LCD readout for the
digital capacitance meter
Did you decide not to build the 4-digit
capacitance meter published in May 1990
because it was mains powered? Did you
want a liquid crystal display too? If so, have
a look at this re-design of the circuit which
uses a 4½-digit LCD panel.
By BARRY NAUJOK
The heart of this new design is the
74C946 (IC3). It is a 4½-digit LCD
counter and decoder/driver which is
used to drive the LCD panel. It replaces the 74C926 (IC3) used in the
original design.
Because the new circuit uses a
liquid crystal display instead of the
4-digit LED display, its power requirement is greatly reduced and it can be
supplied by batteries (or an external
DC plugpack).
Since the design uses batteries, a
"low battery" indicator has been
added to the circuit.
Let's see how the new circuit works.
The nulling circuit, using IC1, is
still the same as the LED version and
the reference oscillator is also the
same.
Gating oscillator
The gating oscillator (IC2, IC8d and
IC7b), wired for monostable operation as before, measures capacitors
up to 19,999µF compared to 9999µF
for the LED version. To charge a capacitor which is twice the value in
the same time as for the previous version, the timing resistors have been
halved to keep reading updates at a
reasonable rate (45 seconds for a
20,000µF capacitor is quite a long
time). But when this gates the reference oscillator (IC4), a capacitor of
say 6800µF would read 3400µF. To
overcome this, the clock frequency
from Slc is fed via exclusive-OR (XOR)
gate IC10c and the associated RC network. IC10c operates as an edge detector and effectively doubles the
92
SILICON CHIP
clock frequency which feeds pin 32
of the 74C946.
Latch and reset
The 74C946 control inputs for
latching and resetting are different to
the 74C926.
To reset IC3, the reset pin (33) must
be brought low (normally it is held
high). Pin 10 ofIC6 has a signal opposite to that required by IC3, so the
signal is inverted by IC9d.
To latch the display, the store pin
(34) must be brought high. To achieve
this, pin 4 ofIC6 has to be inverted by
IC9e.
Over-range indication is also entirely different to the LED version.
The over-range is driven by the Carry
Out (CO) and the half-digit outputs of
IC3.
During each cycle, the CO output
goes high when (and if) a count of
9999 is reached. This goes low again
on the next clock pulse. It always
goes high every 10,000 counts. Because the meter has a "full scale" of
19,999, the over-range needs to trigger on the second CO pulse so the CO
signal is fed into a NAND gate, consisting of D5, D6, Q3 and two 10kn
resistors. The other signal to this NAND
gate, fed via D6, is the half-digit signal which is demodulated from the
backplane.
The output of the NAND gate is
connected to the clock input of a Dtype flipflop, IC7a, and the output is
latched high until it is reset by IC6. If
the output goes high, it is modulated
with the backplane signal (BP; pin 5
ofIC3) by IC10b which drives the"+"
segments on the LCD.
Battery power
The circuit of the LCD version
needs about 9V DC. Ideally, this could
be supplied by 6 AA cells in a suitable holder, inside the case. This 9V
DC supply is fed to two 78L05 5V
regulators (REG1 & REG2) to drive the
circuitry.
If the battery voltage drops below
6.6V (1.1V per battery), the regulator
outputs start dropping below 5V and
also become unstable. This condition
means that some sort of low-battery
indicator is needed, because low batteries can lead to inaccurate capacitance readings. Therefore, battery
indication is provided by IC12, ZD1
and their associated resistors.
The first stage in the battery indicator (IC12a) is a voltage comparator. A
3.3V reference is generated by ZD1
and is fed into the non-inverting input of IC12a. The unregulated power
supply (nominally 9V) is divided by
two by the two 100kQ resistors and
fed into the inverting input of IC1a.
Normally, the output of IC12a is low
as the input at pin 2 is higher than the
3.3V reference. When the battery
supply drops below 6.6V, the output
goes high.
The second stage of IC12 is configured as an XOR gate. One of the inputs
(pin 6) is the backplane signal while
the other is the output of IC12a. The
output of this is used to drive the
"arrow" symbol on the LCD. This
then, is the "low battery" indication.
Brief LCD operation
For anyone who is confused by the
operation of the LCD, it requires an
continued on page 96
Fig.1 (right): the LCD version of the
capacitance meter is similar to the
LED version, but uses an LCD panel
instead of the original 7-segment LED
readouts & a 74C946 LCD decoder/
driver instead of a 74C926 LED driver.
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BOOKSHELF
Computers and Music:
an introduction
Computers and Music, by R. A. Penfold. Published 1989 by PC Publishing, Kent, UK. Soft covers, 138 x
216mm, 174 pages. ISBN 1 870775 07
4.
This book is entitled "Computers
and Music" but it could just as well
have been "Computers & MIDI" because that is largely what it is about,
as you would expect. It is written for
people who know nothing about
computers and so a fair amount of the
book is solely devoted to talking about
computer hardware. It is only when
you are a fair way into the book (page
53) that MIDI gets a reasonable mention.
There are six chapters in all, with
their titles as follows: Computer' Bas-
LCD capacitance meter
- ctd from p.92
AC signal to drive it. The backplane
is an AC signal running at about 55Hz
in this case. If a segment is to go
black, the segment input has to be out
of phase with the backplane. If it is to
be transparent, the segment has to be
in phase so that the potential difference across the segment is zero. These
circuit functions are provided by the
74C946 (IC3).
Parts availability
At the time of writing, no kit is
96
SILICON CHIP
ics, Storage Media, Ports and Peripherals, Real Computers, About MIDI
and Music Software. As well, there
are three appendices: one is a glossary of all the technical terms in the
book, the second is a listing of hexadecimal numbers, and the third is
entitled "checklists" and is in fact a
list of desirable specifications for
computers and equipment to be purchased with musical activities in
mind.
Overall, while the book is reasonably good as a primer for anyone new
to computers, it is not so good as far
as musical applications and MIDI are
concerned. It never quite comes to
grips with the MIDI standard for example, and nowhere that I could find
does it give the technical specifications for MIDI (ie, baud rate, data
frames, number of channels), even
though it talks about the MIDI specification in a number of places.
And if you want to run a MIDI system with your Commodore or IBM
clone, there is very little help.
In conclusion, if you want a basic
primer on computers and some background on musical applications, this
book is OK but it certainly won't make
you an expert.
We received sample copies from
Altronics (Cat. B-2210) and Jaycar
Electronics (Cat. BP-1216) and the
price is $19.95.
available for this LCD version of our
capacitance meter, nor do we expect
that any kit supplier will provide one.
Both the 74C946 and the 4½-digit LCD
panel are available as stock parts from
Dick Smith Electronics. All the other
parts are stock items from most electronic parts retailers.
A PCB artwork and layout plans
are available for $1.50 including postage from the author. Finished printed
circuit boards are also available for
an additional $12. Send all orders,
together with cheque or money order,
to Barry Naujok, 7 Admella Court,
Portland, Vic 3305.
SC
Computer Viruses:
with anti-virus disc
Computer Viruses, Jonathon L. Mayo.
Published 1989 by Windcrest Books,
Blue Ridge Summit, Philadelphia,
USA. Soft covers, 232 x 189, 160
pages with 5.25-inch anti-virus disc.
ISBN 0 8306 3382 0.
Are you frightened of a virus getting into your computer and corrupting all your valuable data? If you are
not, you should be, especially if you
ever borrow a disc from a friend. It
can happen so easily and your
"friend" may not even know that the
disc is infected.
When it happens, you really do
have problems. This book talks about
the subject in easy to understand language and tells you the commonsense
ways of protecting your computer
from infection, before it happens.
Mainly, the safeguard is to backup all
your data and programs and then put
write-protect tabs on all the original
(master) and backup discs. That way,
the computer cannot corrupt the
backed up data if it does suffer an
attack.
As a bonus, the book comes with a
floppy disc with quite a few antivirus programs on it. We did not have
time to check it through and in any
case, we don't have copies of virus
programs (as far as we know)! Even
so, a disc of anti-virus software could
be very handy and so the book is well
worth considering.
Our copy came from Dick Smith
Electronics and is priced at $49.95
(Cat. B 6052).
SC
