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Amaze your friends with this highly visible animated clock project. It’s based on a PICAXE micro, a quartz clock movement and a large quantity of low-cost diodes and LEDs. The results will surprise you! Program & circuit design: RON RUSSO Article & PC board design: CLIVE SEAGER Clive Seager is Technical Director of Revolution Education Ltd, the developers of the PICAXE system. Novel PICAXE LED Chaser Clock I T’S ALWAYS INTERESTING to see the many and varied projects that people create using PICAXE chips. When first introduced to this LED Chaser Clock from Ron Russo, I was at first taken aback with the complexity of the prototype; how could it possibly have been made by hand? It was intricately assembled by wiring and soldering together well over 200 components, all without a PC board! As Ron himself stated, “the wiring is kept to a minimum by the strategic 40  Silicon Chip (piggy-back) placement of the shift registers and the unique way I coupled the two banks of registers with the isolation diodes. The results of this method appear more like a work of art than a rats nest, which is the usual outcome when no circuit board is used.” However, most of us would not have a steady enough hand or the patience to build such a masterpiece. Therefore, this incarnation of the project does include a PC board so that everyone has a chance to build it! As can be seen from the photos, the Chaser Clock is a marriage between an analog quartz clock movement and a circular array of LEDs. A continuous visual display is created by synchronising LED effects with the movement of the clock’s seconds hand. Once every second, a “chaser” LED starts from the 12 o’clock position and appears to move around the dial in an anticlockwise direction towards the seconds hand. When it meets the seconds hand, the LED appears siliconchip.com.au to “freeze” under the pointer (ie, it remains illuminated). So at each tick of the clock, one more LED is illuminated. This creates an arc of light that trails the seconds hand. When the seconds hand reaches 12 o’clock, the arc closes to form a complete circle of light, whereupon all LEDs are turned off and the cycle repeats over. It’s difficult to visualise this effect simply by reading about it, so we’ve posted a short video of Ron’s prototype clock in action on the SILICON CHIP website. You’ll find it in the downloads section for this month. Circuit basics The complete circuit diagram for the chaser clock appears in Fig.1. A PICAXE-08 microcontroller (IC1) drives the whole show with the aid of timing pulses from the analog clock mechanism. The clock is used intact, with a pair of diodes steering the pulses from its coil to input3 of the micro. A 2.5V supply for the clock mechanism is derived from the 5V rail by dividing it down with two 330W resistors, while a 100mF capacitor supplies peak coil current. All 60 LEDs are driven by 74LS164 8-bit shift registers. Every tick of the clock, the PICAXE-08 program performs a few simple calculations and then manipulates the data (A&B), clock (CLK) and clear (CLR) inputs of the shift registers to produce the magical effects mentioned earlier. The 74LS164 shift registers are divided into two paralleled banks, with isolation between the banks provided in the form of series-connected diodes. Registers SR1-SR8 handle the LED chase effect, while SR9-SR16 handle the arc of light that follows the seconds hand. With this circuit arrangement, about 10-15mA of current flows through each LED. No separate current-limiting resistors are required, which greatly simplifies the assembly task. Some might be wondering how this simple scheme is possible; don’t we always need to limit LED current in a logicdriven circuit? The answer is usually yes, of course. However, by design, the 74LS series TTL logic devices incorporate a certain resistance in the upper NPN output transistor’s collector circuit (see Fig.3). In the case of the 74LS164, siliconchip.com.au this amounts to about 120W. Accounting for the impedance of the NPN output and the forward voltage drop of the LEDs and series diodes, the total current sourced from each output shouldn’t exceed about 15mA. This works very well, with the total power dissipation for each IC remaining within safe limits. Note that current will vary between different brands of LEDs and shift registers, so for best results use devices from the same manufacturer throughout. Do not be tempted to substitute devices from another logic family. For example, while 74HC164 devices are pin compatible with the 74LS164, they have an entirely different output structure and will self-destruct in this circuit! Power supply Due mainly to the large number of LEDs, overall current consumption is quite high, starting at about 200mA and increasing to over 900mA as the arc of light grows. The original circuit was developed using an on-board 7805 regulator with a 9V supply but when using this setup the regulator gets very hot. Although a sizeable heatsink would keep the regulator within operating parameters, there is still the possibility of heat damage to surrounding materials. The final version of the clock was therefore developed for use with an off-board regulated 5V power sup- Par t s Lis t 1 PC board 1 2-way 5mm pitch terminal block 1 miniature tactile push-button switch (S1) 1 8-pin IC socket 16 14-pin IC sockets 1 analog clock mechanism (see text) Semiconductors 1 PICAXE-08 (IC1) 16 74LS164 8-bit shift registers (SR1-SR16) 1 1N5817 (or 1N5819) Schottky diode (D1) 2 BAT85 (or 1N5711) Schottky diodes (D2 & D3) 120 1N4148 small-signal diodes (D4-D123) 12 5mm yellow LEDs 48 5mm green LEDs Capacitors 5 100mF 16V 8 100nF polyester Resistors (0.25W 5%) 1 2.2kW 1 470W 2 330W Also required (not in kit) 5V DC 1A (minimum) regulated power supply (eg, Altronics Cat.M 8909) The kit includes two quartz clock mechanisms like the one shown here. These are widely available for just a few dollars. If you want to use a dark clock face design, you can paint the black hands a lighter colour (eg, yellow) to obtain a good contrast. August 2006  41 Fig.1: here’s the (almost) complete circuit diagram for the clock. As noted, we’ve left out some portions to make it easier to follow. A PICAXE micro (IC1) manipulates 16 shift registers to control the light show, using pulses from the clock mechanism for timing. Clever arrangement of the shift registers and LEDs allows the use of the smallest PICAXE device in the range, with the program code consuming only 76 bytes! ply, such as the Altronics M-8909. However, if you prefer to build your own regulated power supply, you’ll find a suitable circuit in Fig.4. Note that any supply should be located no more than 2m from the clock and the power leads should be formed from heavy-duty hookup wire. 42  Silicon Chip Power is connected to the board via a 2-way screw-terminal block. A diode in series with the positive input (D1) is included for protection against accidental reversal of the leads. We’ve specified a 1N5817 Schottky diode to minimise voltage losses, so you should get a reading of close to 4.7V when measuring between the +5V and GND rails on the PC board. Putting it together Assembly is very straightforward, but time consuming! The PC board is a double-sided design (tracks on both sides), so take care that you have siliconchip.com.au the board the right way up. All of the components except the LEDs mount on side with the white silk screen overlay! Put all of the LEDs aside for the moment and begin by fitting the diodes and resistors. Take care to insert each diode around the right way; the cathode (banded) ends must go in as shown on the overlay diagram (Fig.2). Also, don’t mix up the 1N4148 and BAT85 types, which may look very similar depending on the brand supplied. siliconchip.com.au Add all the IC sockets next, orienting the notched (pin 1) end as indicated. Follow with the few remaining components, including the reset switch, terminal block and capacitors. Note that the electrolytic capacitors are polarised devices and must be installed with their positive leads oriented as shown. The final task is to install all of the LEDs. As shown on the overlay diagram (Fig.2), there are two possible positions for the LEDs. Most constructors will probably go for the outer circle but some clock designs may suit the inner circle. Don’t install LEDs in both positions. Before soldering the LEDs, work out carefully what your clock face design is going to look like and how it is going to fit to the PC board – you may wish to mount the LEDs several millimeters off the PC board. Whatever your design, remember that the LEDs are fitted on the reverse side of the PC board to all the other components! August 2006  43 With all the LEDs in place, plug the 74LS164 shift registers into their sockets. It’s very important that the notched end of each IC lines up with the notch in its socket. Clock mechanism This view shows the fully-assembled clock prototype. Note that there have been some changes to the PC board since this photo was taken – just follow the parts layout diagram of Fig.2. The final task is to “hack” the clock mechanism. This requires a steady hand and lots of patience! The aim is to solder four wires to the clock PC board; one to each coil end and two to the battery connection points. Start by holding the assembly over a tray to catch any gears that may drop out. Use a flat screwdriver blade to carefully open the clock’s plastic case by sliding it under the tabs on either side of the casing. Don’t bend the plastic too far or it will snap! You then have to remove the gears to get access to the coil and its small PC board. It’s a good idea to take notes as you remove each piece, as you have to rebuild the gearbox later! Before lifting out the circuit board, study the assembly carefully to see how the two battery connector strips make contact with the PC board. Make a note of which PC board pad connects to the positive side of the battery. Lift out the PC board complete with its plastic surround. You will see two + – Start by removing the back half of the clock’s casing. This can be achieved by sliding a flat-bladed screwdriver under the tabs on either side of the casing. Don’t bend the plastic tabs too far or they will snap! 44  Silicon Chip After removing several of the nylon gears, you’ll be able to lift out the complete coil assembly. Solder light-gauge wires to the coil and battery pads on the PC board, positioned as shown here. Make a note of which wire goes where – so that you’ll be able to hook them up correctly once it’s reassembled. Remove all of the gears and the two battery contacts from the case half. Cut a slot roughly as shown to allow the four wires to pass out of case once it is closed, then reassemble the lot in reverse order. Don’t force the case halves together – it you feel any significant resistance, then a gear has most likely jumped out of position! siliconchip.com.au Fig.2: follow this diagram and the legend on the board when assembling your clock. Don’t mix up the different types of diodes and take care with the orientation of the ICs, diodes, LEDs and 100mF capacitors. Table 1: Resistor Colour Codes o o o o siliconchip.com.au   No. 1 1 2 Value 2.2kW 470W 330W 4-Band Code (1%) red red red brown yellow violet brown brown orange orange brown brown 5-Band Code (1%) red red black brown brown yellow violet black black brown orange orange black black brown August 2006  45 Program Listing: PICAXE Clock symbol Sec = b13 symbol Leds = b12 symbol TmLength = w0 symbol ChaseClock = 0 symbol NewSec = 1 symbol NewMin = 2 symbol ChaseData = 4 'seconds counter 'chase counter 'duration of pulsout to match chase with second 'pin0 'pin1 'pin2 'pin4 (chase data output & reset input for sync) MAIN: pause 100 high NewMin goto RES_MIN 'settling time 'release SR9-SR16 reset (MR) inputs 'go initilise for start of new minute LOOP: input 4 if input4 = 1 then RES_MIN if input3 = 1 then NXT_SEC 'set pin 4 as input 'if sync button is pressed initialise for new minute 'else a high-going pulse on input 3 indicates one second has elapsed, 'so start a new second ' NOTE: Some analog clocks have low-going rather than high-going pulses (change the above line to suit). goto LOOP NXT_SEC: output 4 if Sec = 0 then RES_MIN Sec = Sec – 1 'loop until a clock pulse occurs 'set pin4 to output to send chase data 'if min is up then go reset minute 'else one sec has elapsed so decrement secs ' Calculate optimum time factor so that all intended LEDs are illuminated just before the current ' second is up. A multiplication factor of 90 is optimum for the PICAXE-08. TmLength = 61 – Sec TmLength = TmLength * 90 CHASE: high ChaseData 'set chase data bit high for Leds = Sec To 0 Step –1 'count back starting from relevent second pulsout ChaseClock, TmLength 'pulse with calculated duration to increment clock input of chase 'regs(SR1-SR8). low ChaseData 'after first clock pulse, set data low for rest of chase next Leds 'do rest of LEDs until this sec finished low NewSec pause 1 high NewSec goto LOOP RES_MIN: Sec = 60 low NewMin pause 1 high NewMin Fig.3: the equivalent circuit for each output (O0-O7) of a 74LS164 shift register. The resistance in the collector leg of the high-side transistor greatly reduces the maximum current that these devices can source. 'zero SR1-SR8 outputs, clock SR9-SR16 'make pulse 1ms wide 'return high very fine wires connecting the coil to two small pads near the edge of the PC board. These are only as thick as a human hair, so you may need a bright light and a magnifying glass! Now carefully solder two light-gauge hookup wires (about 150mm long) to the coil connection pads. Study the orientation and dress of the wires in the photos before you begin. The kit for this project includes two PP3 battery snaps that can be sacrificed for their black and red wires. Simply cut off the battery snaps and use the pre-tinned wire ends for the clock connections. Alternatively, you could use rainbow cable for the job. Only hold the soldering iron in 'power up or minute is up or syncro reset . . . 'reset all SR9-SR16 outputs to zero 'make pulse 1ms wide ' As the first 4 outputs of SR9 are not connected to LEDs we must send a dummy run of 4 clock ' pulses to shift regs SR9-SR16 to start at LED60 (SR9 output O4). This will also pulse the reset (MR) ' inputs of SR1-SR8, zeroing their outputs ready for the new minute. for B10 = 1 to 4 pulsout NewSec, 10 next if input4 = 1 then LOOP goto NXT_SEC 46  Silicon Chip 'check input again if sync button still pressed 'else go start a new minute Ron Russo’s prototype was made by hand wiring over 200 components – all without a PC board. The ICs were even piggy-backed! siliconchip.com.au place for a fraction of a second – if a joint is overheated the pad will lift off or the coil wire come adrift. Next, solder the two power wires. Thankfully, these are easier to work with, as the pads are much larger. Be sure to note the positive (+) and negative (-) wires for later identification. Important: it is not necessary to remove the coil and its PC board from the plastic surround. However, you must be very careful not to contact the gear posts that are part of the plastic molding with the barrel of your soldering iron! Remove the two metal battery contact springs from the plastic casing and use a sharp knife to cut a small opening in the plastic to allow the four wires to pass through (see photos). You can now reassemble your clock mechanism, with the aim obviously being to get all of the gears in the right places! Finally, mount the clock to the chaser PC board and solder the four wires in position. Fit the hands and then you’re ready to go! Made a mistake during the assembly or lost a gear? Don’t worry; the kit includes a spare clock mechanism! Fig.4: the clock must be powered from a regulated 5V DC supply with at least a 1A capacity. High-power regulated plugpacks are readily available but if you want to use an unregulated plugpack, you’ll need to add a 5V regulator circuit. Here’s a suitable circuit based on the popular 7805. PICAXE program The PICAXE chip in the kit is supplied pre-programmed with Ron’s original program. However, the fully commented listing is included here for those who wish to experiment. In particular, the timing multiple (90) may need tweaking slightly if your chaser runs too fast or too slow. A programming socket is not included on the PC board. However, it’s a simple matter to reprogram the micro in any of a number of different project boards. If you don’t already have a suitable board and programming cable, check out the Schools Experimenter Starter Pack (part. no. AXE092S) or the PICAXE-08 Starter Pack (part no. AXE-003). Both are available from MicroZed Computers; see the adjacent panel for contact details. Synchronisation to the seconds hand is achieved by pressing the reset button and then waiting for the seconds hand to reach 60. The LED display will be blanked and then start from 1 when the button is released. The clock will stay in sync until power SC is removed. siliconchip.com.au This side of the board looks very bland without the clock face. You don’t get one of these in the kit but you can easily create your own from a favourite photograph or desktop wallpaper (we used “The Matrix” theme wallpaper). The numerals can be added in just about any graphics program and the result printed out on photographic quality stock. Where To Buy A Kit Of Parts The PC board copyright for this project is owned by Revolution Education Ltd. Complete kits (part no. AXE115S) are available from authorized PICAXE distributors – see www.microzed.com.au or phone MicroZed on 1300 735 420. Note: kit does not include clock face (see above). August 2006  47