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The Christmas Star What’s this? A Christmas project in November? Well, we were so taken with this cute little project that we just had to run it this month so that readers would have plenty of time to build it for Christmas. It’s a Christmas “star” which lights up its LEDs in a seemingly endless variety of patterns. Build it to add that “something different” to your Christmas tree. Or put it in the front window to impress the neighbours! The PC board has just one IC to drive 30 LEDs which are arrayed in a five-pointed star. But we can’t show you what the star looks like since the LEDs constantly flash in lots of different ways. Sometimes the LED “points” of the star will flash, then they will rotate, then reverse direction, then the whole star lights up from the centre outwards and so on. The circuit uses just one IC and yet the range of flashing patterns seems endless, although it does actually repeat the sequence after a minute or so. With such a simple circuit producing such a variety of patterns the conclusion is inevitable. Yes, the IC is a microprocessor otherwise it would not be able to produce such a huge variety. The PC board is five-sided and is quite small, suitable for mounting at the top of your Christmas tree if you want. It is powered by a 9V DC plugpack and has an onboard 5V regulator. The microprocessor is an Atmel AT89C2051, a relatively recent derivative of the very common 8051. It comes in a 20-pin plastic package and contains 2K 18  Silicon Chip bytes of program memory, 128 bytes of RAM, 15 programmable I/O lines, on-chip oscillator, two 16-bit counter/timers, six interrupt sources and a full duplex serial port (UART). This all sounds very much like a small 8051 until we add that the program memory is re-programmable Flash with 1000 erase/write cycles, the oscillator runs to 24MHz (double that of the original 8051), the I/O pins can sink 20mA for directly driving LEDs and two I/O pins are connected to an on-chip analog comparator. In other words, it is a somewhat souped-up 8051 and it can drive the LEDs direct, without any other circuit components. This project started just before last Christmas when my daughters asked “Why don’t we have any flashing Christmas tree lights?” So, while they were out shopping, my son and I grabbed a dozen LEDs, some ribbon cable and a microcontroller. While my son wired up the LEDs, I wrote some simple software. It was all installed and running when they came home after spending all my money. That’s why we didn’t have lights before... The smiles on their faces made it all worthwhile! I then resolved to do by LES GRANT* something better for this coming Christmas. And here is the result. Why use a Microcontroller? Using a PC’s parallel port to control external devices is a popular approach these days but I certainly couldn’t afford to tie up a PC for the few weeks leading up to Christmas just to flash a few LEDs; just think of the power bill to run a few LEDs in this way! So, why not use a small microcontroller? They are cheap and easy to use and if the design doesn’t work first time, you simply re-program it. So that is what I did. As already mentioned, the hardware is the Atmel '2051 micro. To make it start thinking, we need a reset circuit and this simply consists of the 10µF capacitor (C7) connected to pin 1. The reset function works because the capacitor briefly holds pin 1 high when power is first applied. Then the capacitor charges up and this causes pin 1 to be pulled to 0V. When the power is switched off, diode D2 forces this capacitor to discharge quickly, ready for the next time power is applied. To set how fast the micro thinks, we need a 12MHz crystal X1 (in fact we used 11.0592MHz) and associated capacitors C1 and C2, connected to pins 4 & 5. Note that the crystal could be replaced by a 12MHz ceramic resonator. This allows the ‘2051 to execute an instruction every 1 or 2µs. As you can see from the circuit of Fig.1, the 30 LEDs are connected in an X-Y matrix. Why 30 LEDs? Engineering is full of trade-offs or com- promises. I wanted a 5-pointed star so the number had to be divisible by 5. For aesthetic reasons, we need an even number of LEDs per point. Six LEDs per point looked “about right”. The next step up would have been 40 LEDs which would have required 13 I/O pins to drive them and a more complicated PC board. We can drive 30 LEDs from only 11 I/O pins using a process called multiplexing. The appropriate combination of LEDs in a column is switched on for a short time (about 2ms in this case). This process is repeated for each column in turn, taking 10ms for a full cycle. Provided the multiplexing is done quickly enough, the persistence of the human eye “fills in the gaps” and we see any combination of LEDs on without any flicker. The minimum practical multiplexing frequency is about 100Hz which is the frequency used by the star. Fig.1: the micro drives the 30 LEDs in 5 x 6 matrix, with 5 colums and 6 rows. There is provision for an optional EEPROM which will store extra patterns in the future. November 1998  19 Ideally, the LEDs should be high brightness types for best effect and in an ideal world would be matched for brightness. Maybe that's going a tad too far . . . The power supply uses the ubiquitous 7805 3-terminal regulator with 0.1µF bypass capacitors at its input and output. Diode D1 provides reverse polarity protection so that you can’t blow up the circuit if the DC supply is connected the wrong way around. The maximum current drawn by the star is about 150mA with all LEDs on but is less than about 50mA for most patterns. The maximum temperature rise of the 7805 when the star is run from a typical 9V DC unregulated plug-pack is about 30°C which is quite acceptable. If run from a 12V DC unregulated plug-pack it gets warmer and therefore should be provided with a small heatsink. Star software TWINKLE, TWINKLE little star. How I wonder how you are. . . Bet you never saw a star twinkle like this one. Its on-board micro drives the LEDs in all sorts of patterns to light up the star. Fig.2: the component overlay, Make sure that you insert all the LEDs correctly: the cathode, or flat side, is oriented to the right in all cases. Don’t insert the micro until you’ve done a voltage check on the board (see text). 20  Silicon Chip In the spirit of Christmas, the basic source code is available free (you may download it from the SILICON CHIP Web site – www.siliconchip.com. au). An extended version that uses the EEPROM for storage is available at minimal cost from Grantronics Pty Ltd. The software was written in C language using the low cost Dunfield Development Systems Micro/C compiler. There is nothing particularly “smart” or “tricky” about the software – it was written to be easy to understand and to encourage use of small micros. Consequently, there are no interrupt routines and no use of the counter/timers, the UART or the comparator although Micro/C can make use of these resources. The software is table driven. This means that the display patterns and sequences are determined by data stored in a table (an array of bytes). There is a simple interpreter that scans through the table to perform the specified operations. The defined byte values are listed in Table 1. Note that there are quite a few undefined values so future expansion is possible. Putting it together Assembly is quite straight forward. You will need a soldering iron with Byte value or range 01 to 30 (0x01 to 0x1e) 33 to 62 (0x21 to 0x3e) 64 (0x40) 65 to 79 (0x41 to 0x4f) 128 (0x80) 129 to 191 (0x81 to 0xbf) 253 (0xfd) 254 (0xfe) 255 (0xff) Operation Turn on LED 1 to 30 Turn off LED 1 to 30 (LED number = byte – 32) Go back to byte after loop start Loop start, count = byte – 64 Delay (use last delay count), each count = 10ms Delay, count = byte – 128, each count = 10ms All LEDs on All LEDs off End of table a fine tip, preferably temperature controlled to about 600°F or 320°C. The first step is to carefully check for shorts between tracks and broken tracks. Fit the smallest parts first, the wire links, followed by the resistors and diodes. Next, fit the crystal (or resonator) and an IC socket for the micro. Then install the transistors, capacitors and LEDs. Pay particular attention to the orientation of the LEDs – they don’t work when installed backwards! Finally, install the 3-terminal regulator and the 2.1mm DC power socket. Don’t insert the micro into its socket just yet. Do another close visual inspection, looking for solder bridges especially Fig.3: actual size artwork for the PC board. on the transistor pads. Then apply power and check for the presence of 5V between pin 20 (+) and pin 10 of the IC1 socket. If all is OK, remove power, plug in the micro (careful!) and apply power again. After a brief pause, the micro starts to do its thing and generates quite a range of patterns which then repeat after a while. At the time of writing, the patterns weren’t quite finalised but there is enough to entertain you. Everyone who saw the prototype thought that it was cute – or words to that effect. The two holes in the board near LED1 may be used to hang the star and the holes near SK1 may be used to secure the plugpack’s cable. Finally, the appearance of the star may be enhanced by placing a piece ELECTRONIC COMPONENTS & ACCESSORIES •  LARGE RANGE OF ICs, RESISTORS, CAPACITORS & OTHER COMPONENTS •  MAIL ORDERS WELCOME! CROYDON STORE ONLY ELECTRONIC DISPOSALS CLEARANCE! •  OPEN FRAME 240V INDUCTION MOTORS 600 WATT AND 900 WATT. 600 WATT - $15 EACH OR 10 FOR $100 900 WATT - $18 EACH OR 10 FOR $120 •  LARGE VARIETY OF DISPOSALS TRANSFORMERS AT GIVEAWAY PRICES! Croydon Ph (03) 9723 3860 Fax (03) 9725 9443 MilduraPh (03) 5023 8138 Fax (03) 5023 8511 M W OR A EL D IL C ER O M E TABLE 1 Truscott’s ELECTRONIC WORLD Pty Ltd ACN 069 935 397 30 Lacey St Croydon Vic 3136 24 Langtree Ave Mildura Vic 3500 SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. November 1998  21 of red cellophane over the front. Parts List Fault finding 1 5-sided PC board, code 08211981 1 2.1mm DC connector (SK1) 1 crystal or ceramic resonator, approx. 12MHz (X1) 1 20-pin IC socket 1 9V DC 150mA plugpack, Jaycar MP-3003 or equivalent Semiconductors 1 AT89C2051 programmed microprocessor (IC1) 1 7805 regulator (REG1) 1 24C16 EEPROM (optional, enhanced version only) 30 red LEDs (LED1-LED30) 5 BC557 PNP transistor (Q1-Q5) 1 1N4002 power diode (D1) 1 1N4148 or 1N914 diode (D2) Resistors (0.25W, 5%) 5 470Ω 6 120Ω NS_16_8 PCB is a system conditioning card with 16 optically isolated inputs set-up for either 12V or 24V operation. The board provides 8 single pole, double throw relays with 10 Amp contact rating. KITS & CARDS NS_DC_DC is a step down converter with an input range 11 to 35V DC and an output of 5 volts DC at 5 Amps, with an output ripple of approx 150mV. There is an IN/OUT 50-way connector isolating the 5V and 12V+ &12V- rails of the PC power supply. This segregates PC’s power when working on prototypes. NSDC_DC1 module used with NS_DC_DC & NSDC_DC4 converters is a 5V to 12V(+/-) step- up converter. The board utilises 743 switch mode IC with 2 x 12V regulators, with output ripple of approx 200mV. NS_UTIL1 prototyping board has 1580 bread board holes access to any 3 groups (0 to 4) on the 50-way cable pinout. Power is available from the 50-way cable format 5 volts at 2 Amps & 12V+ 12V- at 1 Amp. There is provision for array resistors with either a ground or positive common connection. For brochure write to: Reply Paid 68, NORBITON SYSTEMS, PO Box 687, Rockingham WA 6968 http://www.users.bigpond.com/norbiton 22  Silicon Chip Where to buy a kit We understand that the complete Christmas Star kit will be available from all Jaycar Electronics stores from SC this month for $29.95 * Les Grant is the Engineering Director at Grantronics Pty Ltd, electronics design engineers. They can supply the programmed microprocessors for $10 plus $5 for packing and postage. Send remittances to Grantronics Pty Ltd, PO Box 275, Wentworthville, NSW 2145. Phone (02) 9896 7150. NORBITON SYSTEMS NS_LED PCB gives visual access to five groups (0 to 4) of the NS_PC1OX. There is a total of 40 status LEDs. The board offers a 25-way “D” type female socket. The lines are driven by 74244 ICs & configured as a parallel printer port. This socket gives access to printer port kits, eg, stepper motors, LCDs, direct digital synthesis. The future The star is still evolving. That is Capacitors 1 4.7µF 16VW electrolytic 3 0.1µF monolithic or MKT polyester 2 27pF ceramic NS_PC101 card for XT/AT/PCs allows access to 48 I/O lines. There are 5 groups (0 to 4) available on a de-facto industrial standard 50-way ribbon cable used in STEbus and VMEbus 19" rack mount control systems. The board uses 2 x 8255 ICs. Multiple boards can be used if more I/O lines are required. If the 5V DC is not present, check the applied power polarity. The centre pin of the 2.1mm plug (SK1) must be positive. Check that D1 is correctly fitted. Check the tracks from SK1 via D1 and the 7805 to IC1 for breaks or shorts. If one LED does not work, it may be inserted backwards or it may be shorted by a solder bridge between its pads. If one group of adjacent LEDs does not work, check the circuitry and soldering around the appropriate column drive transistor. If several individual LEDs do not work, check the corresponding row drive circuitry. Remember, faulty components are rare, soldering problems are common! part of the attraction of using a micro – it is so easy to change the behaviour by changing the software. And what about that optional EEPROM? Well, an enhanced version of the star will read its data from the EEPROM for much longer sequences. To check out the latest version of the software, log on to the Grantronics web-site at http://www.grantronics.com.au If you don’t have Internet access, send a stamped ($1) self-addressed envelope with an IBM format 3.5" disc to Grantronics and they will send you the current software files. We hope you have as much fun building the star and playing with the software as I did creating it. Enjoy! Protect Your Valuable Issues Silicon Chip Binders REAL VALUE AT   Heavy board covers with 2-tone green vinyl covering $12.95 PLUS P & P   Each binder holds up to 14 issues  SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A12.95 plus $A5 p&p each (Aust. only). Just fill in & mail the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number.