Fullscreen HTML5Med Res (??MB)HTML4

Build this Jumbo Digital Clock Do you need a clock with a very large digital display? This Jumbo Clock uses 7-segment LED displays that are 70mm high. It has battery backup, automatic display dimming at night, AM/PM indication & a 12hour display. By DARREN YATES 16  Silicon Chip OK, I admit it. Digital clocks are now so common that you can go down to your local supermarket and pick one up for around $15. So what? Have you ever tried to repair one of those clocks? Do you how they work? Taking the back off won’t give you any clues on either front. You’re just confronted with a single chip (or more often these days, a single blob) and little else. Embedded inside this blob is a single large scale integration (LSI) chip which con­tains virtually the entire clock circuit. You’ll learn more by staring at a blank wall than looking at that blob! However, when you build your own clock, you get a circuit diagram that shows you how it works and, should anything go wrong, you can fix it yourself without too many problems. And by sticking to discrete ICs, you can buy the replacement parts just about everywhere. More importantly, you learn how the clock works. In partic­ular, you learn about counters and crystal oscillators, and about LED displays and how to drive them. It may cost you more to start off with but it’s always money well spent. The odds are that if you have a well-stocked junkbox, then you’ll have many of the parts already. The Jumbo Clock featured here has the added attraction of having very large display digits. It is designed to hang on a wall and can be easily read at distances of 40 metres or more. It’s just the shot for a factory or small business, or any appli­cation that requires a large viewing distance. CRYSTAL OSCILLATOR ÷16384 IC1 ÷2 IC2a ÷60 IC3 TIME SET MINUTES 12-1 CLOCK PULSE IC9b TIME SET HOURS CLK IN AM/PM LATCH IC8b TEN-HOUR COUNT AND LATCH IC8a,IC9a BCD COUNTER 3 IC7 BCD COUNTER 2 IC6 CIN CO BCD COUNTER 1 IC5 Block diagram The main sections of the clock are shown in the block diagram of Fig.1. It uses an accurate frequency reference which is divided down and used to clock a number of BCD counters and a latch. There are three BCD counters in all – two to count the min­utes and one to count the hours from 0-9. All three counters drive 7-segment LED displays via NPN transistor buffers. The latch provides the 10-hour count and drives two seg­ments of a fourth LED display. Let’s go through the block diagram step-by-step and explain how it all works. Basically, you can think of a clock as a specialised coun­ter that increments once every minute. Unlike a conventional counter, it is presettable and has a somewhat unusual count sequence; eg, it counts from 59 to 00 and from 12 to 1. Let’s begin with the section that generates the pulses. These have to be accurate and that means that we can’t use a simple RC-type oscillator to do the job. This type of oscillator drifts with temperature and any frequency variations can trans­ late into quite large errors. DISPLAY DIMMER IC4d Fig.1: the Jumbo Clock uses a crystal-controlled oscillator (IC1) to generate an accurate reference frequency. This frequency is then divided down & used to clock BCD counters IC5-IC7 & a latching circuit (IC8a & IC9a). These in turn drive four 7-segment LED displays, while IC8b drives the AM/PM indicator. What’s needed is a very accurate frequency reference and this has been obtained by using a digital watch crystal. This type of crystal oscillates at 32.768kHz and this is divided down Main Features • Jumbo-sized 4-digit LED read­ out. • • 12-hour operation. • Automatic display dimming at night. • • • AM/PM indication. Separate hours & minutes settings. Crystal-controlled timing. 12VDC plugpack power supply with back-up battery. by 16,384 to obtain an accurate 2Hz square-wave signal. To obtain one pulse every minute, we need a frequency of 0.0166Hz and so our 2Hz signal must be further divided by 120. This is achieved by first passing it through a divide-by-2 stage and then through a divide-by-60 stage. The resulting 0.016Hz signal is fed into counter 1, which is the 0-9 minutes counter. Its carry out (CO) output goes high on the 10th count and clocks counter 2 which counts the tens of minutes. Because the maximum count that the minutes counter can display is 59, we have to detect the 60th count and this is done by checking counter 2’s display driver outputs. When the 60th count is reached, the first two counters are reset and counter 3 is incremented by one. Finally, the CO output from counter November 1993  17 18  Silicon Chip 39pF 22k 12 13 8 CK R 10 D +V1 E C 10 10 330  330  E C +V1 11 9 47k 680  B Q28 BC548 IC8b Q2 BC558 B Q S Q 14 +V1 VC1 5-30pF X1 32.768kHz 10M 11 11 +V1 9 11 5 DP 11 12 f 7 IC4c 8 e 4 3 12 13 +V1 DISP4 SC23-12EWA IC1 4060 16 2 1 R 4 D 5 3 C E +V2 22k Q6 BC558 B +V1 Q3 BC548 B 100k 6 IC8a 4013 3 CK Q S 6 7 Q E C 47k 4 1 D IC2a Q 2 4013 Q 3 CK 7 5 14 5 8 b S 8 c d B 1k 7 E e R f 15 6 g a 2 c b 1 d DISP3 SC23-12EWA e f 4 7x 330  B 7 g CLEN CLK 16 .001 100k .001 9 8 2 1 99 C +V1 10 E CK D 8 S 8 R 10 Q 13 MINUTES S3 .001 D1 1N914 IC9b .001 +V1 11 11 10k 4081 14 6 4 IC4a 5 11 6 1 15 13 12 Q21-Q27 7xBC548 5 DP C 10 12 13 9 CO a IC7 4026 DISEN 7 IC9a 4013 Q 2OUT CK D 3 3 5 9 R Q2B Q3B 14 4 R TIME SET S1 7 CLKA IC3 ENB 4518 Q4A 14 11 10 10 6 16 1k +V1 100pF a 3 c d IC6 4026 B 1k 1 9 7 CLK 15 g 5 6 B 7 4 8 2 1 .001 10 IC4d CLEN R 7x 330  Q14-Q20 7xBC548 11 6 f 1k 1 2 0.1 DISP2 DP SC23-12EWA 2 E C e D2 1N914  33k LDR1 10 12 13 9 b DISEN 16 47k D3 .001 1N914 HOURS S2 12VDC 500mA PLUG-PACK 3 47k 9 8 47k +V1 47k 47k E OUT 5 +V2 a CO +V2 D7 1N4004 2.2k E C C E GND 7812 Q5 C BC337 B Q1 BC548 B Q4 BC558 IN JUMBO CLOCK 10 E C IC4b 3 100 25VW D4 1N4004 1 c d IC5 4026 100k B 8 7 E C e 16 4 2 7x 330  g 1 B 7 R CLEN DISP1 SC23-12EWA 6 f 11 6 DISEN 3 +V1 100pF Q7-Q13 7xBC548 10 12 13 9 b CLK B 100 16VW C E VIEWED FROM BELOW 9V BATTERY BACKUP D6 1N4004 D5 1N4004 9 8 15 2 10 E C V1 I GO +V1 ▲ Fig.2 (left): all the IC numbers on the circuit diagram are directly related to the circuit diagram. IC5 is the 0-9 minutes counter, IC6, the minutes tens counter, IC7 the 0-9 hours counter, & IC8a & IC9a the 10-hour count & latch circuit. These drive the LED displays via transistors Q2 & Q7-Q27. 3 clocks a latch when a count of 10 hours is reached. This latch not only drives the two segments of the fourth LED display but also drives a display latch to give AM/PM indication. It also provides a reset pulse to counter 3 for the transition from “12” to “1” – more on this later. Time setting is achieved by feeding the 2Hz clock signal directly into counters 1 and 3 so that the minutes and hours can be incremented separately. This makes time-setting a breeze. Circuit diagram Fig.2 shows the full circuit details of the Digital Clock. Note that all the IC numbers on the block diagram can be related directly to the circuit diagram. IC5 is the 0-9 minutes counter, IC6 the minutes tens counter, IC7 the 0-9 hours counter, and IC8a & IC9a the 10-hour count and latch circuit. In greater detail, IC1 is a CMOS 4060 14-bit counter and oscillator which has its frequency set by a 32.768kHz watch crystal. A 39pF capacitor provides the correct loading for the crystal to ensure that it operates correctly, while a 5-30pF trimmer capacitor (VC1) allows the crystal frequency to be trimmed slightly so that the clock keeps accurate time. The output from pin 3 of IC1 is the required 2Hz square-wave signal (ie, the crystal frequency is divided by 214). This signal is divided by flipflop IC9a to produce a 1Hz signal on pin 1 which, among other things, is used to flash the two centre decimal points on the display to divide the hours and minutes digits. The 1Hz signal from IC2a is also fed to a divide-by-60 circuit based on IC3, a 4518 dual BCD counter. Both count­ers inside this IC are connected in cascade, with AND gate IC4a used to detect a ‘6’ output from the second counter. Pin 4 of IC4a drives an RC time constant consisting of a 10kΩ resistor and a .001µF capacitor. Each time IC3 reaches a count of 60, pin 4 of IC4a goes high, the capacitor charges and pin 15 of IC3 is pulled high. Thus, IC3 is reset to 00 a short time after the count of 60 is reached. As a result, each time IC3 counts to 60, pin 4 of IC4a briefly switches high. IC4a thus delivers a 0.016Hz pulse train (ie, one pulse per minute) and this signal clocks minutes BCD counter IC5 via D1. Depending on the count, IC5’s a-g segment outputs then drive LED display DISP1 via buffer transistors Q7-Q13 and their associated 330Ω current limiting resistors. Similarly, counters IC6 and IC7 drive DISP2 and DISP3 via transistors Q14-Q27. IC5’s CO output clocks IC6 (the minutes tens counter) on every 10th count, as described previously. It’s here that we strike the first wrinkle. When IC6 reaches a count of six, two things must happen: (1) IC5 & IC6 must both be reset to zero; and (2) a clock signal must be applied to hours counter IC7. As it turns out, we can easily detect the 6th count by monitoring the “b” and “e” outputs from IC6. When a `6' is to be displayed, the “b” output segment is low and the “e” segment output is high. These two conditions only occur together at the 6th count. Thus, on the 6th count, transistor Q1 will be off and pin 8 of IC4b will be high. Pin 9 of IC4b also goes high on the 6th count and thus pin 10 switches high and clocks hours counter IC7 via D2. IC4b then resets IC6 a short time later via the RC delay circuit connected to its output. Because the time constant of this RC circuit is very small, the observer doesn’t see the ‘6’ appear. The output pulse from IC4b is still long enough to clock hours counter IC7, howev­er. Hours counter This is where things start to get a little tricky. That’s because IC7 must cycle from 1 to 9 to 0 (as in 1am-10am or 1pm-10pm), then from 1 to 2 (as in 11am-12pm or 11pm-12am), then from 1-0 again and so on. This sequence is impossible for a 4026 UP counter to do on its own but it can be done by adding a small amount of extra circuitry based mainly on IC9a. We’ll look at this in some detail shortly. IC8 is a 4013 dual D-type flipflop, with IC8a connected as a latch to drive the leading display. Because this display either shows a ‘1’ or is off, segments “e” and “f” are tied together via 1kΩ resistors and driven by the Q-bar output of IC8a via transis­tor Q2. When Q-bar is low, Q2 turns on and the two segments light to show a “1”. Conversely, when Q-bar is high, Q2 and the seg­ments are off. IC8a is clocked by the CO output of IC7. When IC7 reaches a count of 10, its CO output goes high and Q-bar of IC8a goes low, thus turning on Q2 and RESISTOR COLOUR CODES ❏ No. ❏   1 ❏   3 ❏   7 ❏   1 ❏   2 ❏   1 ❏   1 ❏   4 ❏   1 ❏ 23 Value 10MΩ 100kΩ 47kΩ 33kΩ 22kΩ 10kΩ 2.2kΩ 1kΩ 680Ω 330Ω 4-Band Code (1%) brown black blue brown brown black yellow brown yellow violet orange brown orange orange orange brown red red orange brown brown black orange brown red red red brown brown black red brown blue grey brown brown orange orange brown brown 5-Band Code (1%) brown black black green brown brown black black orange brown yellow violet black red brown orange orange black red brown red red black red brown brown black black red brown red red black brown brown brown black black brown brown blue grey black black brown orange orange black black brown November 1993  19 PARTS LIST 1 PC board, code 04108931, 245 x 215mm 1 red Perspex panel, 250 x 220mm 1 3.5mm DC socket 1 12VDC 500mA plug pack 1 PC mount 9V battery holder 1 light dependant resistor (LDR1, Jaycar Cat. RD-3480) 3 pushbutton momentary switches (S1,S2,S3) 4 25mm tapped spacers 4 10mm x 3mm machine screws 1 32.768kHz watch crystal (X1) 1 9V battery 12 PC stakes Semiconductors 1 4060 oscillator/14-bit counter (IC1) 3 4013 dual D flipflops (IC2,IC8,IC9) 1 4518 dual 4-bit BCD counter (IC3) 1 4081 quad 2-input AND gate (IC4) 3 4026 decade counter/display drivers (IC5-IC7) 24 BC548 NPN transistors (Q1,Q3,Q7-Q28) 3 BC558 PNP transistors (Q2,Q4,Q6) 1 BC337 NPN transistor (Q5) 1 7812 3-terminal regulator 4 SC23-12EWA commoncathode 7-segment 70mm LED displays (DISP 1-4) 3 1N914 signal diodes (D1-D3) 4 1N4004 silicon diodes (D4-D7) the “e” and “f” segments of the leading hours digit. Now let’s see how IC7 cycles through its count sequence. As already discussed, clock pulses are applied to IC7 at regular 1-hour intervals via diode D2. Assume for the moment that the time is currently 1:59; ie, IC7 is at a count of “1”. When the next clock pulse arrives, IC7 goes to a count of 2 (ie, we have 2:00 on the displays) and this causes the “2OUT” pin (pin 14) to go low. This low transition is ignored by the clock input of IC9a, since this flipflop can only change state when its clock input goes from low to high (provided its Reset input is low). When the next clock pulse occurs, IC7 goes to a count of “3” and pin 14 of IC7 goes high again. This high is applied to the clock input of IC9a but IC9a ignores the clock pulse on this occasion. That’s because its reset input (pin 4) is held high by the Q-bar output from IC8a. However, when the count in IC8a and IC7 reaches 13, Q-bar of IC8a is low. IC9a thus switches its Q output (pin 1) high on receipt of the clock pulse and this resets both IC7 and IC8a. Q-bar of IC8a now goes high again and turns off transistor Q2 and the leading digit (ie, the leading digit is blanked). At the same time, IC7 is reset to “0”. But we don’t want the hours units display to show “0”; we want it to show a “1” instead. That’s achieved by using the Q-bar output of IC8a to clock IC9b when it switches high to turn off the leading hours digit. When that happens, IC9b’s Q output switches high and feeds a clock pulse to IC7 via D3 to that IC7 immediately advances to a count of 1. IC9b then resets itself almost immediately via the RC time constant on its pin 13 output. In summary then, the hours count­ers (IC7 & IC8a) count to 12 and are reset to 0 on the 13th count. IC7 is then immediately clocked to produce a “1” on the display. This all happens very quickly so that, as far as the observer is concerned, the display goes straight from “12:59” to “1:00”. Q3, IC4c and IC8b are used to drive the AM/PM indicator. Q3 inverts the 2OUT output from IC7 and drives one input of AND gate IC4c, while the Q output of IC8a drives the other input Capacitors 2 100µF 25VW electrolytic 1 0.1µF 63VW MKT polyester 6 .001µF 63VW MKT polyester 2 100pF ceramic 1 39pF ceramic 1 5-30pF trimmer capacitor (VC1) Resistors (0.25W, 1%) 1 10MΩ 1 10kΩ 3 100kΩ 1 2.2kΩ 7 47kΩ 4 1kΩ 1 33kΩ 1 680Ω 2 22kΩ 23 330Ω Where to buy the parts Kits for this project will be available exclusively from Jaycar Electronics Pty Ltd, who sponsored the design. 20  Silicon Chip This view shows the completed Jumbo Clock with the Perspex cover in place. The time-setting switches & the LDR (which controls the display dimming) are at top right. LED BRAKE LIGHT INDICATOR This “brilliant” brake light indicator employs 60 high intensity LEDs (550-1000mCd) to produce a display that is highly visible, even in bright sunlight. The intensity produced is equal to or better than the LED brake indicators which are now included in some late model “upmarket” vehicles. The LED displays used in most of these cars simply make all the LEDs turn on every time the brakes are applied. The circuit used in this unit can perform in this manner and, for non-automotive applications, it can be customised to produce a number of sweeps (110) starting at the centre of the display and with a variable sweep rate. It not only looks spectacular but also attracts more attention. All the necessary “electronics” is assempled on two identical PCBs and the resulting overall length of the twin bargraph dis­play is 460mm. It’s simple to install into a car since only two connections are required: Earth and the brake­ LASER SCANNER ASSEMBLIES These are complete laser scanners as used in laser printers. Include IR laser diode optics and a very useful polygon scanner ( motor-mirror). Produces a “fan” of light (approx. 30 deg) in one plane from any laser beam. We provide information on polygon scanner only. Clearance: $60 400 x 128 LCD DISPLAY MODULE – HITACHI These are silver grey Hitachi LM215XB dot matrix displays. They are installed in an attractive housing and a connector is provided. Data for the display is provided. BRAND NEW units at a low: $40 LASER OPTICS The collimating lens set is used to improve the beam (focus) divergence. The 1/4-wave plate and the beam splitter are used in holography and experimentation. All are priced at a fraction of their real value: 1/4 wave plate (633nM) ..............................$20 Collimating lens sets ..................................$45 Polarizing cube beam splitters ....................$65 GREEN LASER TUBES We have a limited supply of some 0.5mW GREEN ( 560nm) HeNe laser tubes. Because of the relative response of the human eye, these appear as bright as about a 2mW red tube: Very bright. We will supply this tube and a suitable 12V laser power supply kit for a low: $299 CCD ELEMENT BRAND NEW high sensitivity monolythic single line 2048 element image sensors as used in fax machines, optical charachter recognition and other high resolution imaging applications: Fairchild CCD122. Have usable response in the visible and IR spectrum. Supplied with 21 pages of data and a typical application circuit. $30 INFRARED TUBE AND SUPPLY These are the key components needed for making an INFRARED NIGHT VIEWER. The tubes will convert infrared light into visible light on the phosphor screen. These are prefocussed tubes similar to type 6929. They do not require a focus voltage. Very small: 34mm diameter, 68mm long. All that is needed to make the tube light connecting wire. The case for the prototype unit which would be suitable for mounting on the rear parcel shelf, was mainly made from two aluminium “L” brackets that were screwed together to make a “U” section. A metal rod and its matching holders (commonly available from hardware shops) are used for the supporting leg. $60 for both the PCBs, all the onboard components & instruc­tions: the 60 LEDs are included! We also have available a similar kit that does not have the sweeping feature. It produces similar results to the commercial units installed in cars: all the LEDs light up when power is applied. $40 for both the PCBs and all the onboard components. This kit is also supplied with the 60 LEDs and it uses different PCBs, that have identical dimensions to the ones supplied in the above­ mentioned kit. operational is a low current EHT power supply, which we provide ready made or in kit form: powered by a 9V battery and typically draws 20mA. INCREDIBLE PRICING: $90 For the image converter tube and an EHT power supply kit! All that is needed to make a complete IR night viewer is a lens an eyeiece and a case: See EA May and Sept. 1990. ALUMINIUM TORCHES – INFRARED LIGHTS These are high quality heavy-duty black anodised aluminium torches that are powered by four “D” cells. Their focussing is adjustable from a spot to a flood. They are water resistant and shock proof. Powered by a krypton bulb – spare bulb included in cap. $42 Note that we have available a very high quality INFRARED FILTER and a RUBBER lens cover that would convert this torch to a good source of IR: $15 extra for the pair. PASSIVE NIGHT VIEWER BARGAIN This kit is based on an BRAND NEW passive night vision scope, which is completely assembled and has an EHT coaxial cable connected. This assembly employs a high gain passive tube which is made in Russia. It has a very high luminous gain and the resultant viewer will produce useful pictures in sub-moonlight illumination. The viewer can also be assisted with infrared illumination in more difficult situations. It needs an EHT power supply to make it functional and we supply a suitable supply and its casing in kit form. This would probably represent the best value passive night viewer that we ever offered! BECAUSE OF A SPECIAL PURCHASE OF THE RUSSIAN-MADE SCOPES, WE HAVE REDUCED THE PRICE OF THIS PREVIOUSLY ADVERTISED ITEM FROM $550 TO A RIDICULOUS: $399 This combination will be soon published as a project in EA. NOTE THE REDUCED PRICE: LIMITED SUPPLY. Previous purchasers of the above kit please contact us. 24VDC TO MAINS VOLTAGE INVERTERS In the form of UNINTERRUPTABLE POWER SUPPLIES (UPS’s).These units contain a 300W, 24V DC to 240V 50Hz mains inverter. Can be used in solar power systems etc. or for their original intended purpose as UPS’s. THESE ARE VERY COMPACT, HIGH QUALITY UPS’s. They feature a 300W - 450W (50Hz) SINEWAVE INVERTER. The inverter is powered by two series 12V 6.5Ahr (24V). batteries that are built into the unit. There is only one catch: because these NEW units have been in storage for a while, we can not guarantee the two batteries for any period of time but we will guarantee that the batteries will perform in the UPS’s when these are supplied. We will provide a 3-month warranty on the UPS’s but not the batteries. A circuit will also be provided. PRICED AT A FRACTION OF THEIR REAL VALUE: BE QUICK! LIMITED STOCK! $239 ATTENTION ALL MOTOROLA MICROPROCESSOR PROGRAMMERS We have advanced information about two new STATE OF THE ART microprocessors to be released by Motorola: 68C705K1 and 68HC705J1. The chips are fully functional micros containing EPROM/OTPROM and RAM. Some of the features of these new LOW COST chips include: *16 pin DIL for the 68HC705K1 chip * 20 pin DIL for the 68HC705J1 chip * 10 fully programmable bi-directional I/O lines * EPROM and RAM on chip * Fully static operation with over 4MHz operating speed. These two chips should become very popular. We have put together a SPECIAL PACKAGE that includes a number of components that enable “playing” with the abovementioned new chips, and also some of the older chips. IN THIS PACKAGE YOU WILL GET: * One very large (330 x 220mm) PCB for the Computer/Trainer published in EA Sept. 93; one 16x2 LCD character display to suit; and one adaptor PCB to suit the 68HC705C8. * One small adaptor PCB that mates the programmer in EA Mar. 93 to the “J” chip, plus circuit. * One standalone programmer PCB for programming the “K” chip plus the circuit and a special transformer to suit. THE ABOVE PACKAGE IS ON SPECIAL AT A RIDICULOUS PRICE OF: $99 Note that the four PCBs supplied are all silk screened and solder masked, and have plated through holes. Their value alone would be in excess of $200! A demonstration disc for the COMPUTER/TRAINER is available for $10. No additional software is currently available. Previous purchasers of the COMPUTER/ TRAINER PCB can get a special credit towards the purchase of the rest of the above package. PLASMA BALL KIT This kit will produce a fascinating colourful changing high voltage discharge in a standard domestic light bulb. The EHT circuit is powered from a 12V supply and draws a low 0.7A. We provide a solder masked and screened PCB, all the onboard components (flyback transformer included), and the instructions at a SPECIAL introductory price of: $ 25 We do not supply the standard light bulb or any casing. The prototype supply was housed in a large coffee jar, with the lamp mounted on the lid – a very attractive low-cost housing! Diagrams included. LASER DIODE KIT – 5mW/670nm Our best visible laser diode kit ever! This one is supplied with a 5mW 670nm diode and the lens, already mounted in a small brass assembly, which has the three connecting wires attached. The lens used is the most efficient we have seen and its focus can be adjusted. We also provide a PCB and all on-board components for a driver kit that features Automatic Power Control (APC). Head has a diameter of 11mm and is 22mm long, APC driver PCB is 20 X 23mm, 4.5-12V operation at approx 80mA. $85 PRECISION STEPPER MOTORS This precision 4-wire Japanese stepper motor has 1.8 degree steps – that is 200 steps per revolution! 56mm diameter, 40mm high, drive shaft has a diameter of 6mm and is 20mm long, 7.2V 0.6A DC. We have a good but LIMITED supply of these brand new motors: $20 HIGH INTENSITY LEDs Narrow angle 5mm red LED’s in a clear housing. Have a luminous power output of 550-1000mCd <at> 20mA. That’s about 1000 times brighter than normal red LED’s. Similar in brightness SPECIAL REDUCED PRICE: 50c Ea or 10 for $4, or 100 for $30. IR VIEWER “TANK SET” ON SPECIAL is a set of components that can be used to make a complete first generation infrared night viewer. These matching lenses, tubes and eyepieces were removed from working tank viewers, and we also supply a suitable EHT power supply for the particular tube supplied. The power supply may be ready made or in kit form: basic instructions provided. The resultant viewer requires IR illumination. $180 We can also supply the complete monocular “Tank Viewer” for the same price, or a binocular viewer for $280: Ring. MINI EL-CHEAPO LASER A very small kit inverter that employs a switchmode power supply: Very efficient! Will power a 1mW tube from a 12V battery whilst consuming about 600 mA! Excellent for high-brightness laser sights, laser pointers, etc. Comes with a compact 1mW laser tube with a maximum dimension of 25mm diameter and an overall length of 150mm. The power supply will have overall dimensions of 40 x 40 x 140mm, making for a very compact combination. $59 For a used 1mW tube plus the kit inverter. OATLEY ELECTRONICS PO Box 89, Oatley, NSW 2223 Phone (02) 579 4985. Fax (02) 570 7910 MAJOR CARDS ACCEPTED WITH PHONE & FAX ORDERS P & P FOR MOST MIXED ORDERS AUSTRALIA: $6; NZ (Air Mail): $10 November 1993  21 VC1 Q3 100k 1 D3 100k .001 .001 1k 1 1 S2 47k Q6 47k Q1 47k 1 IC6 4026 IC7 4026 IC8 4013 LDR 47k 33k Q4 100pF 9V BATTERY IC4 4081 IC9 4013 S1 100pF 1 D2 100k 10M 39pF D5 D1 10k XTAL .001 1 IC3 4518 .001 .001 1 10k 7812 1 IC2 4013 4.7k D7 IC1 4060 47k I G O D6 1 100uF 22k 100uF D4 S3 IC5 4026 .001 47k DISP4 DISP3 DISP2 Q8 Q7 330  Q9 330  Q10 330  Q11 330  Q12 330  Q13 330  Q14 330  Q15 330  Q16 330  330  Q17 Q5 330  1k Q18 330  Q19 330  Q20 330  Q21 330  Q22 330  Q23 330  Q24 330  Q25 330  Q26 330  Q27 330  680  330  330  Q28 1k Q2 2.2k 22k DISP1 0.1 DC SUPPLY SOCKET Fig.3: all the parts for the Jumbo Clock are mounted on one large PC board. Take care when installing the LED displays, as DISP2 & DISP4 must be installed upside down (see text). Power for the circuit comes from a 12V DC plugpack supply, while a 9V battery powers the timekeeping circuitry during blackouts. (pin 12) of the AND gate. Pin 11 of IC4c thus clocks IC8b every 12 hours to toggle the AM/PM indicator. The AM/PM indicator itself is actually the decimal point on the leading digit. A very simple trick is used so that it appears in the top lefthand corner of the display – the display is in­stalled on the PC board upside down! minutes. The circuit works like this: when S1 is pressed, 2Hz clock pulses from IC1 are coupled through to S2 and S3. If S2 is now pressed, these 2Hz pulses are differentiated by a .0015µF capaci­tor and fed to pin 1 of IC7 to increment the hours display. Similarly, if S3 is pressed, the minutes 0-9 counter is clocked. Time setting IC4d, Q4, Q5 and an ORP12 light dependent resistor (LDR1) provide the automatic dimming function for the LED displays. The LDR and its series 33kΩ resistor form a variable voltage divider, the output of which Pushbutton switches S1, S2 and S3 perform the time setting function. To set the time, S1 (TIME SET) is held down and then either S2 pressed to set the hours or S3 pressed to set the 22  Silicon Chip Display dimming depends on the ambient light level. This output is fed to one input of AND gate IC4d. The other input of IC4d is driven by a 512Hz square-wave signal derived from pin 4 of IC1. If the ambient light level is high, the resistance of the LDR is low and the output from IC4d is also low. Conversely, if the light level is low, the LDR’s resistance is high and IC4d gates through the 512Hz squarewave signal from IC1. IC4d drives PNP transistor Q4 via a 47kΩ base current-limiting resistor. When IC4d’s output remains low (ie, the light level is high), Q4 turns on and thus Q5 also turns on and the displays are driven at a 100% duty cycle to provide maximum brightness. Conversely, when the light level is low, IC4d switches Q4 and thus Q5 on The three time-setting switches are mounted by soldering their pins to PC stakes, as shown here. Make sure that the switches are correctly oriented (flat side to top of board) – see Fig.3. The LDR is mounted with its leads left at full length & can be installed either way around. and off at a frequency of 512Hz. Q5 in turn switches the displays on and off at this frequency to reduce the display brightness. Note that the jumbo-sized 70mm LED displays used in this project have the same pinouts as the smaller types but each segment contains five LEDs in series. This makes it necessary to use transistors Q7-Q27 in order to obtain sufficient display brightness. Power supply Power for the circuit is derived from a 12V DC plugpack supply. The incoming DC is fed via reverse polarity protection diode D4 to a 3-terminal 12V regulator. Two separate supply rails are then derived from the output of the regulator via isolating diodes D5 and D7. The +V1 rail powers all the timekeeping circui­try and the driver transistors for the LED displays, while the +V2 rail powers the dimming circuit which in turn controls common digit-driver transistor Q5. A 9V backup battery is used to supply the timekeeping cir­cuitry if the mains fails. This battery is isolated from the +V1 rail via D6 which is normally reverse biased. When the mains fails however, D6 becomes forward biased and the battery takes over and supplies power to the +V1 rail. During this time, diode D5 is reverse biased and so Q5 is off and the LED 24  Silicon Chip displays are blanked. This was done to conserve the batteries in the event of a long blackout. The LED displays come back on again and show the correct time as soon as the mains power is restored. Construction All the components for the digital clock are installed on a single PC board coded 04108931. Fig.3 shows the parts layout on the board. Before installing any of the parts, check the board carefully for etching defects (eg, shorted or open-circuit tracks). There shouldn’t be any problems here but it’s always best to make sure. When you’re satisfied that everything is correct, you can start construction by installing PC stakes at all external wiring points and at the switch mounting positions. This done, install the wire links, resistors and capacitors. Make sure that the wire links are straight so that they don’t short against other parts. You can straighten the link wire if necessary by clamping one end in a vice and then stretching the wire slightly by pull­ing on the other end with a pair of pliers. The semiconductors can now be installed on the PC board, followed by trimmer capacitor VC1 and the 32.768kHz watch crys­tal. Be sure to use the correct part at each location and check that all parts are correctly oriented. In particular, check the transistor type numbers carefully and note that all the ICs face in the same direction. The 3-terminal regulator is installed with its metal tab towards the adjacent power diodes (see Fig.2 for the pin connection details). LED Displays Now for the four LED displays. These are installed directly on the board but there is a catch – displays 2 and 4 must be installed on the board upside down (ie, their decimal points must be at top left – see Fig.3). The other two LED displays (1 & 3) are installed in the usual manner (ie, decimal points at bottom right). Push all the displays down onto the board as far as they will go before sol­dering their pins. Once the displays are in, the board can be completed by installing the pushbutton switches, the battery holder and the LDR. The LDR can be installed either way around and should be soldered in with its leads at maximum length, so that it sits about 25mm above the board. The three pushbutton switches are mounted directly on top of the previously installed PC stakes. Be sure to orient the flat side of each switch body as shown in Fig.3 and make sure that the are vertical and don’t lean to one side. A red Perspex cover was fitted to the prototype to enhance the appearance of the LED displays and to hide the circuitry. This cover measures 250 x 220mm and is mounted on the board using four tapped 25mm spacers and 3mm screws. You will need to mark out and drill a mounting hole in each corner of the cover, plus clearance holes for the time-setting switches and the LDR. The clearance holes are best made by first drilling small holes and then enlarging them to size using a tapered reamer. SILICON CHIP BINDERS BUY A SUBSCRIPTION & GET A DISCOUNT ON THE BINDER (Aust. Only) Testing These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers and are made from a dis­ tinctive 2-tone green vinyl that will look great on your bookshelf. ★ High quality. ★ Hold up to 14 issues (12 issues plus catalogs) ★ 80mm internal width. ★ SILICON CHIP logo printed in gold-coloured lettering on the    spine & cover. Yes! Please send me ________ SILICON CHIP binder(s) at $A14.95 each (incl. postage in Australia). NZ & PNG orders please add $5 each for postage. Not available elsewhere. Enclosed is my cheque/money order for $­__________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. Signature­­­­­­­­­­­­_________________________ Card expiry date______/______ Name _____________________________________________________ Street _____________________________________________________ Suburb/town __________________________ Postcode______________ SILICON CHIP PUBLICATIONS PO Box 139, Collaroy, NSW 2097, Australia. Phone (02) 979 5644 Fax: (02) 979 6503. ✂ Now for the smoke test. Connect the DC plugpack supply and switch on – you should immediately get a readout on the displays, although it might not make much sense at this stage. That’s because the 4026 counters can switch on in a random mode and produce incorrect symbols. To correct the displays, all you have to do is press the time setting buttons (ie, Time Set + Hours and Time set + Minutes) until the counters are clock­ ed and revert to a valid condition. If the clock doesn’t work, switch off and check for wiring errors. In particular, check for incorrect parts placement on the PC board and for shorts between soldered joints on the back of the board. If the displays don’t make much sense, check for shorts between the display segments and that the displays have been correctly oriented (displays 2 & 4 must be installed upside down). If all is well so far, connect the 9V battery back-up battery, set the time and switch off the mains power. The display should now go out but the timekeeping circuitry should continue to function. Leave the mains power off for a few minutes, then switch it back on again. The display should now come back on and show the cor­ rect time. Check that diodes D5 and D6 are correctly oriented if you strike problems here. Finally, check that the display dimming feature works by covering the viewing hole for the LDR. The accuracy of the clock can be adjusted by monitoring it over a 24-hour period and tweak­ing VC1 on a trial and error SC basis. November 1993  25