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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. - nFO pF JlE,.,. :- 1oopF .i. STY RO I CALE_J VR2 6 5k 1k $ "'"J 33 ,_: 1MS I 5 ':' 1 ~ J.. ~ lN 4002 07 1.,. L 95 0kHz J: 10 16VWI RBLL .,. . - ... 8 ; 7.2-9V I IC4 7555 I 0.1.., I I ic1 1 V, F I L-. 1¾ .,. 1 .,. · -:- HfflI1 ":"' -:- 22 + 25VWI R2 15 IC5 4518 uF nF . uF i 9.5kHz .,. 0.1I .L, CK vcc 14 I 16 8 .,. IC6 4017 I I --♦♦ H ~ · .,,.- - I _ t='--<--+5V 1 OOk .,. 47 + 16VWI Z01 ~ 400mW .,... J I 1 ~ 8 I ? 2 4 15 R 4 10 100k 10k __.11 ~•• •l .,,._+-_ ~ .,. 0.1I. · - >'--W,,,_➔--=I 11 9 12 05 1 31 EN 34 ST 33 R 32 CK vcr.---.... IC9e IC9d 10 II I I IC7b I ., IC2 7555 I I ,-------+----;_ ___, O -y; 1N4002 D3 +5V t=.a......1P-----~----.----Y.>1w--• .,. 100k 0412 14 I .,. ~~~~~I· ,.,/ + s v ~ S1d 16 8 .,. 6 - , 1% 100k 511l 1% 10k + v 5 - DP3 5.1k ,. . . . -- "----. 510 I F: 820 P pF_,./ R1 .,. °uF S1a =i= 01 v f 5;),~t 2 1 EN1 CKl 1000 + 25VWI - POWER S3 0.1 .L, • 7555 3 .._,,,..--,-,-11 2 .__ __, _ I J. OoF NULL I 5 L { . L~~~tt . . r··~:~) 'i 1, ,,-, ,,... r • _ .1 B~ ? 100k I I [11 100k i b ~VER-RANGE ~ + 5v I OP41 c52d DP3 IC10d I ~~ tt 35 29 w 9 e3 . /4 1 13 :: : 13 g311-'-"----R3 a 20 35 a4 b4 21 34 b4 4 22 7 t4 cd 4 23 6 d4 24 5 4 ~: 26 36 e 25 37 g4>-'-----,g4 e3 5 BAzirinv 38 1 4 8 12 16 33 28 BP BP DP1 DP2 DP3 OP4 l 1 l2 27 3 1/2DE-+----,--t1/20 a1 37 21 a1 b1 38 20 b1 19 39 c1 c1 d1 40 18 d1 ~ 39 17 el 12 ~ 4 22 l1 1 3 23 91 91 25 32 32 6 24 b2 7 b2 15 02 02 8 9 14 • 2 d 10 13 • 1C3 e2i,:.=.----=te2 24151 LCD 12 26 74C946 12 12 (DSE) g2 11 27 92 a ;; ~o . 3 b3 b3 15 11 03 03 d ~~ 10 d3 28 CO I ? 06 2x 1N914 ~ ~ 10k* 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