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Protect your licence & avoid expensive fines with this . . . School Zone Speed Alert By JIM ROWE 40 Have you been booked for driving through a school zone at well above the 40km/h limit? It’s easy to do and can be a very expensive mistake. Here’s a project to help you stay out of trouble. It keeps track of the time and flashes a warning LED during the morning or afternoon 40km/h time periods. 36  Silicon Chip KM/H ZONES on the roads near schools can be a real hazard to your wallet and your licence. You must remember to slow down during the specified morning and afternoon time periods. It is very easy to forget and since many of these zones now have fixed radar speed cameras, you can be hit with a big fine without even knowing it. It’s true that most school speed zones are marked with suitable signs, to warn motorists when they are entering and leaving them. But as yet, only some of the signs have flashing lights to warn when the 40km/h limit actually applies. What this project does is keep track of the time and it flashes a warning LED when the 40km/h speed limit applies in school zones. The project itself is housed in a small jiffy box and is intended to go inside your glove box. However, the warning LED is connected to the main box using a short length of figure-8 cable and can be mounted in a small hole in the lower part of your dashboard or in a small bracket mounted just below the dash. It should be inconspicuous but able to attract your attention when it’s flashing. The LED flashes brightly enough to attract your attention in daylight siliconchip.com.au but not so bright as to dazzle you and disturb your driving. What it does Fig.1: the circuit is based on a PIC 16F628A-I/P microcontroller. This runs the firmware program, scans pushbutton switches S1-S9 and drives LED1 and the four 7-segment displays in multiplex fashion via transistors Q1-Q11. You can think of the project as a special kind of alarm clock. It keeps the time but it has four “alarm time” settings, two for the start and finish of the morning 40km/h period and the other two for the start and finish of the afternoon speed limit period. The LED flashes not just at the start of the 40km/h speed periods but all through their duration. The heart of the project is a PIC16F628A-I/P microcontroller. The program inside the PIC is already provided with the start and finish times of the morning and afternoon 40km/h zone times common throughout Australia: from 08:00 to 09:30 in the morning and from 14:30 (2:30PM) to 16:00 (4:00PM) in the afternoon. Each of these “alarm time” settings can be changed if you wish but in most cases this won’t be necessary. All you’ll normally have to do is set the current time and whether or not it is “normal” or daylight saving time. There’s also a button to enable or disable the warning LED, so you can turn it off for weekends and school holidays. These settings are easily changed, as we will explain later. How it works Fig.1 shows the circuit. It consists of the PIC micro, four 7-segment LED displays, 10 pushbuttons, one LED and little else. All the real work is done by the firmware program inside the PIC micro. Indeed, most of the circuitry is there simply to allow you to set the clock’s time and functions, using the pushbuttons and the 7-segment displays. At other times the displays are turned off, to save energy. The four 7-segment displays are driven by the PIC micro in multiplexed fashion, with the segments driven from port pins RB0-RB6 via switching transistors Q1-Q7, while the digits are driven from RA0-RA3 via transistors Q8-Q11. Switches S1-S9 are also scanned by the PIC in multiplex fashion, again using RB0-RB6 as output lines and RB7 and RA4 as input lines. The remaining switch (S10) is provided as a Reset button, to allow the PIC firmware to be returned to its original “factory settings” if it should siliconchip.com.au April 2009  37 S9 S8 S1 HOURS MIN SET TIME Q4 PN200 4004 D1 10k 78L05 R1 + S4 S5 S6 AM STRT AM FIN PM STRT PM FIN DLS ON/OFF NOTE: COMPONENTS AND TERMINAL PINS IN RED ARE FITTED UNDERNEATH BOARD (S7) + 22pF S3 – 12V + Q5 PN200 Q7 470 F 470 F S2 battery instead of the 12V car battery is the value of resistor R1, ie, the charging resistor for the warning LED flashing circuit. That’s because LED1 is the only part of the circuit that operates directly from the battery input voltage, via polarity protection diode D1. The flashing circuitry for LED1 works as follows. Resistor R1 allows the 470μF capacitor connected to the anode of LED1 to charge up to the nominal battery voltage, which takes a few hundred milliseconds. LED1 does not draw current during this time because it is controlled by switching transistor Q8 and this is normally kept off by the PIC. When the LED is to be flashed (to warn the driver that they are now in a 40km/h time zone), the PIC turns on Q8 for about 100ms. This allows LED1 to conduct through the 120Ω current-limiting resistor, emitting a bright pulse of light and discharging the 470μF capacitor in the process. The PIC then turns off Q8 to switch off LED1, keeping it off for at least 900ms to allow the capacitor to recharge, ready for the next flash. This charging and discharging sequence only happens during the 40km/h zone times, with LED1 flashing brightly once per second. It’s a simple arrangement but one that gives bright flashing despite a relatively low average current drain. Because switching transistor Q8 is shared by both DISP1 and LED1, this results in LED1 flashing unavoidably whenever you are changing the clock’s settings or functions, ie, whenever DISP1 is operating. So you’ll soon notice that LED1 flashes whenever settings are being made or changed. This might be a bit confusing until you get used to it. Note that the 7-segment displays DISP1-DISP4 do not operate when LED1 is flashing during a 40km/h zone time. c 10k 10k 10k Q6 120 PN200 PN100 Q8 SCHOOL ZONE 012SPEED 09002 ALERT b e d REG1 1k 1k 1k 1k X1 4MHz 1k 1k 1k 1k IC1 PIC16F628A prog 1k 4.7k 10k 10k 4.7k 22pF PN100 4.7k 100nF PN100 Q9 Q10 Q11 10k 4.7k S10 EC8291 KC5472 20090210 4.7k + RESET PN100 120 PN200 PN200 Q2 10k 120 PN200 Q1 10k Q3 100 F 120 120 120 120 TRELA DEEPS ENOZ LOOHCS 2745CK 1928CE 8002 C g f 120 a DISP1 DISP2 PN200 8888 DISP3 DISP4 LED1 K S7 A R1: 3.9k FOR 9V 4.7k FOR 12V ALERT ON/OFF ON SOCKET STRIP SPACERS Fig.2: follow this parts layout diagram and the photo below to assemble the School Zone Speed Alert. Note that the three electrolytic capacitors are mounted on the back of the PC board. Here’s a view of the completed PC board assembly. Switch S7 is mounted on socket strip spacers so that its button protrudes through the front panel. ever become “confused” (or rather, if you yourself become confused). The PIC has a 4MHz oscillator using crystal X1 and the two 22pF capacitors. This is necessary so that the clock keeps good time – you don’t want it drifting too much otherwise it will not perform its duty correctly – warning you when the school speed zones apply! The PIC and all its associated cir- cuitry runs from a regulated 5V supply line derived from the battery input via a 78L05 low-power 3-terminal regulator (REG1). This allows the project to be powered from a 9V alkaline battery inside the box or from the vehicle’s 12V battery (note: an internal 9V battery won’t last long, so the vehicle’s battery is preferable). The only change that needs to be made when you decide to use a 9V Construction As you can see from the photos, all Table 1: Resistor Colour Codes o o o o o o No. 9 6 1 9 8 38  Silicon Chip Value 10kΩ 4.7kΩ 3.9kΩ 1kΩ 120Ω 4-Band Code (1%) brown black orange brown yellow violet red brown orange white red brown brown black red brown brown red brown brown 5-Band Code (1%) brown black black red brown yellow violet black brown brown orange white black brown brown brown black black brown brown brown red black black brown siliconchip.com.au Parts List This view shows how the three electrolytic capacitors are mounted. The leads to the warning LED and the battery are secured by looping them through a cable tie that goes through two holes in the PC board. of the circuitry and components used in the project (apart from LED1 and the battery) are mounted on a small PC board. This fits inside a standard UB3 size utility box measuring 130 x 68 x 44mm. If you wish to power the unit from a 9V alkaline battery, this can be fitted inside the same box and under the PC board. The PC board is coded EC8291 and measures 123 x 61mm. It is doublesided with plated-through holes, to avoid the need for any wire links and to also make assembly as straightforward as possible. Fig.2 shows the parts layout on the PC board. To build the unit, just follow this diagram, along with the silkscreening on the board itself, and you shouldn’t have any problems. You can begin assembly by fitting the resistors, followed by the 18-pin socket for IC1 and the four PC board terminal pins (two for the battery input and two for the wires from LED1). These pins are all fitted on the rear of the board, by the way. Note that there are pads and holes for two further pins next to those for LED1. We’ve provided the extra pin locations to give you the option of having pushbutton S7 mounted away from the board if you wish, rather than (or as well as) having it mounted directly on the board. This is because most of the time S7 is the only switch that needs to be accessed, to enable the warning LED for school day driving or disable it for weekend/holiday driving. Alternatively, you may prefer to siliconchip.com.au have S7 mounted remotely, perhaps near LED1, so it can be accessed without needing to open the glove box to reach the main unit. The next step is to fit the 10 pushbutton switches, with reset switch S10 at the lefthand end of the board and all of the others (except S7, if you prefer to have it connected remotely) in a line along the front. All of the switches except S7 mount directly onto the board, so that when the board is mounted behind the box lid/front panel, their actuators do not protrude through the matching access holes – preventing accidental “operation” inside the glove box. These switches are accessed via a ballpoint pen, toothpick or similar stylus, which isn’t a problem since they’re only accessed very occasionally. But since S7 will need to be accessed more often, it’s mounted on four small socket-pin standoffs, so that its actuator protrudes through the hole in the front panel for easy fingertip use. Solder S7’s terminals to the pins after mounting it, to prevent it from coming loose. Once the switches are in, install the two 22pF disc ceramic capacitors (to the right of IC1) and the 100nF multi­ layer monolithic (just above pin 1 of the IC socket). Then you can fit the three electrolytic capacitors but note that all three of these are polarised, so they need to be orientated as shown in Fig.2. Note that all three electrolytic capacitors are mounted on the underside of the PC board, with their leads soldered to the pads on the top of the 1 UB3 jiffy box, 130 x 68 x 44mm 1 PC board, code EC8291, 123 x 61mm 10 SPST micro tactile pushbutton switches, vertical PC-mount, 6mm actuator (S1-S10) 1 4MHz quartz crystal, HC-49US case (X1) 1 18-pin IC socket 4 M3 x 9mm tapped spacers 4 M3 x 6mm machine screws, countersunk head 4 M3 x 6mm machine screws, pan head 4 1mm diameter PC board pins 1 4-pin SIL socket strip (to provide the standoffs for S7) 2 1m lengths of figure-8 cable 1 3AG in-line fuseholder 1 3AG 500mA fuse 1 cable tie 1 100mm length 2.5mm-dia. heatshrink tubing Semiconductors 1 PIC16F628A-I/P microcontroller, programmed (IC1) 1 78L05 +5V regulator (REG1) 7 PN200 PNP transistors (Q1-Q7) 4 PN100 NPN transistors (Q8-Q11) 1 1N4004 diode (D1) 4 FND500 7-segment common cathode LED displays (DISP1-DISP4) 1 5mm high-intensity LED (LED1) Capacitors 2 470μF 16V electrolytic 1 100μF 16V electrolytic 1 100nF monolithic ceramic 2 22pF NPO disc ceramic Resistors (0.25W 1%) 9 10kΩ 9 1kΩ 5 4.7kΩ 8 120Ω 1 3.9kΩ (9V) or 4.7kΩ (12V) for R1 Where To Buy A Kit The development of this project has been sponsored by Jaycar Electronics who own the copyright of the design and firmware. Kits (Cat. KC-5472) will only be available from Jaycar and its dealers. board. This is because they are a little too tall to fit in the 9mm space between the PC board and the lid, when the two April 2009  39 Specifications Flashes a high-brightness LED approximately once per second during the 40km/h school zone periods (normally 08.00-09:30 and 14:30-16:00). User settings: current time, normal & daylight saving time modes, and ability to disable the warning LED (eg, at weekends and school holidays, etc). The starting and finishing times can also be changed. Timekeeping accuracy: close to the frequency accuracy of the quartz crystal oscillator; ie, within about 40 parts per million or better than about one second in seven hours. Current drain: (1) in silent running mode – less than 4mA; (2) in setting mode – about 50mA average; (3) in warning mode (LED1 flashing) – about 5mA average. are assembled together. The capacitors can either be mounted verically as shown on Fig.3 or horizontally (ie, with their leads bent through 90°) as shown in the photos. In the latter case, secure them in place using silicone adhesive, to prevent them from vibrating and coming adrift. After the electrolytics are in place, fit the four 7-segment LED displays. These go in the upper centre of the board. Make sure that you orientate each one so that its small circular decimal point LED is at lower right – otherwise the display won’t function correctly. Next, fit protection diode D1, regulator REG1 and all the discrete driver transistors (Q1-Q11). Orientate all these parts as shown in the overlay diagram and also take care to place the four NPN (PN100) transistors in the positions shown for Q8-Q11, below the 7-segment displays. The PNP (PN200) transistors are used for Q1-Q7. Now fit crystal X1. A crystal in an HC-49US low-profile case fits on the top of the board. It must be installed slightly proud of the board, to avoid the possibility of its case shorting to an adjacent pad. Alternatively, if the crystal is in the taller HC-49U case, it must be fitted on the underside of the board like the electrolytic capacitors. In this case, use a thin insulating plastic or cardboard washer to act as a spacer to again make sure that the crystal’s metal case can’t short any of the PC board’s tracks or pads. The final step in the board assembly is to plug the pre-programmed PIC micro (IC1) into its socket, taking care both to orientate it correctly and also to plug it in without straining or bending any of the pins. Once this is done you can attach the board assembly to the rear of the box lid, using four M3 x 9mm tapped spacers and M3 machine screws. Countersink head screws are used to attach the spacers to the lid while pan-head screws are used to attach the PC board to the spacers. If you live in a dusty environment, you may wish to apply an optional 70 x 23mm rectangle of thin clear plastic film (like photocopier film or laminator pouch material) to the underside of the lid behind the display window (to keep out dust). The film can be held in place using glue around the edges. Having mounted the board, connect the remote warning LED using a length of figure-8 cable – see Fig.3. A second length of figure-8 cable is then used to connect the board to a 9V or 12V power supply. Fit some short lengths of 2.5mm-diameter heatshrink sleeving over the terminations on the PC board, to prevent the leads from vibrating a breaking the solder connections. Note that you will need to drill an The PC board is mounted on the back of the case lid via M3 x 9mm tapped spacers and secured using M3 x 6mm machine screws. 40  Silicon Chip exit hole in the side of the case for these cables. Setting up To test and set up the unit, first connect a 9V or 12V battery, depending on the value chosen for R1. It should now be running, even though you shouldn’t see anything on either the displays or LED1 at this stage. Are you currently in daylight saving or not? If you are, you need to press switch S6 with a ballpoint pen or toothpick; hold it down for about one second. The displays should flash the message “dSon” once and LED1 should also flash briefly. If you are not currently in daylight saving, you don’t need to do this operation until daylight saving does begin. Next, set the current time. This is done by first pressing S1, again holding it down for a second or so. This causes the displays to flash the unit’s current time setting about once per second. As noted above, LED1 will also flash briefly along with DISP1. The initial reading on the displays should be 0001 or 0002, depending on how long the unit has been connected to the battery and running. When first powered up (or reset), it resets to midnight, or 00:00. The idea now is to use switch S9 to increment the hours digits to their current time value, and then S8 to increment the minutes digits to their current time value. In both cases, you simply hold the switch down and the PIC will increment the reading at a rate of about once per second. This makes it easy to get to the current time values and simply release the switch when they are reached. When you have the current time on the displays, press S1 again and hold it down for about a second. The displays will flash the message “SEt”, to show that the time has been set. The displays and LED1 then turn off. Enabling LED1 The final step is to decide whether you want to enable or disable the operation of warning LED1 – which will depend on whether it is currently a school day or not. If it’s not a school day and you therefore don’t want to enable LED1, there’s no need to do anything more because when the School Zone Speed Alert is powered up (or reset), LED1 is disabled by default. If it is a school day and you do wish siliconchip.com.au INLINE FUSEHOLDER WITH 500mA FUSE + b e d SCHOOL ZONE SPEED ALERT C 2008 c EC8291 KC5472 g f a TO 12V SUPPLY IN FUSEBOX 100 F – + 470 F + HEATSHRINK SLEEVING 470 F + LED1 A 20090210 K Fig.3: the School Zone Speed Alert is connected to a permanent 12V supply at the fusebox in the vehicle’s cabin. Note the 500mA inline fuse in series with the positive lead. to enable LED1, simply press switch S7 and hold it down for a second or so. It should flash twice briefly, to indicate that it has been enabled. In most cases, this is all you need to do to set up your School Speed Alert. Normally, there is no need to worry about switches S2-S5, because these are only needed if the school crossing time zones in your area are different from the unit’s default time settings of 08:00 - 09:30 for the mornings and 14:30 - 16:00 for the afternoons. Only if you are in an area where the times are different from these, will you need to change the two start and finish time settings using switches S2-S5. These are used in a very similar way to the current time settings switch (S1) and as before, in conjunction with the hours incrementing switch S9 and the minutes incrementing switch S8. You simply press and hold down the appropriate setting button until the current setting is shown on the displays, then change the setting hours and minutes using S9 and S8. Then you press the initial setting button once again, holding it down until the displays flash “SEt” and go dark again. Installation Once set up, the unit can be connected to the vehicle’s 12V supply. Use figure-8 cable to do this and make the connection to a permanent 12V source in the fusebox inside the cabin. Be sure to include a 500mA inline fuse in series with the positive lead (at the fusebox end) and take care with the SC polarity of LED1 – see Fig.3. siliconchip.com.au Using The School Zone Speed Alert The School Crossing Alert is intended to be placed in your car’s glovebox but with the warning LED (LED1) fitted in a recess in the dashboard or on a small bracket below it, so its flashing can attract your attention. There’s normally nothing to do in terms of operating the unit, apart from enabling the warning LED for driving on school days and disabling it for weekends and school holidays. Both operations are done simply by opening up the glove box and pressing switch S7, holding it down for a second or so. The LED will flash twice briefly when it has been toggled into “warning enabled” mode but it remains off when it has been toggled into disabled mode. Daylight saving The only other operation that needs to be done from time to time is to toggle the unit into daylight saving mode when daylight saving begins, or back into “normal time” at the end of daylight saving. Both operations are done using switch S6, pressing it and holding it down for a second or so. The displays will flash “dSon” when you toggle into daylight saving mode, or “dSoF” when you’re returning to normal time. The firmware inside the PIC has been programmed to keep time with much the same accuracy as a normal quartz clock or watch. As a result, you shouldn’t need to correct its current time setting very often – once every couple of months, at most. Because the timekeeping accuracy of the unit does depend on the frequency accuracy of the 4.00MHz crystal though, some units may need the time to be corrected more frequently. This is done in exactly the same way as you originally set the time, using switch S1, switches S9 and S8 and then S1 again to save the new setting. Trimming the crystal If your unit turns out to have a crystal whose frequency error is sufficient to require more frequent resetting, there is another option. This involves replacing the 22pF NPO ceramic capacitor nearest IC2 on the PC board, with a 6.2-30pF trimmer capacitor such as the Jaycar RV-5716 (green). This trimmer should be fitted on the rear of the board, to allow it to be adjusted more easily. The trimmer will then allow you to set the crystal frequency closer to 4.000MHz, to bring the timekeeping into line. It will be easiest to find the right setting for the trimmer by using a frequency counter to monitor the frequency at pin 15 of the PIC. If you don’t have access to a frequency counter, you will have to set the trimmer by trial and error. Run the unit for a few hours, then check the current time setting. If it’s slow, reduce the trimmer’s setting slightly and try again. Conversely, if it’s fast, increase the trimmer’s setting. April 2009  41