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Looking for something different this Christmas? Try our multi-coloured, multi-pattern LED Christmas Tree. It will look great at the top of your Christmas tree or in the front window. By Les Grant* November 1999  31 I N NOVEMBER 1998, we published the Christmas Star as a novelty project and it proved extremely popular. This year, our “just for fun” festive season project is in the shape of a Christmas Tree but the display is a lot more diverse and interesting because it uses bi-coloured LEDs. Not only can each LED produce 16 different colours, the LED Christmas Tree has a fascinating range of ever-changing patterns. In fact, considering that the LEDs are red/green types, you will wonder how they can produce such a range of colours; some of them are quite odd. As with last year’s Christmas Star project, this circuit uses just one IC (OK, one-and-a-bit!) and yet the patterns it produces are seemingly endless. How does it do it? Yes, you guessed it. The Tree is controlled by a microcontroller but this one is different. While it can be programmed by most “high-end” (expensive) chip programmers, it can also be programmed (and re-programmed) by a PC parallel port with minimal hardware. This makes it ideal for hobbyists. If you have been avoiding microcon-trollers because of the cost of the programming hardware, now there is no excuse! And most of the development soft- ware can be downloaded free from the Internet – that avoids another excuse! Circuit description Fig.1 shows the circuit. The key to understanding any circuit is “divide and conquer” – break it down into functional blocks. There are three main blocks in the Tree circuit. The first, the power supply, is straightforward. 9V DC is applied from a plugpack to socket SK1. Reverse polarity protection is provided by diode D1. The 3-terminal 7805 regulator (REG1) then provides a 5V rail for the LEDs and the logic. Bypass capacitors C4 and C5 ensure that the 7805 remains stable. Next is the microcontroller IC1. In the Christmas Star and Heart of LEDs (May 1999) projects, we used the Atmel AT89C2051. However, its I/O port structure is not quite suitable for this application so we have used the similar Atmel AT90S2313. See the section entitled “What’s in the AT-90S2313” for a description of the microcontroller. IC2 is a 24C16 serial EEPROM where the pattern data is stored. While IC1 has some EEPROM on chip (128 bytes), this was not enough for the number of patterns we wanted to provide. The final circuit is the LED matrix. At first glance, the PC board looks like it contains 32 LEDs. In reality, there are 64 LEDs as each is a bi-colour LED capable of glowing red or green. 2-pin bi-colour LEDs were chosen to reduce the number of PC board tracks and microcontroller output pins required. 3-pin LEDs would have been easier to drive but would have required more output pins from IC1. To enable IC1 to control so many LEDs with relatively few output pins the LEDs are multiplexed. Multi-plexing is a switching technique whereby each column of LEDs is activated for a short time during which the appropriate rows are driven. This means that individual LEDs are only turned on for a short time. Provided the rate at which the LEDs are turned on is fast enough, our eyes don’t see any flicker. So for multiplexing in this circuit, we connect the LEDs in a matrix of four columns and eight rows to give a total of 32 LED packages. That enables us to drive the whole matrix with just 12 output pins from IC1. Note that while there are only four columns of LEDs, we have to drive each column twice in each multiplex cycle so that we can activate the red and green LEDs. Consequently, each LED’s timeslot is just 12.5% of the total. This is a practical minimum duty-cycle for adequate brightness from the LEDs. The 100Ω resistors R6-R13 set the What’s in the AT90S2313? The AT90S2313 is a member of the Atmel AVR family of microcontrollers which range from tiny 8-pin packages to a 64-pin feature-packed “monster”. Here is a short summary of the features of the ’2313: • • • • • • • • • • • • • • • • • 118 instructions, most single-clock cycle execution 32 8-bit general purpose working registers Up to 10 MIPS throughput at 10MHz 2k bytes (1k words) of In-System-Programmable Flash for program storage (endurance 1,000 erase/write cycles) 128 bytes of SRAM 128 bytes EEPROM (endurance 100,000 erase/write cycles) May be locked for program and EEPROM data security 1 8-bit timer/counter with separate prescaler 1 16-bit timer/counter with separate prescaler, compare and capture modes and 8, 9 or 10-bit PWM On-chip analog comparator (rail-to-rail inputs) Programmable watchdog timer with separate on-chip oscillator SPI serial interface (for in-system programming only) Full duplex UART Low power idle and power down modes External and internal interrupt sources 15 programmable I/O lines in a 20-pin package 2.7 - 6.0V (4MHz parts) or 4.0 - 6.0V (10MHz parts) 32  Silicon Chip Fig.1: the micro drives the 32 bi-colour LEDs in a 4 x 8 matrix with 4 columns and 8 rows. Each row and column is driven by complementary emitter-follower pairs which can sink or source current. This is necessary because the bi-colour LEDs need to be driven in both directions. November 1999  33 peak LED current to about 14mA. Because there can only be eight LEDs on at any time, the maximum current drawn by the Tree is about 150mA. Any 9V DC plugpack rated at 250mA or more should be suitable. Do not use a 12V plugpack otherwise you will cook the 5V regulator. Unfortunately, the microcontroller can’t drive the LEDs directly because its maximum current ratings would be exceeded. So each output pin is buffered by a transistor connected as an emitter-follower. Because each LED package has two LEDs connected in inverse parallel, the emitter-followers have to be “bi-polar” so they can both source and sink current. So two transistors are used for each output and they are connected as complementary emitter-followers so that they can source or sink current. Software The software for the Tree was written in C and compiled by the Dunfield Micro/C compiler which is available from Grantronics. As each byte of pattern data is read in, it is processed by a simple interpreter. Each byte is an instruction such as “set colour to red” or “set LED 22 to the current colour” or “pause for 500ms”. All the complex light patterns are built up from these and similar simple instructions. If you want to know more about the instruction codes, you can download the software from www.grantronics. com.au Down on the assembly line With all the technical stuff out of the way, let’s get the soldering iron going and start building. Your solder- Fig.2: the component overlay for the Christmas Tree. Make sure that you insert each LED to match the overlay otherwise the colour patterns will not be correct. In every case, the flat on the LED faces the closest outside edge of the PC board. ing iron should be temperature-controlled (about 600°F or 320°C) with a fine tip. First, check the PC board for shorts Parts List 1 PC board with Christmas Tree shape 1 4MHz crystal (X1) 1 20-pin IC socket 1 8-pin IC socket 1 9V 250mA DC plugpack 1 2.1mm PC mounting DC socket (or to suit plugpack) 8 100Ω 0.25W resistors 1 1µF 16VW electrolytic capacitor 3 0.1µF monolithic capacitors 2 27pF ceramic capacitors 34  Silicon Chip Semiconductors 1 AT90S2313 programmed microcontroller (IC1) 1 24C16 programmed EEPROM (IC2) 1 7805 regulator (REG1) 12 BC547 NPN transistors (Q1,3,5,7, 9,11,13,15,17,19,21,23) 12 BC557 PNP transistors (Q2,4,6,8, 10,12,14,16,18,20,22,24) 32 red/green (bicolour) LEDs 1 1N4002 silicon diode (D1) 1 1N914, 1N4148 signal diode (D2) between tracks and broken tracks. As usual, start with the small items such as wire links and resistors. Next, fit the IC sockets, crystal, small capacitors, regulator and the diodes. The regulator should be bolted to the PC board. The transistors should be fitted next. All the BC547s face one way and all the BC557s face the other way. Now you can fit the LEDs. Be careful to insert them the right way and don’t apply too much heat as the leads are very short when the LED is pushed down against the board. By the way, make sure each LED is installed the right way around. While no damage will result if you do put a LED in the wrong way around, the resulting colour pattern won’t be right. You will notice that each LED position on the PC board has a circular * Les Grant is the Engineering Director at Grantronics Pty Ltd. They can supply the programmed microcontrollers and EEPROMs for $15 plus $5 for packing and postage. Send cheque or postal order to Grantronics Pty Ltd, PO Box 275, Wentworthville, NSW 2145. Phone (02) 9896 7150. Complete kits for the Christmas Tree will also be available from all Jaycar Electronics stores. workmanship, connect a 9V DC plugpack. No LEDs should light. Measure between pins 10 & 20 (+) of IC1. You should have +4.8V to +5.2V. If all is well, remove power and plug in IC1 and IC2. Make sure they are correctly oriented and be careful not to bend any of their pins as you plug them into the sockets. Turn your Tree on and the display sequence should start within a few seconds. If it doesn’t work... Use this same-size photograph in conjunction with the PC board overlay at left when assembling the Christmas Tree and you should have no problems. Be careful that the two types of transistors aren’t mixed up! outline with a flat on one side – put each LED in so that it matches the outline. Finally, C3 and the DC power connector should be fitted. Testing Carefully check your soldering – use a magnifying glass and a good light. Mistakes found now are less embarrassing than damaged components later! Don’t plug in the two DIL ICs yet. Do a quick continuity check using your multimeter’s diode check range between pin 10 and every other pin of IC1. There should be no shorts or diode junctions. Reverse the probes and you should see diodes (base-collector junctions) on the 12 pins that connect to the LED matrix. A similar test should be performed with pin 20 as the common pin. This may seem like a lot of work but a solder blob shorting an I/O pin to 0V or +5V may damage IC1 and spoil your Christmas! When you are satisfied with your Modern electronic components are very reliable and faulty new components are very rare. All microcon-trollers and EEPROMs programmed by Grantronics are individually tested so problems with these parts are unlikely. The reality is that the most common causes of problems are soldering, a wrong component or wrong component orientation. So the first step in sorting out any problems is to thoroughly check your workmanship. After that, we need to get more logical. If a few LEDs don’t work, are they all in a single column or row? Maybe they only glow red and not green? The column drivers go high and the rows go low for red and vice versa for green. To help with fault finding, the first few patterns are simple “all one colour” displays. The patterns get more SC interesting after that. AVR Resources on the Internet Manufacturer’s data sheets, application notes, free development software and sample source code are available at: http://www.atmel.com Sample startup code written by Dave Van Horn for the Atmel STK200 Started Kit: http://www.dontronics.com/8515.html More sample code and an FAQ http://www.avr-forum.com/ Email list with an active group of AVR enthusiasts Send an email to atmel-request<at>pic.co.za with the word JOIN in the body of the email. November 1999  35