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s i h t d l i Bu Jumbo LED cloc clo This Jumbo clock has large red LED displays for high visibility in your home, in the office or in a factory. It uses readily available CMOS ICs and runs from a 12V supply. You could even use it in a boat or caravan. By JOHN CLARKE “Tempus Fugit” as they say in Latin, or “Time Flies” in English. Whichever language you prefer, it is hard to ignore this clock with its large red LED displays. In fact, they are 57mm high but the readout is so easy to read it looks larger than it really is. If you’re shortsighted, this is the clock for you. These days, clocks are available in virtually any form. You can have talking watches or clocks; digital or analog readouts with liquid crystal, LED, vacuum fluorescent or mechanical displays; oval, square, round, triangu40  Silicon Chip lar or odd shaped dials; and features such as alarm, calendar, world time, and stopwatch and timer functions. There are even “backward” clocks avail­able. What ever happened to the simple digital clock that was easy to read? Well, here it is. The SILICON CHIP Compact Jumbo Clock uses four 7-segment LED displays to provide 12-hour time; 24hour time is not an option. The only gimmicks, if you could call them that, are a colon flashing once a second and an AM/PM indicator. The display also dims in darkness so that it is not over-bright at night. The circuit is crystal-controlled and has battery backup in case of power failure. The Jumbo Clock is housed in a cutdown plastic instrument case to make it quite compact considering the large display size. A red Perspex panel forms the front of the box while at the rear are two time-setting switches and a DC input socket for a 12V DC plugpack supply. Speed-up feature Model railway enthusiasts who want a “fast clock” will be interested in the Jumbo Clock, as it can be built to run at up to 12 times normal speed. For more information on this subject , refer to the December 1996 issue. Block diagram Fig.1. shows the block diagram for the Jumbo Clock. There are two “minutes” counters to provide the requisite ock Fig.1: block diagram for the Jumbo Clock. There are two “minutes” counters to provide the requisite 0-59 count for the minutes displays, plus one counter and a flipflop for the hours displays. All three counters count in 4-bit binary code and this is fed to 7-segment decoders to drive three of the four LED displays. 0-59 count for the minutes displays and one counter plus a flipflop for the hours displays. All three counters count in 4-bit binary code and this is fed to 7-segment decoders to drive three of the LED displays. The fourth display is driven from the flipflop via a buffer stage. Timing is set by a 32.768kHz crystal oscillator, IC1, which is internally divided to produce a 2Hz output. This is further divided by two for the 1Hz colon driver and by 120 for the one minute signal for the first minutes counter, IC4. At each one-minute clock pulse, the minutes counter increments by one. Each time IC4 reaches the count of 0 (after a 9), its output clocks the second minutes counter, IC6. Thus, DISP2 shows the next digit in its count. When the count of “6” is reached, it is detected in IC11a and IC11b which clears IC6 back to “0”. Thus, DISP2 only counts from 0-5 then back to 0. When IC6 is preset to 0, the hours counter IC8 is clocked to increment DISP3. When IC8 reaches the count of 0 (after the 9), the output clocks flipflop IC10a. IC10a’s Q-bar output then drives the “1” digit of DISP4 via IC12c and IC12d. DISP4 and DISP3 now show a “10”. When IC8 reaches the count of 2 (in other words a 12 is displayed), the IC12b and IC10b circuit turns the AM/PM LED off if it was on, or on if it was off. When IC8 reaches the count of 3 (after the hours display reaches 12), the “3 detect” gates IC11c & IC11d clear flipflop IC10a. DISP4 is then switched off and the Q output drives the load input of IC8 which preloads a 1 into the counter. DISP3 Main Features • • • • • • • • • • • Large red (57mm high) 7-segment LED displays 12-hour display (4-digit readout) Compact plastic housing based on a standard case Flashing colon between hours & minutes digits AM/PM indicator Display automatically dims in darkness Crystal accuracy Hours and minutes set switches on rear panel Battery backup in case of power failure (no display) Runs from a 12V DC plugpack or battery Facility to speed up clock to x2, x3, x4, x6, x8 & x12 March 1997  41 The display board is soldered to the main PC board at right angles, as shown here. Tack solder a couple of the end connections and test fit the assembly in the case before soldering the remaining connections. now shows a 1. The count sequence there­fore changes from 12 to 1, as it should for 12-hour time. Setting the hours is achieved using switch S2 which triggers the “6 detect” output. This clears IC6 and clocks IC8. The minutes setting switch S1 resets the divide by 120 circuit which clocks IC4. The crystal oscillator divider is also reset so that the clock can be synchronised to the exact time from the beginning of the minute. Dimming of the display is controlled by an LDR (light dependent resistor) and transistor Q1. As the ambient light increases, the resistance of the LDR is reduced so it turns Q1 on harder to brighten the display. Circuit description Now let’s have look at the full circuit diagram of Fig.2. It comprises a total of 12 low-cost ICs, four large 7-segment displays, plus several resistors, capacitors, diodes and a 32.768kHz crystal. IC1 is a 4060 14-stage divider with provision for a crys­ tal oscillator at its input pins. A 10MΩ resistor is connected between pins 10 and 11 to bias the internal inverter to linear operation, while the 32.768kHz crystal 42  Silicon Chip is connected between the same pins but in series with a 330kΩ resistor. The 330kΩ resistor and trimmer VC1 prevent the crystal from operating in “overtone” mode (ie, at a multiple of the wanted frequency) by virtue of the RC time con­stant. The resistor also reduces the signal level applied to the crystal while the trimmer allows a small frequency variation for precise timekeeping. IC1 divides the 32.768kHz signal at its pin 10 by 16,384 (212) to pro­ vide a 2Hz square wave at pin 3, the Q14 output. This is fed to IC2 and IC3. These are 4526 programmable count­ers which are set to give a total division of 120. IC2 and IC3 have four preload inputs called DP1, DP2, DP3 and DP4, at pins 5, 11, 14 & 2 respectively. For our circuit, IC2 has DP4 set high to give a division factor of 8. For IC3, DP1, DP2 and DP3 are set high to give a division factor of 112. The two factors are added together to give a total division of 120. Note that, as part of the design provision for speeding up the clock for railway modellers, other division ratios can be used – see Table 1. The divided output from IC3 is applied to the clock input of IC4. IC4 counts from 0-9 and its “Carry Out” signal at pin 7 is used to clock IC6. IC5, IC7 and IC9 are 4511 latched BCD-to-7-segment decoder drivers. They take the 4-bit BCD (binary coded decimal) outputs from counters IC4, IC6 and IC8 and convert it to drive the 7-segment lines of the common cathode LED displays, via 390Ω resistors. Counting to 60 While IC4 is used as a conventional decade counter, counting from 0-9 in BCD, IC6 needs to count up to six and then flick back to zero. This is achieved by using the presettable inputs on the 4029 – J1, J2, J3 & J4 (for jam-load inputs) – at pins 4, 12, 13 & 3, respectively. With all these inputs tied low, the preset value is 0 (in BCD). When IC6 counts up to 6, its Q2 and Q3 outputs both go high and so the output of NAND gate IC11a goes low. This is inverted by IC11b which applies a high load signal to the L input, pin 1. This then presets IC6 back Fig.2 (right): the complete circuit for the Jumbo LED clock. IC1, IC2 & IC3 divide the 32.768kHz crystal by a factor of 1,966,080 (16,384 x 120) to provide one pulse per minute for the minutes counters. March 1997  43 This rear view of the Jumbo LED Clock shows the DC input socket (right) and the hours and minutes time setting switches. Power can be supplied from either a 12V battery or a 12V DC plugpack. to 0, the very instant that the 6-count is reached. This means that IC4 and IC6 actually count to 59 (for the minutes count) before being preset back to 00. The 1kΩ resistor and .001µF capacitor at the pin 9 input of IC11b provide a short time delay to ensure that the load signal is sufficiently long for the counter to function correctly. The load input also clocks counter IC8. When IC8 counts up to 9 and then to 0, its carry out (pin 7) clocks flipflop IC10a. The low data level at pin 5 (the D input) is transferred to the Q output and the Q-bar output goes high. The two segments to display the “1” digit on DISP4 are now driven via gates IC12c and IC12d. Displays DISP4 and DISP3 now show 10. When IC8 is clocked to a count of 2, its Q2 output goes high and this is ANDed with the high Q-bar output of IC10a in IC12b. The resulting high output from IC12b toggles IC10b. Hence, each time the clock shows 12:00, the Q output of IC10b toggles. This drives the AM/PM LED indicator which is the decimal point of DISP4. LEDs 3 and 4 are in series with the AM/ PM drive to allow the dimming circuit to function correctly on all display segments, but more on this later. At the count of 3, the Q1 and Q2 outputs of IC8 both go high and the pin 3 output of IC11c goes low and the output of IC11d goes high. This sets flipflop IC10a so that its Q output is high and its Q-bar output is low. Thus, the displayed “1” in DISP4 goes off and the Q output of IC10a pulls the load input (pin 1) of IC8 high, via a 0.1µF capacitor. The J1 input (pin 4) of IC8 is high and so IC8 is preloaded to a 1. Hence, when IC8 reaches a count of 3, instead of DISP4 & DISP3 displaying “13”, DISP4 is turned off and DISP3 shows “1”. The count sequence for DISP4 and DISP3 is therefore 1-9, 10, 11, 12 (AM/ PM indication) and then 1 again. Power-on reset At switch-on, counters IC4, IC6 and IC8 are preloaded so that the display reads “1.00”. For IC4, the load input (pin 1) is momentarily held high via the 1µF capacitor. This loads a 0 into the counter. The 10kΩ resistor releases the load by charging the capacitor to ground. IC6 is preset via the 1µF capacitor at pin 8 of IC11b initially being discharged. This produces a high at IC11b’s output to preload a 0. RESISTOR COLOUR CODES  No.    1    1    5    2    1  26    1 44  Silicon Chip Value 10MΩ 330kΩ 10kΩ 1kΩ 470Ω 390Ω 10Ω 4-Band Code (1%) brown black blue brown orange orange yellow brown brown black orange brown brown black red brown yellow violet brown brown orange white brown brown brown black black brown 5-Band Code (1%) brown black black green brown orange orange black orange brown brown black black red brown brown black black brown brown yellow violet black black brown orange white black black brown brown black black gold brown PARTS LIST 1 PC board, code 04302971, 224 x 94mm 1 PC board, code 04302972, 252 x 76mm 1 self-adhesive label, 89 x 49mm 1 plastic instrument case, 260 x 190 x 80mm 1 red Perspex sheet, 252 x 76 x 1.5mm 4 SC23-12EWA 57mm 7-segment common cathode LED displays (DISP1-DISP4) (Jaycar Cat. ZD-1850) 4 5mm red LEDs (LED1-LED4) 3 AA cell holders 3 AA nicad cells 1 DC panel socket 1 12VDC 500mA plugpack 2 snap action PC board mounting pushbutton switches (S1,S2) 1 LDR (LDR1) (Jaycar Cat RD3480 or equivalent) 1 32.768kHz watch crystal (X1) 1 10kΩ horizontal trimpot (VR1) 1 300mm length red hookup wire 1 300mm length green hookup wire 1 900mm length 0.8mm tinned copper wire 1 3mm screw, washer & nut 4 self-tapping screws 8 PC stakes Semiconductors 1 4060 14-stage ripple carry bina- Time setting The hours display of the clock is set by pressing button S2. This discharges the 1µF capacitor at the pin 8 input of IC11b. Thus, IC6 is preloaded to a 0 and IC8 is clocked. Upon releasing S2, the 1µF capacitor charges and IC11b's output goes low again. Thus every time S2 is pressed, the hours display is incremented. Capacitors 1 2200µF 25VW PC electrolytic 4 1µF 16VW PC electrolytic 9 0.1µF (100n or 104) MKT polyester or monolithic ceramic 1 .001µF (1n0 or 102) MKT polyester 1 8.5-50pF trimmer (VC1) 1 22pF NPO ceramic Resistors (0.25W, 1%) 1 10MΩ 1 470Ω 0.5W 1 330kΩ 28 390Ω 5 10kΩ 1 10Ω 1 1kΩ The AM/PM indicator can be set by counting to 12:00. The minutes display is set by pressing S1. This applies a reset to IC1, IC2 and IC3. A positive pulse is applied to the clock input of IC4 on each reset. Note that counter IC1 is reset to ensure that on setting the minutes, the seconds are also reset. The clock is thus reset to begin counting at the beginning of the min­ ute; ie, as soon as S1 is released. The colon between the hours and minutes displays is formed with the decimal points of DISP2 and DISP3. The 1-second pulse output of IC2 is buffered using IC12a to drive the decimal points via two series-connected LEDs (LED1 and LED2) and 390Ω resistors. Note that if the clock is set to run at a x2 or x4 speed using Electronic Projects For Cars 5 $8.9 PLUS P & $3 P Available only from Silicon Chip Price: $8.95 (plus $3 for postage). Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. Use this handy form  The 1µF capacitor at pin 13 of IC11d produces a momentary high at the set input of IC10a. This sets its Q output high to produce a load signal to IC8 and thus preloads a 1. The low Q-bar of IC10a prevents the “1” digit in DISP4 from lighting. Thus on power up, the clock resets to 1:00. The AM/ PM indicator could be either on or off at power on. ry counter (IC1) 2 4526 programmable divide-by-N 4-bit binary counters (IC2,IC3) 3 4029 presettable binary counters (IC4, IC6 & IC8) 3 4511 BCD-to-7-segment decoders (IC5,IC7 & IC9) 1 4013 dual D flipflop (IC10) 1 4093 quad 2-input NAND Schmitt trigger (IC11) 1 4081 quad 2-input AND gate (IC12) 1 BD682 PNP Darlington transistor (Q1) 1 15V 1W zener diode (ZD1) 1 1N914, 1N4148 signal diode (D1) 1 1N4004 1A diode (D2) Enclosed is my cheque/money order for $________ or please debit my  Bankcard  Visa  Mastercard Card No: ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ March 1997  45 Fig.3: this diagram shows the component layout of the main PC board and wiring for the backup battery. Take care to ensure that each IC is correctly oriented. 46  Silicon Chip the pin 2 and pin 1 outputs of IC1, the colon will flash at a 2Hz or 4Hz rate. Display dimming Transistor Q1 drives the common cathodes of all four LED displays. It is connected as an emitter-follower so that the voltage at the emitter follows the base voltage. The base voltage is set by trimpot VR1 and the LDR. As the ambient light increases, the resistance of the LDR is reduced and Q1 turns on harder to brighten the display. In darkness, the resistance of the LDR increases and so Q1 is not turned on quite as hard and the display dims. VR1 allows adjustment of the dimmed display brightness. The dimming effect is dependent on the voltage drop across the LED display segments. For the main segments, there are four LEDs in series to produce an even light distribution over the lit element. Because the decimal point is smaller, only two LEDs are in series. We have compensated for this lower display voltage drop by adding two LEDs in series for the colon decimal points (LEDs 1 & 2) plus two more for the AM/PM indicator (LEDs 3 & 4). These extra LEDs are not seen in the clock display but are still illu­minated on the main PC board where they are mounted. Power The clock circuit is normally powered from a 12VDC plug­pack. These usually produce more than 15V when unloaded and so a 15V zener diode has been included to protect the ICs from overvoltage. A 220µF capacitor and nine 0.1µF capacitors dotted around the PC board provide power supply decoupling. The backup battery consists of three 1.2V nicad cells in series. These are kept charged via a 470Ω resistor from the 12V supply. If the plugpack is disconnected or the mains power is off, the battery feeds power to the circuit. Note that the voltage is too low for the displays to light, but sufficient to keep the ICs going. When power is restored, the time displayed will be correct. The battery is protected against reverse connection of the plugpack supply via D2, while ZD1 protects the clock circuit. The 10Ω resistor feeding ZD1 is likely to go open circuit if the reverse polarity connection is maintained Fig.4: the display board accommodates the four LED readouts and the LDR. Note that DISP2 and DISP4 are mounted upside down so that the decimal points are at the top of the display. The LDR should be mounted so that its surface lines up with the front of the LED displays. This device is the sensor for the automatic dimming circuitry. March 1997  47 This is the view inside the case with the top and the red Perspex front panel removed. The three 1.5V backup batteries are mounted in single-cell holders which are attached to the rear panel. but, apart from this, there will be no other damage. Construction The Jumbo Clock is built on two PC boards which are mounted at right angles to each other. The main PC board is coded 04302971 and measures 224 x 94mm, while the vertical display PC board is coded 04302972 and measures 252 x 76mm. It is housed in a plastic instrument case which has been reduced in depth so that its over­all measurements are 260 x 80 x 118mm (W x H x D). The parts layout diagram for the main PC board is shown in Fig.3 while the display board is shown in Fig.4. Begin construction by checking the PC boards for shorts between tracks, breaks in tracks or undrilled holes. Fix any board defects before proceeding. Next, insert and solder in all the links as shown on the overlay diagram. Be sure to install the links on the display board before placing the displays in position. Note that DISP4 and DISP2 are mounted upside down as indicated by the position of the decimal point. The LDR is mounted so that its face is about level with the front of the displays. Insert the resistors and PC stakes next. The PC stakes are required for mounting the time-setting switches and for the power supply connections. This done, install the capacitors, COMPACT JUMBO CLOCK SET MINUTES HOURS (SET LAST) (SET FIRST) + 48  Silicon Chip + DC INPUT 12VDC <at> 500mA + Fig.5: this full size artwork can be copied and attached to the rear panel. making sure that the electrolytics are inserted the right way around; ie, with correct polarity. Next, install the diodes, zener diode and LEDs and make sure that each is oriented correctly. The same comment applies when installing the ICs. Note that the 4511 ICs (IC5, IC7 and IC9) are oriented differently to the other ICs. Transistor Q1 is mounted horizontally with its metal face towards the PC board. We inserted a metal washer between the transistor and PC board before securing it with a screw and nut. The washer will allow the small amount of heat generated to dissipate more readily. Finally, wire in the time-setting switches, the adjustable trimmer capacitor VC1, trimpot VR1 and the crystal. Make sure that the switches are correctly oriented. Modifying the case To make a reasonably compact case, we took a standard plastic instrument case measuring 260 x 80 x 190mm and reduced its depth to 119mm. This can be easily done using a hacksaw. You will need to mark the cutting line on each case half and then cut between the integral slots. After you have finished with the hacksaw you can use a file to clean up the cuts. Note that the part that must be removed does not have the speaker slots Fig.6: here are the actual size artworks for the two PC boards. Check your boards carefully for defects before installing any of the parts. March 1997  49 The PC board assembly is secured using four self-tapping screws. These go into integral plastic pillars moulded into the base of the case. Notice how the display PC board slides into the rearmost slot at the front. in the base. The rear plastic panel will need to be chamfered slightly around the edges since the new rear slot is slightly narrower than the original. Remove all the integral mounting pillars in the base of the case, except for the four in the corners (these support the PC board). This can be done by using a large drill. The cut down case halves still join together neatly and are retained using the original two screws. Next, place the main PC board in position and locat­e it over the mounting pillars. This done, slide the display board into the rearmost front slot and mark the rear of this board where the main PC board makes contact. You can now remove both PC boards and tack solder them together at the large copper pads, making sure that they are at right angles. Finally, check the assembly in the case again to make sure that everything is correct before soldering all the matching pads. It is a good idea to apply a liberal fillet of solder to the large copper pads to improve mechanical strength. The rear panel can now be drilled to accept the DC socket and switches S1 and S2. Attach the DC socket and cell holders as shown, using contact adhesive or double-sided adhesive tape. Finally, wire up the socket and holders as shown in Fig.3. Testing Only time will tell if the circuit is working or not (Er .. sorry about that!). Rotate VR1 fully clockwise, apply power and check that the displays show 1:00. If there is no display at Table 1: Clock Speed Options IC2 IC3 Speed IC1 to IC2 Link Pin 2 Pin 5 Pin 5 Pin 11 x1 Pin 3 to pin 6 H L L L L H H H x2 Pin 2 to pin 6 H L L L L H H H x3 Pin 3 to pin 6 H L L L L H H H x4 Pin 2 to pin 6 H L L L L H H H x6 Pin 2 to pin 6 H L L L L H H H x8 Pin 1 to pin 6 H L L L L H H H x12 Pin 1 to pin 6 H L L L L H H H 50  Silicon Chip Pin 11 Pin 14 Pin 2 Pin 14 all, disconnect the power and check for reversed supply connections or incorrectly placed com­ponents. If all is well, the colon should flash at a one-second rate. You should be able to increment the hours and minutes with the time-setting switches. Check that the minutes digits count from 00 to 59 then 00 again and that the hours digits count from 1 to 12. Verify that the AM/PM indicator lights on alternative 12:00 time. Optional speed-up Table 1 shows the modifications required for faster than normal clock operation. Note that the PC board has been designed so that you only need to cut the narrowed tracks leading to the IC1 output and the IC2 and IC3 DP inputs, before applying solder bridges to make the new contacts. Most of the changes are indicated on the PC board pattern. Note that for time­keeping rates beyond x4, you have to modify the linking to IC1 and to either or both IC2 and IC3. Finally, insert the cells in their holders and adjust VR1 so that, when you place your finger on the LDR, the display dims (the final adjustment should be made in the dark). Trimmer capacitor VC1 can be adjusted if the clock needs to run slightly faster or slower in order to keep the correct time. If you have a frequency meter, it can be connected to pin 9 of IC1 and VC1 adjusted for a reading of exactly SC 32.76800Hz.