7 holes across * 6 tracks

22swg multicore sheaved 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


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


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


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


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


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


               Track F   ----O----O----O----O----O----O----O----


         Now comes the fairly easy bit. First we will put in the transistor TR1

as this is the most complicated piece here. If you hold the transistor with the

legs pointing down and the half round area of TR1 to your LEFT  then  you  can

place it into the board. Fine. Hold the veroboard off the table with the tracks

below and then take TR1 and place the leg FURTHEST from you into hole  B5,  the

middle  leg  into C5 and the leg nearest to you will go into hole D5. Now don't

push that all the way down, leave some room other wise you may break  the  legs

when  soldering, or if you have to remove it. For future reference, the legs on

this BC184L NPN transistor are as follows. The leg in B5 is  called  the  BASE.

The  middle  leg  in C5 is called the COLLECTOR and the leg in D5 is called the



         Ok. Now we will add R1. This can go either way round, bend the leadout

wires  of this resistor 90 degrees so that if you hold the main body, both legs

hang down. Place one leg into hole A3 and the other leg into hole E3.  Now  you

can  push  R1 in nearly all the way, solder it in. Bend the leadout wires of R2

the same way and place one leadout wire into hole B6 and the other leadout wire

into hole F6.


         Now for the LED or L1. This has a positive and negative side to it. If

you look into the LED you will see two very small pieces of metal, if you  look

closely  you  will  see  what  also  looks like a thin sliver of wire. However,

look closely at the two pieces of metal, one is smaller than the other. This is

the Anode or the positive side. The large piece of metal is the Cathode or  the

negative side. Place the positive side of L1 into hole E2 and the negative side

into hole C2. Just push them in enough to solder them in so you  have  a  light

that is quite high.


         That is the electronic components fitted in. Now we will add the probe

leads. Take a piece of wire, it doesn't matter how long, about  1ft  is  ideal.

Remove about 1/2 a centimeter  from either end, twist the loose wires at either

end so there are no strays and then tin them. Do this by running the  soldering

iron  over  the ends and putting a small amount of solder on the ends. You will

need to tin nearly every bit of wire you use. Do the same to another 1ft  piece

of  wire  so you end up with two separate pieces of wire with each end  covered

in solder. Place one end of one of the wires into hole A1 and solder it in  and

the other wire into hole F1 and solder this in.


         Now  to  add the  power. If you have a separate 9 volt DC PSU then you

won't need to do this, you can wire directly into these holes, but  be  warned,

you do so at your own risk!


         If  you are using a 9 volt radio battery and connecting lead, take the

red + lead and tin the end, then place it into hole A7, solder it in. Now  take

the black lead and place in hole D7 and solder it in.


         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 '*' are wires. Now  you  can

plug your battery onto the battery lead.


                             1    2    3    4    5    6    7


               Track A   ----*----O----:----O----O----O----*----


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


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


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


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


               Track F   ----*----O----O----O----O----:----O----




         The  LED should not light up, If it does then make sure that the probe

wires, (A1 and F1) are not touching each other. If they are not touching,  then

disconnect  the  battery and check for these, (1). T1 is fitted the correct way

round. Other then that check everything.


         If the LED does not light up then touch the probe wires  (A1  and  F1)

together,  the LED should now glow brightly. If it does not then disconnect and

check these few things. (1). The battery supply  or  PSU  supply  is  connected

round  the  right way. (2). That L1 is connected with the positive and negative

lead in the right place. (3). That T1 is fitted the correct  way  round.  Other

than that check everything.


         If  the  LED  lights up only when the probe wires are touched together

then everything is ok. You can now test this a bit more fully,  You  can  touch

each  end  of the wire with your fingertips, it doesn't matter whether you lick


them or not the effect is almost the same. Go to a dry plant pot and stick  the

leads  into  the  soil, the LED should not glow at all or if it does, then very

dimly. It is a very sensitive device, test it to see if the walls in your house

or outhouse are wet, even if they feel dry to the touch.


                         OTHER THINGS TO TRY WITH THIS



         If your'e lucky enough to own a digital or needle test  meter then you


can  set  it  to a low voltage range, about 3 volts will be the maximum reading

your likely to get, remove L1 and place the '+' and '-' leads  of  your  tester

into this. You will then get a reading that you can work by.


         This  can  also  be  connected  to  the  analogue port on the BBC. Any

channel will do but the  output voltage will have to be lowered down to about 1

volt  to  be able enable your program to work accurately. If you're intrested,

Imay take  this  up  at  a later date when I turn your attentions to the BBC

Userand Analogue ports.


                                 HOW IT WORKS



         So how does this work? Well it is  very  simple  to  understand  which

makes  it  simple  for  even  the  beginner  to understand. When the battery is

connected, current is  passed to two points in the circuit,  one  is  R1.  This

lowers  the current to the LED so that the LED won't blow. The second is to one

of the probes.


         Negative doesn't have a current, it is an opposite. However,  for  the

sake  of  argument, negative is fed into the Emitter of T1 where it waits until

the probes come into partial or complete contact whereby the current  from  the

first probe is passed through the second and to R2 which reduces the current so

that T1 won't blow.


         After the current has passed through R2, the current at the Base of T1

rises  from  0.5v  to  between 0.6v and about 3.0v depending on the dampness in

this case. When the Base current rises, it opens a gate that allows current  to

pass  from the Emitter to the Collector thereby allowing the negative 'current'

to pass to the LED lighting it up.


         With no connection between the two probes the battery should last  for

years when contact is made, only about 10 mA is used each time.


                                  Next Month.



         I have shall  do  an  electronic  switch for the next issue. This will

work  in  that  you  can  have a buzzer which will be quiet until two wires are

touched for  any length of time from  a fraction of a second onward. Even  when

the  two  leads  are  disconnected  the  buzzer  will  sound until the power is

switched off.


         If you have any circuits you would like me to attempt for you,  please

feel  free to ask and  I will try my best to see if I can make them for you. If

they're interesting, I may make them into a project for  the  mag  and  put your

name forward as the person whos idea it was.