Specifications
CIRCUIT IDEAS
accuracy, which is quite reasonable and
adequate for some practical applications.
The bulb (which is 230V, 100W, fila-
ment type) shown in the figure is
mounted on a wooden box with a bulb
holder. Connection to the mains supply
is obtained using long wires, which are
terminated into a 3-pin power plug that
ensures non-reversibility of live and
neutral leads. The bulb drops the volt-
age to a safer level at the terminating
probe. Resistor R1 limits the current.
Voltage V1 is measured across resistor
R1 using AC voltmeter.
The probes, which are equidistant
from each other (about 45.7 cm, or 18-
inches, apart) have a height (below
ground level) of about 30.5 cm (12
inches) and a diameter of 1.25 cm (or
½-inch). The probes may be made of
iron, stainless steel, or copper. (The au-
thor used copper probes.)
The ends of the probes are connected
securely to the wires by means of bat-
tery terminal lugs (generally
used in automobiles) or power supply
terminals (used in UPS, DC-AC
converters, etc). The probes P2 and
P3 can be fixed on hylam strips
measuring 75cm(20-inch)x5cm(2-inch)x
5mm(0.2-inch). Hylam strips are gener-
ally available from switchboard dealers.
To measure the conductivity (
),
the probes are driven into the ground,
as shown in the figure, and the circuit
is powered with adequate safety mea-
sures against any electric hazard. Volt-
ages V1 across resistor R1 and V2
across probes P2 and P3 are measured
and noted.
The earth conductivity is then cal-
culated as:
Conductivity = 21xV1/V2 millimhos/
metre (m
/m)
Please note:
•
For poor soil with very little
moisture and bio-fertility, the conduc-
tivity ranges from 1 to 8 millimhos per
metre.
•
For average soil, the values range
from 10 to 20 millimhos per metre.
•
For fertile and good conductive
soil, the conductivity ranges from 80 to
100 millimhos per metre.
•
For very saline soil, or salt water
with very good conductivity, the
values might be as high as 5,000
millimhos per metre.
Caution. It should be noted that
the polarity of the AC (phase and
neutral) leads should never be reversed,
to prevent any dangers to human/ani-
mal lives.
S.C. DWIVEDI
T
o better understand the circuit,
one needs to have some knowl-
edge of electronics, computer
programming, and the computer’s par-
allel port.
You will of course need a computer,
12-volt power supply (preferably a bat-
tery eliminator), stepper motor, ULN2003
chip, and some connecting wires.
The circuit can be easily assembled
on a breadboard. It is very important
that you work with the smallest stepper
motor available in the market, such as
the one used in
a floppy drive.
If you go in for
the large ones
used in CNC
machines,
there is a
chance of dam-
aging the PC’s
parallel port.
The second
thing to men-
tion is that the
colours of the
wires of the
stepper motor
are non-stan-
dard.
The paral-
lel port of the
PC is the most
flexible way of
getting the
computer to communicate with the out-
side world.
The parallel port is generally used to
interface printers, but we have used it to
interface the stepper motor. The parallel
port consists of 25 pins, but it can only
transmit 8 bits of data at a time. The
reason for the large number of pins is
that every data pin has its own ground
return pin. There are other pins for vari-
ous other functions. We have used only
four data pins and a ground pin.
SHOBHAN KUMAR DUTTA
STEPPER MOTOR CONTROL
VIA PARALLEL PORT
203