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PC-controlled moving message display Have you got an old PC sitting around gathering dust? You can use it to control this moving LED message display which plugs into the PC’s printer port. All you have to do is type the message in on the keyboard. Design by JOHN WESTERN Moving LED message displays are common in shops and clubs and are very effective as advertising signs. Now you can have your own by build­ ing this unit. All you need to drive it is an old XT (or better) computer and you can easily set the unit up to repeat a message or a number of messages. While LED message displays use a variety of formats, this one employs characters which are seven LEDs high by five LEDs across and these move along the LED array at a fixed rate. As presented here, the display con­ sists of a 48 x 7 LED matrix arranged 10  Silicon Chip on a single large PC board. This board basically consists of three 16 x 7 LED modules: a master module which also contains the necessary parallel port interface circuitry, and two extension modules. Each module has enough LEDs to display three characters, which means that the basic unit can display up to nine characters at any given time. If you wish, you can increase the display width by adding another one or two extension modules. This will allow either 12 or 15 characters to be displayed at any one time. The additional extension modules are sim­ ply cut from a second PC board and connected to the lefthand end of the message display using 12 wire links. Conversely, you can reduce the display width by cutting off one of the extension modules, to give a 32 x 7 (6-character) LED display. Of course, the length of the message is not limited by the number of char­ acters that can be displayed at any one time. Basically, you can make the message as long as you like. In opera­ tion, the leading characters appear on the righthand side of the display and scroll across to the lefthand side before disap­pearing off the “edge”. The mes­ sage continues scrolling until all the characters have been displayed and can easily be set up so that it repeats. Our prototype was built to the stand­ ard 9-character config­uration; ie, a sin­ gle PC board with three modules. This is housed in a folded smoked-Perspex case to produce an attractive display. It is powered by a 12V AC plugpack and is connected to the parallel port of the PC via a DB25 socket mounted on one end of the board. How it works Each character to be displayed is produced by turning on all the appro­ priate LEDs in a row for a short period of time. This is repeated for each of the seven rows, to make up the character in a multiplexed fashion. Because this happens at a very high rate, all the LEDs appear to be turned on at the same time. Fig.2 shows the circuit diagram of the Moving LED Display. Each row of LEDs is driven by a Darlington tran­ sistor pair con­sisting of a BC549 and a BC639; ie, Q1 & Q2 for row 1, Q3 & Q4 for row 2, etc. These seven Darlington transistor pairs are in turn driven by the printer port data lines. Note that each line from the printer port is filtered by an RC network con­ sisting of a 47Ω resistor and a 220pF capacitor. These filters prevent noise pulses from disturbing normal opera­ tion of the display. The LED columns are controlled by separate BC549 transis­tors (Q15, Q16, etc), in turn driven by 74LS164 shift registers (one for each group of eight columns). These shift registers accept the serial data applied to their A & B data inputs and convert it to paral­lel format at their Q0-Q7 outputs. So each shift register con­trols eight transistors Fig.1: this diagram gives a breakdown of the basic operation just to light one LED. In this case, we want to light the LED at row 4 column 3 (ie, R4,C3). This involves clocking a logic 1 into the shift register and then moving it until the third output goes high, represented here by the closed switch between the shift register and C3. Switch SW3 is then closed to light the row and thus eight LED columns. Note that, for the sake of clarity, our circuit only shows the first eight LED columns, their corresponding transistors (Q15-Q22) and one shift register (IC1). The circuitry for each successive eight columns is identical, with pin 13 of IC1 clock­ing the data inputs of the next shift register, and so on down the chain. IC1 is driven by one of the parallel printer port data lines, while two other data lines drive the clock and reset pins (pins 9 & 8). Basically, data is shuffled into IC1 in serial fashion and its appropriate Q outputs go high, thereby turning on the corresponding column transistors. One of the row data lines is then briefly taken high to light the required LEDs. In greater detail, the character to be displayed is broken down into the required pattern of dots for each row. Initially, the shift registers are all cleared by applying a pulse to the MR line. This sets all outputs to a logical low condition, turning all columns off. The required data is then applied to the A & B data inputs and the CLK line pulsed to move the data into the first shift register. Successive data is sub­ sequently applied in a similar fashion until the required pattern of dots for a particular row is set up in the shift registers. Once the data is ready, the row is turned on for a short period of time after which the shift register is cleared (reset) and the process starts again for the next row. Fig.1 gives a breakdown of the basic operation just to light one LED. In this case, we want to light the LED at row 4 column 3 (ie, R4,C3). This involves clocking a logic 1 into the shift regis­ ter and then moving it until the third output goes high, represented here by the closed switch between the shift register and C3. Switch SW3 is then closed to light the row, in this case the single LED at R4,C3. February 1997  11 Where To Buy The Parts The parts for this design are available from Oatley Elec­tronics, PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563; fax (02) 9584 3561. The options are as follows: Complete Kits (does not include case) (1) PC board, all on-board parts, software on 3.5-inch disc, a surplus plugpack & bright red, green or amber LEDs (you specify): $165 (2) Above kit with super bright LEDs (narrow viewing angle): $200 Shortform Kits & Accessories (3) PC board only plus software on 3.5-inch disc: $75 (4) 336 bright LEDs (red, green or amber – please specify): $45 (5) 336 super bright LEDs: $90 (6) Suitable small 10.6V 1.4A surplus switchmode power supply in case: $12 Note 1: none of the above options includes a case or the Perspex channel shown in the photos. Please add $6.00 p&p to any combination. Note 2: the PC board associated with this design is copyright Oatley Electronics. In addition, the software supplied is copy­right John Western and must not be altered in any way or used for other purposes without permission. Note, however, that the basic circuit of Fig.1 works in the opposite sense to the circuit of Fig.2. In reality, the shift registers drive transistors and these provide logic lows, while the printer port data lines and their associated Darlington transistors pull the rows high. To sum up, the printer port data lines pull each row high in succes­ sion to light the appropriate LEDs. And in between times, the shift reg­ isters are reset and new data appro­ priate for the next row is clocked in. Add to this the fact that the display moves from left to right and you can see that the timing process is quite complicated. Fortunately, that’s all taken care of by a machine language program which is called LEDs.COM. This program manipulates all the control lines from the printer port to control the LED dis­play. Power supply The display is powered from a 9-12V DC plugpack rated at 1A. The DC rail from the plugpack is applied to REG1, which delivers a regulated 5V rail to power the LED arrays and the shift reg­isters. The 10µF and 1µF capacitors at the input and output of REG1 are there to ensure regulator stability. In addition, the supply pins of all the 12  Silicon Chip shift registers are filtered using 0.1µF capaci­tors Construction This design is available as a com­ plete kit of parts from Oatley Electron­ ics, who own the copyright on the PC board (see pricing panel). The board is double-sided with plated-through holes which means that there are no links to install. It is also solder-masked and carries a screen printed overlay to make the job of assembly as straightforward as possible. As mentioned earlier, the basic con­ figuration is a 3-sec­tion board with a 48 x 7 LED array. Each section (or module) contains 16 LED columns plus a pair of matching shift registers. In addition, the master module carries the DB25 socket plus the Darling­ton transistors and power supply components. Fig.3 shows the parts layout on the PC board. Note that this only shows the master module plus part of the first exten­sion module. The pattern of LEDs, shift register ICs and other parts simply repeats towards the left. Begin the assembly by installing the resistors and capaci­tors, then add the transistors and the ICs. The use of IC sockets is recommended here, since a dud IC (rare) is very difficult to remove if it is soldered directly to a double-sided board. Take care with the polarity of the ICs – they are all installed with the notched end to­ wards the right. Similarly, take care to ensure that the transistors are all correctly ori­ ented and note that Q2, Q4, Q6, Q8, Q10, Q12 & Q14 (ie, the transistors immediately adjacent to the LED rows) are all BC639s. Now for the LEDs. There are 336 LEDs in all, so installing them will take some time. The main thing to watch out for here is to ensure that they are all correctly oriented. You can identify LED polarity in two ways: (1) the anode lead is the longer of the two; and (2) the cathode lead is adjacent to a small flat section on the bottom lip. Push the LEDs down onto the board as far as they will go before soldering their leads. Once all the LEDs are in, you can install the DB25 socket and the 7805 regulator (REG 1). The latter is installed with its leads bent at right angles and its metal tab bolted to the PC board along with a small finned heatsink. The prototype board was installed in a smoked Perspex chan­nel (470mm long x 150mm high) and secured using machine screws and nuts at the back. This Perspex channel was bent up by a local plastics supplier. Alternatively, you can make up a suitable wooden or metal case with a Perspex viewing window for the LED arrays. Software & testing The software for the Moving LED Display comes on a 3.5-inch floppy disc and consists of six main files plus a brief readme file. The files leds3.com, leds4.com and leds5.com are for dis­ plays with from three to five modules (including the master module), while the ledset.com file configures the basic setup. First, copy the correct leds_.com file to the hard disc (or to another floppy), along with the ledset.com file. Next, rename the copied leds_.com file to leds.com, then run ledset.com Fig.2 (right): the Moving LED Display is controlled via the PC’s parallel port. The rows are driven by Darlington transistor pairs, while the data in the shift registers (IC1, etc) controls the column switching transistors (Q15, Q16, etc). February 1997  13 Silicon Chip BINDERS These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. ★ High quality ★ Hold up to 14 issues ★ 80mm internal width ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A14.95 (includes postage in Australia). NZ & PNG orders please add $A5 each for postage. Not available elsewhere. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number.  Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Bankcard  Visa  Mastercard Card No: _______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ 14  Silicon Chip Fig.3: follow this diagram when installing the parts on the PC board. Note that only the master module and part of the first extension module are shown here. The pattern of LEDs, shift register ICs and other parts simply repeats. Note that a heatsink should be fitted to REG1 (see photo). The basic PC board includes a master module (at right) plus two extension modules to make up a 48 x 7 LED array. Up to two extension modules, each with a 16 x 7 LED array, can be added to the lefthand end of the board. Note the heatsink fitted to the 7805 regulator (REG1). Fig.4: the ledset.com program lets you configure the ledsx.com program to suit your computer. The values shown are good starting points for a 133MHz Pentium machine (see text). to configure the display driver to the required parameters. Fig.4 shows the setup that appears when ledset.com is run. There are three parameters that can be varied: (1) the printer Port address; (2) the Delay; and (3) the Duty cycle. The latter sets the speed at which the message move across the screen, while the Delay sets the period between messages. The up and down arrow keys select the parameter to be altered. In most cases, the default printer port address of 0378H will be cor­ rect. If not, the address can either be gleaned from the system BIOS or by running the Microsoft Diagnostics program (type msd at the command prompt). Often, too, the address will be dis­ played at some stage during the com­ puter’s boot sequence. If you are using Windows 95, double click the System icon in Control Panel, then click the Resources tab, select the printer port and click Properties and Resources to view the address. The numbers for Delay and Duty will depend on the speed of the PC used. A Delay of 00500 and a Duty of 001 are good starting points for an XT but these numbers should be increased for higher speed PCs. We found that a Delay of 02500 and a Duty of 250 produced good results on a 133MHz Pentium machine. Note that you have to type in each digit in an entry, start­ ing from the left­most digit, until the number is correct. The display can now be plugged into the PC and the leds.com program run from the DOS prompt. The mes­ sage to be displayed must be included on the command line; eg, to display the message DOES YOUR DISPLAY WORK?, you type leds does your display work? at the command prompt. Note that all characters are displayed in upper case, regardless as to how they are typed. The above command will display the message once before returning con­ trol to DOS. If you want the message to be displayed repeatedly, you simply use a full stop as the first character of the message. For example, the command leds .silicon chip will repeatedly cycle the message SILICON CHIP across the display. The display can be stopped at any time by pressing Ctrl C on the computer keyboard. Assuming that the unit works cor­ rectly, you can now experi­ment with the Delay and Duty values in the ledset.com program. If a row of LEDs fails to light, check the associated Darling­ton transistor pair. Similarly, if a column of LEDs fails to light, check PARTS LIST 1 double-side PC board with plated-through holes, 414 x 107mm (incl. three modules) 1 smoked Perspex channel case with Perspex window or (see text) 1 mini U-shaped heatsink to suit TO220 regulator, 19 x 19 x 11mm 1 PC-mount DB25 male socket 1 DB25 cable, male-to-female 1 9-12V DC 1A plugpack supply 6 14-pin IC sockets Semiconductors 6 74LS164 shift registers 7 BC639 PNP transistors 55 BC549 PNP transistors 1 7805 5V 3-terminal regulator 336 LEDs Capacitors 1 10µF 25VW electrolytic 7 0.1µF monolithic ceramic 10 220pF ceramic Resistors (0.25W, 5%) 10 10kΩ 48 68Ω 48 4.7kΩ 10 47Ω the associated column switching transistor. Finally, a batch file can be used to allow a sequence of messages are to be displayed continuously. An example of this is as follows: :start leds message 1 leds message 2 leds message 3 goto start These lines must be created in an ASCII text editor and the file saved with a bat extension; eg, message.bat. February 1997  15