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Design by SCOTT MELLING* *Grantronics Pty Ltd. A LED Clock with a difference Here’s a LED digital clock with a difference – a circular 60-LED array which chases anticlockwise each second to build up a count of seconds until it gets to 60, whereupon the chase starts all over again. The effect is mesmerising. Have you even been accused of being a clock-watcher? Whether you have or not, there is a definite risk of being entranced (enchanted?) with this new LED digital clock. You tend to ignore the central 4-digit display and just concentrate on that magical circular LED performance. At the beginning of each minute, each successive LED races anticlockwise around the periphery to take up its position as the seconds count builds up. As the seconds count nears 26  Silicon Chip 30, each new LED only has to traverse half the circle and so each LED makes its circuit slightly slower than the last until finally, as the count approaches 60, the last few LEDs make the transit very slowly indeed. But each LED transit around the circle, whether it covers the whole 360° or just a few, takes exactly one second. So you find yourself wondering: just what fancy machinations have been pulled to achieve that? The answer is, of course, that there is a fancy microcontroller calling all the shots. But even knowing that and having considered all the programming that must have gone into it, you still tend to sit there mesmerised by this clock. You just have to see it but be warned – when you do, you will probably want one! Apart from that magic circular LED array, this wall clock also has a 4-digit readout with 12 or 24-hour operation. It also features an alarm with piezo sound and opto output, a battery backup for time-keeping and alarm functions, AC mains synchronisation and crystal timebase for precise timing, an efficient switchmode supply powered by a 12V AC plugpack and a high quality double-sided, screenprinted and solder-masked PC board with plated-through holes. What more could you want? The hours and minutes display consists of the four large digits in the siliconchip.com.au How The Seconds Chaser Works centre of the clock face. It can be set to display either 12 or 24-hour time, depending on the position of a single jumper link (JP1). On the righthand side of the minutes digits is an AM/ PM indicator LED and this is active for PM hours if the 12-hour display mode is chosen. The timing uses a crystal oscillator for short term accuracy with the chaser control and is synchronised to the mains AC cycles when present for long term accuracy. The clock’s alarm features need explaining. Apart from the piezo buzzer that can be set to sound as an alarm, there is an optocoupler output which allows the clock to trigger an external device. Once both or either of these outputs has been enabled and becomes active, they can be reset by pressing any of the three time-setting buttons on the back of the clock. There is provision to connect a backup battery to the clock for periods when the mains power fails. When running from the backup battery, the LEDs are not illuminated, to enhance battery life. Unlike many other designs, however, the alarm output and opto output will still activate during a mains power failure. The battery backup circuit also includes a charging facility so that NiMH or Nicad cells can be used. There are three buttons on the back of the clock labelled UP, MODE and DOWN. The functions of these buttons vary depending on which “mode” the clock is currently displaying. Four seconds into the minute. At the start of each second, the chase LED starts at the top and travels anticlockwise around the clock face, as indicated by the green arrow. Eighteen seconds into the minute. The chaser LED is shown here travelling anti-clockwise past the 40s mark. Note: the green arrow is not part of the clock display. Thirty seconds into the minute. The chase LED is now really starting to slow down, since it has much less distance to travel in 1s. Forty-seven seconds in and the chase LED is getting slower and slower. It now travels less than 45° of arc in one second. Coming down the straight . . . the chase LED moves very slowly during the last few seconds of the minute. Finished – 60 seconds is up and the minutes digit “ticks” over. The seconds LEDs now go out and the chase sequence starts again. Menus and setup When AC power is first applied, the clock will power up and proceed to run, beginning with a default time of 0:00 and 0 seconds. This is the “run” mode, identified by the standard LED chasing pattern described above. In all other “utility” modes, as set by the MODE button, there is a very different chase sequence to indicate you are not in “run” mode. In the “time-hours set” mode, the hours digits display as “Ch”. You can then press the UP and DOWN buttons to set the clock’s hours. In “time-minutes set” mode, the hours digits display as “C” and pressing the UP and DOWN buttons allow the clock’s minutes to be set. In “alarm/opto enable” mode, the UP button toggles the alarm on and off. When enabled, the hours digits siliconchip.com.au show “AL.” The DOWN button toggles the opto output on and off. When this is enabled, the minutes digits reads “Au.” In “alarm-hours set” mode, the hours digits display “Ah” while in “alarm-minutes set” mode, the hours digits display “A .” Using the UP and DOWN buttons in both modes allows the clock’s hours or minutes display to be set for alarm triggering. The same comments apply to the “opto-hour set” mode (display “Hh”) and “opto-minutes set” (display “H”). June 2005  27 (LD61) is enabled (ie, it lights after 12 o’clock midday). Conversely, shorting pins 2 & 3 with a jumper converts the clock to 24-hour operation and disables the AM/PM indicator LED. Programming header As well as JP1, Fig.1 also shows a 6-way pin header connected to pins 9, 6, 8 & 7 of the microcontroller. This header was included during development to allow for in-circuit programming of the microcontroller and has been retained for those who like to experiment. Most people will not want this facility, in which case the pin header can be left off the PC board. Power supply The LED clock comes as a complete kit of parts and includes a double-sided plated-through PC board with a solder mask and silk-screened overlay. When triggered, both the audible alarm and the opto output are disabled by pressing any of the pushbuttons. Circuit description The brain behind the operation of the clock is an Atmel ATMEGA851516PC microcontroller – see Fig.1. It runs at 8MHz, which gives approximately 8MIPS throughput with a machine cycle of 125ns – eat your heart out Microchip! The PC board layout was actually designed for the now obsolete Atmel AT90S8515, together with its MC34064P-5 under-voltage sensor (U4), but the ATMEGA8515 is a dropin replacement. It also incorporates an on-chip under-voltage detector which has made the MC34064P-5 redundant. The ATMEGA8515-16PC will operate happily down to 2.7V, relying on its own internal brownout detector. The short-term timing of the clock is derived from an 8MHz crystal but this may drift slightly over several months. To help combat the drift problem, the micro samples the AC mains supply, comparing this cycle count every hour to the expected cycle count for the 50Hz (or 60Hz) AC supply. If it is close to being in sync, the assumption is that there has been some small drift and the micro is re-synchronised. If there is a large difference, the assumption is that the AC mains supply 28  Silicon Chip is not present or was not present for a part of the last hour’s operation, and the synchronisation process is skipped for that time around. LED arrays All of the LEDs on the clock face, except the LED that sits in parallel with the buzzer, are in three 5 x 7 matrices. Each LED in each matrix is individually controllable except in the case of the digits where each segment is controllable. Seven bits of ports A, C and D on the ATMEGA8515 are used to drive three ULN2003 7-way Darlington transistor drivers (ie, driving three matrices), allowing the clock to multiplex up to 21 LEDs on at any one time. There are five BD682 PNP transistors on the supply side of the LED arrays, breaking it into parts that can be time division multiplexed with about a 20% duty cycle. The base cycle time used is 1ms, so each LED (if required) is on for 1ms in every 5ms. The associated 220W resistors limit the current in any LEDs that are active. Display format The 3-way pin header labelled JP1 is used to control the display format – ie, whether the clock shows 12-hour or 24-hour time. If pins 2 & 3 are left open circuit, the clock operates in 12-hour mode and the AM/PM indicator LED Power for the clock circuit is derived from a 12VAC plugpack and bridge rectifier DB1. The resulting unfiltered 16-17V rail from DB1 is then fed via diode D1 to a 2200mF filter capacitor and to pin 1 (Vin) of an LM2575 switching regulator, U5. This IC produces a regulated +5.8V rail for driving the LEDs. Schottky diodes D4 & D5 and the associated 47W resistor provide a simple charging circuit for a 4-cell NiMH or Nicad backup battery. It also allows the micro to be powered from the main +5.8V DC rail (via D4) when available and then automatically fall back to the backup supply when the main source fails. The added voltage drop across D4 (about 0.3V) also puts the microcontroller’s supply well below its absolute maximum rating of 6V. During a mains failure, the microcontroller continues to run and power is also available to the opto output and piezo buzzer but the power-hungry LED array is not powered. This allows maximum backup battery life and still preserves operation of the alarm functions. Mains synchronisation signal The unfiltered 100Hz signal from DB1 is also fed to the base of transistor Q7 to derive the mains synchronisation signal. This pulses Q7 on and off at a 100Hz rate, which in turn drives pin 4 (PB3) of the microcontroller (U3). The internal software in U3 processes this signal to derive the mains synchronisation signal for the 8MHz crystal oscillator. In effect, the clock relies on the mains for its long term siliconchip.com.au siliconchip.com.au June 2005  29 Fig.1: an ATMEGA8515-16 microcontroller (U3) is at the heart of the LED clock. It performs all the timekeeping functions and drives the LEDs via Darlington transistors Q1-Q5 and three ULN2003A Darlington transistor arrays (U1, U2 & U6). Fig.2: the circle LEDs are multiplexed by the microcontroller (U3), with Darlington transistors Q1-Q5 used to provide buffering and switching for the individual groups. Q1-Q5 also switch the digit LEDs. 30  Silicon Chip siliconchip.com.au siliconchip.com.au June 2005  31 Fig.3: the four digit displays in the centre of the clock each consist of 28 individual LEDs (ie, four LEDs to each digit segment) Fig.4: the parts layout for the top of the PC board. Install a shorting link on pins 2 & 3 of JP1 for 24-hour operation. accuracy but falls back to the crystal oscillator during a power failure. Alarm outputs When the alarm is triggered, the microcontroller switches its OC1B output (pin 29) high. This logic high then turns on transistor Q6 which sounds a small piezo buzzer and turns on the alarm indicator LED (LD70). 32  Silicon Chip At the same time, PD0 (pin 10) also goes high and this activates the optocoupler (OC1). As mentioned before, its output can then be used to control a low-voltage external device. Assembly Before starting the assembly, it’s a good idea to carefully inspect the supplied PC board and the parts lay- out diagram (Fig.4). In particular, pay special attention to the screw terminals mounted on the rear of the board – the supply and back-up terminals are labelled in copper and are hard to see under the solder mask. The PC board is double-sided with plated-through holes and a solder mask. This makes the assembly easy – there are no feed-through links to siliconchip.com.au Fig.5: the parts layout for the back of the PC board. The capacitors & choke L1 can be secured using hot-melt glue. install and you only have to solder the component leads on one side of the board. Note, however, that a few parts are mounted on the back of the board, which means that soldering takes place on the top (LED side) of the board. The main thing to watch out for with this project is the large number of posiliconchip.com.au larity sensitive parts – particularly the LEDs. And because the board is platedthrough, removing a part that’s already been soldered in will be extremely difficult and risks damaging the board. The rule is: check and double check before soldering. Apart from that, the assembly is quite straightforward and should only take a few hours. Begin the assembly by installing all the resistors on the board. To save any confusion, it’s best to install all those with the same value at a time. It’s also a good idea to install them all with the tolerance band facing the same way, as this makes it easier to check the assembly later on. Once the resistors are in, you can install the diodes, taking care to ensure June 2005  33 Above: the completed PC board is secured to the case using two M3 x 6mm screws and nuts, located at the 3-o’clock and 9-o’clock positions. This view shows the parts on the back of the PC board. Be sure to mount the two electrolytic capacitors exactly as shown, so that they clear the battery compartment. 34  Silicon Chip each device is installed in the correct location and is correctly orientated. D1 & D2 are 1N4007s, while D3-D5 are 1N5819s (don’t get them mixed up). That done, install the two BC547 transistors (Q6 & Q7), the bridge rectifier (DB1), the optocoupler (OC1) and the IC sockets. Push the transistors down onto the board as far as they will comfortably go before soldering their leads and watch the orientation of the bridge rectifier. The IC sockets should all be orientated so that their notched ends match the parts layout (this will make it easier when it comes to plugging the ICs in later on). Note that the socket for U6 (and the IC itself) faces in the opposite direction to the other sockets. Don’t fit the ICs into the socket just yet, though – that step comes later after the power supply has been tested. There’s just one wrinkle when it comes to fitting the socket for the microcontroller (U3) – the 6-way pin header for in-circuit programming mounts on the rear side of the board, directly under this socket. This pin header can be omitted in the vast majority of cases, since the microcontroller comes pre-programmed. If you do need the programming header, it will need to go in before the IC socket – just flip the board over and solder it in. The optocoupler (OC1) solders directly to the board. Be sure to install it with its notched end towards U3, as shown on Fig.4. Once it’s in, install the adjacent 3-pin header (JP1). Next, install the crystal (X1), followed by the five BD682 Darlington transistors (Q1-Q5). The latter are all installed by first bending their leads downwards through 90° about 4mm from their bodies, with the labels facing up. They are then installed so that they lie flat against the PC board, before soldering the leads. The LM2575T switching regulator (U5) is installed in similar fashion. As before, bend its leads down through 90° about 4mm from its body, then mount it in position and fasten its metal tab to the PC board using an M3 x 10mm screw and nut. That done, its leads can be soldered and trimmed in the usual manner. Note: don’t solder U5’s leads before bolting it to the PC board. If you do, the leads may be unduly stressed as the screw is tightened, which could fracture the PC board tracks. siliconchip.com.au Par t s Lis t 1 188mm-diameter double-sided PC board with black solder mask 1 clock case to suit PC board 1 330mH 3A ferrite choke (L1) 1 8MHz crystal (X1) 1 mini piezo buzzer (PC mount) 3 2-way PC-mount screw terminal blocks 4 AAA 1.2V rechargeable cells (NiMH or Nicad) 1 4 x AAA cell holder 3 miniature momentary contact PC-mount switches (SW1SW3) 3 M3 x 6mm screws 3 M3 nuts 1 3-way pin header 1 jumper shunt 1 black cable tie, 150 x 3mm 3 16-pin DIL IC sockets 1 40-pin IC socket The clear plastic bezel is fitted with a dark filter and simply clips into position via a couple of locating lugs. Once it’s in place, the filter is sandwiched between the bezel itself and the 5mm LEDs. The ceramic and monolithic capacitors are the next in line. Follow these with two 10mF tantalum capacitors. The latter are polarised, so make sure their positive leads go towards the top of the board. Installing the LEDs Now the real fun begins – you have to install no less than 176 LEDs. OK, so this job is a bit tedious but if you install them in groups of seven or eight, it won’t take long at all. As mentioned before, you really have to watch the orientation of the LEDs – put them in the wrong way around and the little blighters won’t work. The cathode lead is the shorter of the two (see Figs.1-3) and this corresponds to the flat edge shown on each LED outline in Fig.4. Note that, depending on the manu- facturer, each LED may actually have a flat side to also indicate the cathode. However, the LEDs supplied with the prototype were completely round, so don’t count on this. Basically, it’s just a case of pushing each group of LEDs all the way down onto the PC board and splaying their leads slightly to hold them in place for soldering. Be sure to double-check their orientation before actually applying the solder – get one (or more wrong) and it will be difficult to remove! Flip side Now for the parts on the reverse side of the PC board – see Fig.5. Flip the board over and install the three 2-way screw terminal blocks, followed by the piezo buzzer and the three pushbutton switches (SW1-SW3). Make sure the Where To Buy A Kit Of Parts This project was developed by Grantronics Pty Ltd for Jaycar Electronics and the design copyright is owned by Jaycar Electronics. A kit of parts is available from Jaycar for $A129.00 – Cat. KC-5404. This includes the clock case, the battery holder, the PC board and all on-board parts but does not include a plugpack supply or the rechargeable batteries. The 12VAC plugpack supply (Cat. MP-3020) is available for $22.95. siliconchip.com.au Semiconductors 3 ULN2003N Darlington transistor arrays (U1,U2,U6) 1 ATMEGA8515-16PC microcontroller – pre-programmed (U3) 1 LM2575T-ADJ switchmode regulator (U5) 1 PS2505-1 optocoupler (OC1) 5 BD682 PNP Darlington transistors (Q1-Q5) 2 BC547 NPN transistors (Q6,Q7) 1 WO4 bridge rectifier (DB1) 2 1N4007 silicon diodes (D1,D2) 3 1N5819 Schottky diodes (D3-D5) 164 high-brightness 3mm red LEDs 12 high-brightness 5mm red LEDs Capacitors 1 2200mF 25V PC-mount electrolytic 1 1000mF 10V PC-mount electrolytic 2 10mF 16V tantalum 3 100nF monolithic (code 104) 2 33pF NPO ceramic (code 33) Resistors (0.25W, 1%) 1 100kW 1 1.8kW 1 68kW 7 330W 1 6.8kW 14 220W 5 4.7kW 57 120W 1 3.3kW 1 47W June 2005  35 mount the electrolytic capacitors, the choke or the buzzer on the top of the board. They will interfere with the dark filter when the clear plastic bezel is later clipped into position if you do. Assuming everything is OK, switch off and install the chips into their sockets, taking care to ensure that they are all correctly orientated. Be careful when handling the chips, to avoid damage from static electricity. Don’t touch the pins and be sure to discharge yourself by touching an earthed metal object before touching the ICs. Note that U6 faces in the opposite direction to the others. Note also that pin 9 of U6 must NOT go into its corresponding socket pin. This pin can either be cut off using a pair of sidecutters or splayed out so that it runs down the outside of the socket– ie, this pin must NOT make any connection to the circuit (OK, we admit it – we made a mistake on the PC board). That done, connect the backup battery pack and re-apply power from the AC plugpack. The clock should immediately show 0:00 and the seconds LED should start chasing anti-clockwise. It’s then just a matter of setting the time and checking out all the functions using the pushbutton switches, as described earlier. After that, it’s simply a matter of securing the PC board inside the case using the M3 screws and nuts provided and clipping the front bezel into place. It’s up to you whether or not to use the dark filter material supplied. If you do decide to use it, it must be cut into a neat circle exactly 197mm in diameter, to fit inside the bezel. When the bezel is fitted, the filter is sandwiched between it and the 5mm LEDs and held firmly in position. Leave the filter out if you want the display to be really bright. Finally, if one or more LEDs fails to light, check its orientation. If a group of LEDs fails to light, check the corresponding BD682 driver transistor and its associated base bias resistors. SC Fit the stickers The rechargeable battery pack fits neatly in the battery compartment and can be secured using adhesive tape. Make sure it’s connected the right way around. buzzer goes in the right way around (ie, positive terminal to the left). That done, install the 2200mF and 1000mF electrolytic capacitors and the 330mH choke (L1). Note that, in both cases, the capacitor leads are bent down by 90°, so that their bodies lie flat against the PC board. Pay attention to the polarity of the capacitors and position them exactly as shown in Fig.5, so that they will clear the battery compartment A blob of hot-melt glue or epoxy adhesive can be used to secure them in position. Similarly, use hot-melt glue to secure the choke or you can secure it using a plastic cable tie – just loop the cable tie through the holes on either side. By the way, don’t be tempted to A number of adhesive labels are supplied with the kit and these indicate the switch functions and the connections to the screw terminal blocks. The ones for the screw terminal blocks are affixed directly to the PC board. Be sure to get these correct – if the 12V AC plugpack is connected to the back-up battery terminals, it will blow every chip on the board! The switch function labels are affixed to the back of the case, above the access slot. They are, from left to right: “Down”, “Mode” and “Up”. That’s it – the PC board assembly is complete and you’re now ready for the smoke test. Well, actually there shouldn’t be any smoke but you know what we mean! Testing Before fitting the ICs, it’s best to check that the supply regulator (U5) is working correctly. To do this, apply power from a 12VAC plugpack and check the voltage at the anode of D4 – you should get a reading of close to +5.8V. D4’s cathode should be at about +5.3V and this voltage should also be present on pin 40 of U3’s socket. The tab of the LM2575 regulator makes a convenient ground point. If you don’t get anything at D4’s anode, check the voltage at the cathode of D1 – you should get a reading of about 16-17V DC. Table 1: Resistor Colour Codes o o o o o o o o o o o   No. 1 1 1 5 1 1 7 14 57 1 36  Silicon Chip Value 100kW 68kW 6.8kW 4.7kW 3.3kW 1.8kW 330W 220W 120W 47W 4-Band Code (1%) brown black yellow brown blue grey orange brown blue grey red brown yellow violet red brown orange orange red brown brown grey red brown orange orange brown brown red red brown brown brown red brown brown yellow violet black brown 5-Band Code (1%) brown black black orange brown blue grey black red brown blue grey black brown brown yellow violet black brown brown orange orange black brown brown brown grey black brown brown orange orange black black brown red red black black brown brown red black black brown yellow violet black gold brown siliconchip.com.au