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If you want others to notice what you have to say, try using the Spacewriter. Simply wave it from left to right to automatically display a message that appears to materialise out of thin air. By JOHN CLARKE This novel gadget is ideal for games nights, outdoor ev­ents, spy activities and for just having fun. Called the Space­ writer, it can display up to four separate messages, each up to 10 characters long. The messages are programmed in via the print­er port of a PC, after which the Spacewriter operates indepen­dently of any other equipment. The Spacewriter comprises a length of 32mm diameter conduit with a single column of seven LEDs at the top end. The lower end has two push­ button switches to select the message, while a switch on the bottom end cap selects between Record, Spacewrite and Off. By waving the Spacewriter from side to side the appropriate LEDs light up in sequence and a message magically appears to be writ­ ten in “space”. If you want to select a new message, no problem - just press one (or both) of the two pushbutton switches on the side. And if you tire of the existing messages, you can quickly program in a new batch using your PC. How it works Fig.1 shows how we can use a single column of seven LEDs to make up the first three characters of the alphabet. To display an “A”, the Spacewriter Fig.1: this diagram shows how a single column of seven LEDs can be used to make up the first three letters of the alphabet. It’s simply a matter of lighting the appropriate LEDs at the appropriate times as the column travels from left to right. 54  Silicon Chip momentarily lights LEDs 2-7 to form the left hand side of the letter. These then extinguish. A moment later, the LEDs are positioned further to the right and the next part of the letter is displayed by lighting LED1 and LED4. This process continues until the entire letter has been displayed, after which “B” and “C” are displayed in similar fashion. In practice, the display process relies on the Spacewriter being swung from left to right so that each successive part of the character is located just to the right of the last. Our observation of the complete display depends on persistence of vision where­by we continue to see an image for a short time after it has gone. Block diagram Take a look now at Fig.2 – this shows the block diagram of the Spacewriter. The basis of the circuit is the memory block, which stores the requisite LED codes to make up the messages. Each memory location stores the code for one column of each character. Each memory location is sequent­ ially accessed using coun­ters IC3 and IC4. These increment the address at a rate set by a clock circuit based on IC2. During this process, the memory data lines from IC1 switch the LEDs on and off via driver transistors Q1-Q7. Flipflop IC5 resets the counter and stops the clock (IC2) via its reset input when the counter (IC3 & IC4) reaches its end of count value. IC5, in turn, is reset via mercury switch S2 which closes when the Spacewriter begins travelling from left to right. This allows clock IC2 to start again and so the counter stage begins counting again to Features • • • • • • Writes messages of up to 10 alphabet characters in “space” Four separate messages can be stored & displayed Messages programmed via a PC printer port Operates independently from the computer once programmed Adjustable write speed Battery powered shuffle data out of the memory. For the display to be readable, the entire message must be spelt out during a single left-to-right sweep of the LEDs in space. This means that the clock rate must be set to suit the person using the Spacewriter. If the clock rate is too slow then the characters will appear to be stretched. Conversely, if it is too fast, the characters will appear squashed. Ultimately, if the clock rate is really fast compared to the Spacewriter swing time, all that will be seen is a single column of flashing LEDs. VR1 sets the clock rate and is adjusted to prevent any significant smearing of the display as it travels in space. Another parameter which requires adjustment is the delay before the message starts after the mercury tilt switch closes. If it starts immediately, the first characters will appear to be squashed or will not be discernible at all. And if the message begins too late, the display will start too far to You just wave the Spacewriter back and forth to display a message that appears suspended in thin air (computer processed photograph). the right and may not be completed before the swing is finished. VR2 sets this delay parameter and is also adjusted to suit the user. As well as driving the LEDs (via Q1-Q7), the data lines for the memory are also connected to a computer printer port for programming. During this process, counters IC3 & IC4 are clocked under software control, with S1 selecting the strobe signal from the printer port. The printer port also provide the read /write selection for IC1 and provides the necessary reset signals for the counters. Circuit details Refer now to Fig.3 for the circuit details of the Spacewriter. It consists of just five ICs, several transistors, diodes and LEDs, a 3-terminal regulator and a handful of other passive parts. IC1 is the memory which stores the character information. This is a TMS6264L 8Kb x 8-bit memory which May 1997  55 means that it has eight data lines and 8192 spaces. Since we are using only 64 locations for each of the four possible stored messages, the memory size far exceeds our requirements. However, the device was chosen because of its low cost compared to smaller static RAM devices. As shown, data lines D1-D7 from IC1 drive transistors Q1-Q7 via 2.2kΩ resistors. Q1-Q7 in turn drive the Spacewriter LEDs (LEDs1-7) via 15Ω current limiting resistors. The data lines also connect to the PORT.A printer port of a PC for programming. IC3 & IC4 are the counters and these drive address lines A3, A1, A5, A4, A2 & A0 of IC1. The A12, A6 and A7 inputs are normally tied low via 10kΩ resistors but can be pulled high via switches S3 and S4 to access data in another memory block. Note that the address lines are not in any particular se­quence and the labelling shown is the convention of the 6264 device. The address lines can be in any order since we are pro­ gramming and replaying data in the same sequence. IC2 is a 7555 timer configured to operate in astable mode. This clocks counters IC3 & IC4 when switch S1 is in the Space­write position. The clock frequency is set by the RC components connected to pins 6 & 7 and is adjusted using VR1. IC2’s pin 3 output also drives the E1-bar input (pin 20) of IC1 via a 56  Silicon Chip .056µF capacitor. This is a select pin which sets the data lines in a high impedance state and shuts down the memory when it is high. We have used this feature to produce a short on-time for the LEDs when pin 3 of IC2 is low (this prevents the display from smearing). When pin 3 of IC2 goes high, IC3 and IC4 are clocked to the next address and the E1-bar input of IC1 goes high to disable the memory. IC3 and IC4 are presettable up/ down counters which have been set to count in binary. In addition, the two counters have been cascaded by connecting the carry out (pin 7) of IC3 to the carry in (pin 5) of IC4. The presettable jam inputs at pins 4, 12, 13 and 3 (corre­sponding to J1, J2, J3 and J4) are all tied low so that when the Preset Enable (PE) input at pin 1 is pulled high, the Q outputs all go low. This resets the counter to zero. Initially, IC2 is reset when the output of NAND gate IC5c pulls pin 4 low. In greater detail, IC5c and IC5b together form an RS flipflop. When IC5b’s output (pin 11) is low, IC5c’s output (pin 3) is high and vice versa. These outputs are set and reset by low-going pulses to pins 12 & 2, respectively. When mercury switch S2 closes, the 1µF capacitor at the input of Schmitt NAND gate IC5d charges via VR2. The output of IC5d then goes low and briefly pulls pin 2 of IC5c low via a Programming When the circuit is connected to a PC printer port, the D1-D7 lines of PORT.A are used to apply the character codes to memory IC1. Control over this operation is enabled using the W-bar input at pin 27 of IC1, the PE inputs of IC3 & IC4, and the clock input to IC3 via switch S1b. These signals use the D0 output of PORT.A and the -D0 and -D1 outputs of PORT.C, respec­tively. Initially, counters IC3 & IC4 are reset using -D0. The requisite codes are then applied to the data inputs of IC1 with the W-bar input low to write the data to the memory. The clock signal from -D1 increments the memory locations. This entire programming process is controlled by software (either SPCWRI.EXE or SPCWRI.BAS). The user Fig.3 (right): the final circuit consists of just five ICs, several transistors, diodes and LEDs, a 3-terminal regulator and a handful of other passive parts. IC1 is the memory chip which stores the character information  Fig.2: the block diagram of the Spacewriter. The memory block stores the LED codes to make up the messages and each memory location is sequentially accessed using coun­ters IC3 and IC4. The memory data lines from IC1 switch the LEDs on and off via driver transistors Q1-Q7. .001µF capacitor. This capacitor then quickly charges again via its associated 220kΩ resistor and pin 2 of IC5c goes high again. As a result, pin 3 of IC5c briefly goes low and then high again to reset IC2. It also resets IC3 and IC4 by applying a pulse to their reset enable (PE) inputs via a .001µF capacitor. IC2 now applies clock signals to the pin 15 inputs of IC3 and IC4 via S1b. At the 64th clock pulse, the Q3 output of IC4 (pin 14) goes high. This high is inverted by IC5a and a low-going pulse is applied to pin 12 of IC5b (part of the RS flipflop) via a .001µF capacitor. The flipflop now toggles, with pin 11 of IC5b going high and pin 3 of IC5c going low. IC2 is thus held in the reset condition and clocking ceases. D4 is included to prevent the pin 1 inputs of IC3 & IC4 from going below ground potential when pin 3 of IC5c switches low. Similarly, D1 & D2 protect the inputs of IC5b & IC5c when the outputs of IC5a & IC5d go high. D3 quickly discharges the 1µF time delay capacitor when the mercury switch opens, to reset the delay circuit. May 1997  57 This view shows the completed PC board prior to final installation in the tube. Note how the mercury switch has been oriented at a 45° angle to IC1. This is necessary to ensure that it only closes when the Spacewriter stops at the end of the lefthand arc. simply boots the program and types in the messages on the keyboard. Power is derived from a 9V battery via switch S1a and this is fed to 3-terminal regulator REG1 to derive a regulated 5V supply for the circuit. Note that REG1 is a low-power device to minimise the drain from the battery. The quiescent current is nominally about 4.5mA with the mercury switch open and about 6.7mA when it is closed. The 10µF capacitors at the input and output of REG1 prevent instability and improve transient response of the regulator. In addition, a 10Ω resistor is included between the +5V rail and the LEDs to decouple them from the rest of the circuit. Construction The SILICON CHIP Spacewriter is built on a PC board coded 08305971 and measuring 292 x 18mm. This is housed in a 400mm length of 32mm conduit with end caps. An adhesive label is at­tached to the lower end cap to indicate the switching positions, while a second dress label is attached to the side of the con­duit. The software is available in Quick Basic and also as an execut­able (.exe) file which does not require Basic. The executable version only operates with a printer port located at hexadecimal 0378-037A. Begin construction by checking the PC board for breaks and shorts between tracks. Check also that the PC board will slide inside the conduit and file it down to size if necessary. Fig.4 shows the parts layout on the PC board. It is neces­sary to install the links first, as some of these are located under the ICs. The ICs can then be installed, taking care to orient them correctly as shown on the diagram. The diodes can go in next but note that D4 and D5 are mounted end on. The resistors are all mounted end on as well (see Table 1 for the resistor colour codes). The transistors and REG1 should be pushed down onto the board so that their lead lengths are only about 3mm long. When these parts are in, install Table 2: Capacitor Codes ❏ ❏ ❏ ❏ ❏ Value IEC Code 0.1µF 100n .068µF   68n .056µF   56n .001µF    1n the seven LEDs. These must all be mounted so that the top of each LED is 15mm above the PC board. This is best done by cutting a strip of cardboard 15mm wide and then using this as a gauge to adjust the LEDs. Note that you may need to adjust the LEDs later on, so leave a couple of millimetres spare when you trim their leads. The mercury switch is mounted flat against the PC board but must be oriented at a 45° slant to IC1 as shown. This en­sures that it only closes when the Spacewriter stops at the extremity of the lefthand arc. The capacitors can now be mounted, using Table 2 to deci­ pher the Table 1: Resistor Colour Codes ❏ No. ❏  3 ❏  7 ❏  8 ❏  7 ❏  1 58  Silicon Chip Value 220kΩ 10kΩ 2.2kΩ 15Ω 10Ω 4-Band Code (1%) red red yellow brown brown black orange brown red red red brown brown green black brown brown black black brown EIA Code 104 683 563 102 5-Band Code (1%) red red black orange brown brown black black red brown red red black brown brown brown green black gold brown brown black black gold brown Fig.5: the PC pattern is shown here at 71% of actual size. It can be enlarged to full size on a photocopier set to a 1.41 enlargement ratio. values of the MKT types. The electrolytics (ie, those labelled 1µF and 10µF) must be oriented as shown. They must also be pushed all the way down onto the PC board to allow clearance inside the conduit tube. Next, install trimpots VR1 & VR2 and the two pushbutton switches (S3 & S4). Note that the latter must be oriented so that their flat sides face towards REG1. Finally, go back over the assembled PC board and check that all parts have been installed correctly and that all the solder joints have been made. Drilling the conduit The next step in the assembly is to drill the conduit to accept the LEDs and the switches. Begin by drilling seven 5mm holes for the LEDs. These holes must be in a straight line 6.3mm apart and beginning 30mm from the top end of the conduit. The two switch holes go on the same line but are drilled to 10mm diameter and are located 280mm and 295mm from the top edge of the conduit. Next, make a slot in the conduit to accept the 25-pin D socket (to connect the printer cable). This slot is positioned directly opposite the LED holes and must be positioned low enough to avoid fouling the end cap. The D socket is secured using two self-tapping screws and you will need to drill holes for these as well. The other end cap must be drilled to accept the slider switch knob and Fig.4: install the parts on the PC board and complete the wiring as shown here. Note that the wiring for the DP3W slider switch varies according to the type of switch you have, so be sure to check this carefully. May 1997  59 The Spacewriter is programmed from a PC printer port via this D25 socket which is located immediately behind the LEDs. the associated securing holes. Use the label as a guide to drill and file the necessary holes. Now check that the PC board fits into the conduit neatly and that the LEDs and pushbutton switches mate correctly with their respective holes. The PC board is secured in position by the end caps. In addition, a nylon screw is threaded into a hole in the conduit directly opposite the push­ button switches. This screw presses against the back of the PC board and ensures that the board cannot move when the switches are pressed. Don’t make the hole for this nylon screw too big – it must be a tight fit. We also drilled holes to allow screwdriver The slider switch is mounted on the bottom end cap. access to trimpots VR1 and VR2. Once everything fits correctly, remove all the parts and paint the conduit black. This increases the contrast between the LEDs and the background and makes the message easier to read. Wiring The wiring to the D25 socket and the slider switch is all run using rainbow cable – see Fig.4. Note that Fig.4 shows the wiring details for two different slider switches. That’s because the DSE P7614 has its wiper contacts at one end of the switch while the Altronics S2030 has its wiper contacts towards the centre. The table in Fig.4 lists the various wire lengths. Cut the leads to length and solder them to the PC board first. The wires to the D25 socket are then passed through the socket cutout in the conduit and soldered to the relevant pins. Similarly, the wires for the switch and battery clip exit from the bottom end of the conduit. Connect the switch leads and don’t forget the wire that runs from pin 14 of the D25 socket to the corresponding switch terminal. The battery clip leads will have to be extended so that they have an overall length of 150mm. This will allow the battery to be slid into the tube with the clip towards the end. Be sure to cover the joins in the wires with insulation tape. Finally, we soldered a 20mm dia­ meter loop of tinned copper wire to the strip of copper labelled “pull out here” at the end of the PC board. This makes it easy to remove the board from the tube, should the need arise. Testing Programming the Spacewriter is easy. You just boot the software and follow the on-screen instructions to enter four different messages, each up to 10 characters long. Note that the letters always appear in upper case format. 60  Silicon Chip It’s best to run a few preliminary checks on the unit before final assembly. Connect the battery, switch on and check that there is 5V between pins 14 & 28 of IC1, pins 1 & 8 of IC2, and pins 8 & 16 of both IC3 & IC4. There should also be 5V bet­ween pins 7 & 14 of IC5. If you don’t get the correct voltages, switch off imme­diately and locate the fault before proceeding. If everything checks out correctly, shake the board so that the mercury switch briefly closes. Check that the LEDs flash on when you do this (the pattern will be quite random at this stage). If all is well, disconnect the battery and adjust both VR1 and VR2 to their midpoint settings. This done, the board assembly can pushed into the conduit and the D25 socket and slider switch installed. The battery is installed through the bottom end of the conduit (near the slider switch), with its clip nearest the end cap. Don’t forget the nylon screw that presses against the back of the PC board immediately behind the pushbutton switches. Using the software To check the address of the printer port in Windows 95, double-click the System icon in Control Panel, click the Device Manager tab, select the printer port from the list of devices, click Properties and select the Resources tab. PARTS LIST 1 PC board, code 08305971, 292 x 18mm 1 self-adhesive label (for bottom end cap) 1 Spacewriter software (Spcwri.bas, Spcwri.exe) 1 400mm length of 32mm diameter conduit 2 32mm conduit end caps (Clipsal No. 262/32) 1 DP3W slider switch plus screws, Altronics S2030 or DSE P7614 (S1) 1 mercury switch (S2) 2 momentary pushbutton PC mount switches (S3,S4) 1 25-pin “D” panel socket 1 100kΩ (104) horizontal trimpot (VR1) 1 500kΩ (504) horizontal trimpot (VR2) 1 3mm x 18mm Nylon screw 2 self-tapping screws to secure D socket 1 600mm length of 5-way rainbow cable 1 300mm length of 10-way rainbow cable 1 300mm length of 0.8mm tinned copper wire 1 9V battery 1 9V battery clip Semiconductors 1 TMS6264L low power 8K x 8-bit static RAM (IC1) 1 7555, LMC555CN or TLC555 timer IC (IC2) 2 4029 CMOS 4-bit up/down counters (IC3,IC4) 1 4093 quad Schmitt NAND gate (IC5) 1 78L05 low-power 5V regulator (REG1) 7 BC338 NPN transistors (Q1-Q7) 5 1N914 switching diodes (D1-D5) 7 5mm high intensity red LEDs (LED1-LED7) Capacitors 4 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 0.1µF MKT polyester 1 .068µF MKT polyester 1 .056µF MKT polyester 3 .001µF MKT polyester Resistors (0.25W, 1%) 3 220kΩ 7 15Ω 7 10kΩ 1 10Ω 8 2.2kΩ Miscellaneous Black paint, solder, D25 plug-toplug lead As mentioned above, the software is supplied as both an executable (.exe) file and as a Quick Basic file (.bas). The .exe file can be copied to your hard disk and you simply type SPCWRI at the DOS prompt to load the program. Alternatively, you can double-click the SPCWRI.EXE file in the Windows File Manager or Explor­er. After that, it’s simply a matter of following the on-screen instructions to program the unit. Note that this program uses a printer port address at 0378. If you need to check what printer ports you have, type MSD at the DOS prompt. Alternatively, for Windows 95, double-click the System icon in Control Panel, click the Device Manager tab, select the printer port from the list of devices, click Properties and select the Resources tab. If you don’t have a printer port on 0378, the Basic program can be used instead. This is run in Quick­ Basic using the “File Run” command. The advantage of the Basic program is that the printer port address can be chang­ed if required. To program the unit, first connect the Spacewriter to the printer port of the PC using a D25 plug-to-plug lead. This done, switch the Spacewriter to the RECORD position, type in a message of up to 10 characters and press ENTER. The LEDs on the Spacewriter will flash and you then switch to the SPACEWRITE position before disconnecting the D25 lead. Warning: switching the unit OFF erases all recorded messages. Now wave the Spacewriter in front of you to see if the message appears. You will probably need to adjust the clock rate and delay using VR1 and VR2 – just adjust them until the mesSC sage appears correct. Where To Get the Software The software for this design is available from Silicon Chip Publications for $7.00 (includes disc) plus $3.00 p&p – see order form page 33. May 1997  61