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Pt.2 By JIM ROWE GPS Driver For The 6-Digit GPS Clock Based on the GlobalSat EM-408 GPS module, this compact GPS receiver/driver board mates with the 6-digit display described in Pt.1 to form a self-contained GPS clock. It can also be housed separately and used to provide NMEA 0183 time and date information to a PC, via a serial port. T HE 6-DIGIT GPS Clock Display described in last month’s issue of SILICON CHIP was originally conceived as an attachment for the author’s GPSBased Frequency Reference (MarchMay 2007). The idea was that since the NMEA 0183 stream of GPS time, date and navigational data was available from the GPS receiver in the Frequency Reference, we’d provide a “smart” display unit to receive this data stream, extract the time information and display it in either its native UTC form or converted to local time. However, before the design was published, we realised that it would also be of interest to many more people than those who had built the GPSBased Frequency Reference. That’s because it could be turned into a fully 36  Silicon Chip self-contained GPS Clock simply by building a GPS receiver and display driver module into the same enclosure. And by taking advantage of one of the low-cost GPS receiver modules currently available, this could be done surprisingly cheaply – with the complete clock costing less than $200. Not bad for a clock offering you very close to “atomic time” (and updated every second), wouldn’t you say? So that’s the story behind the little GPS Clock Driver board described here. Its main application is to convert last month’s 6-Digit Display into a self-contained GPS clock. Alternatively, it can be used to feed the GPS/ NMEA 0183 data stream into a PC via a standard serial port. You can then use a freeware software program such as “GPS Clock” to process and display the data – see panel. How it works As mentioned above, the heart of the new driver board is the GlobalSat EM-408 “GPS Engine” module. This module was also used in Geoff Graham’s GPS-Synchronised Analog Clock, described in the March 2009 issue. The EM-408 is quite small, measuring just 36.4 x 35.4 x 8.3mm. Despite this, it includes a built-in microwave “patch” antenna and is very sensitive (-159dBm). This allows it to operate reliably indoors using just the patch antenna, without requiring an external antenna or cabling. The current drain is also surprisingly modest, at siliconchip.com.au D4 K A OUT 100 F 16V 100nF Vcc EM-408 GPS RX MODULE En Rx Tx Gnd 5 1 3 4 430 1k 2x1k (PATCH ANTENNA) ADJ 240 SET 3.3V K IN D1 D2 D3 REG1 LM317T +3.3V A K A K A + 12V DC INPUT – 1000 F 25V VR1 5k 6 (Rx) 1 2 7 10k B 2 C 3 8 Q1 BC338 9 E 4 NMEA DATA OUTPUT TO DISPLAY (OR PC) 5 DB9F SOCKET D1–D4: 1N4004 A K LM317T 5 4 3 2 1 SC  2009 EM-408 BC338 CLOCK DRIVER USING EM-408 GPS RECEIVER B E OUT ADJ C OUT IN Fig.1: the circuit uses an EM-408 GPS receiver module and an adjustable LM317T regulator to provide a 3.3V supply. The output from the GPS module appears at pin 4 and is inverted by transistor Q1 to drive the display board. just 44mA continuous from a 3.3V supply rail. As a result, all we have to do to use it as a clock driver is to provide it with a source of 3.3V DC power plus a simple buffer stage to interface its NMEA 0183 data stream output to the serial data input of the clock display (or a PC). Fig.1 shows the circuit details. In operation, the driver board operates from the same +12V DC supply used for the display board via its own 3.3V regulator circuit (REG1). REG1 is an LM317T adjustable regulator and is configured in standard fashion, with trimpot VR1 used to set the output voltage to 3.3V, as required by the EM-408. Diodes D1-D3 provide both supply polarity protection and an additional 1.8V voltage drop from the 12V source to reduce the power dissipation of REG1. Diode D4 protects REG1 from reverse current damage. As shown in Fig.1, the EM-408’s Vcc input (pin 5) is connected to the +3.3V line, while the En input (pin 1) is pulled high via a 1kΩ resistor to the same line, to enable it. Also pulled up via a 1kΩ resistor is the Rx input (pin 3), which is provided on the EM-408 to allow it to be fed with NMEA settingsiliconchip.com.au Parts List 1 PC board, code 07106091, 122 x 57mm 1 GlobalSat Technology EM-408 GPS Engine module with cable (Altronics K-1131) 1 short length of double-sided adhesive foam tape 1 PC-mount 2.5mm DC connector (optional – see text) 1 PC-mount DB9F connector (optional – see text) 1 M3 x 6mm long M3 pan-head screw 4 M3 x 30mm screws, countersink head 9 M3 nuts 1 5kΩ horizontal trimpot (VR1) Semiconductors 1 LM317T adjustable regulator (REG1) 1 BC338 NPN transistor (Q1) 4 1N4004 diodes (D1-D4) up commands in some applications. We don’t need to do this in the present project, because it comes set up to do what we want by default – Capacitors 1 1000µF 16V RB electrolytic 1 100µF 16V RB electrolytic 1 100nF monolithic ceramic Resistors (0.25W 1%) 1 10kΩ 1 430Ω 3 1kΩ 1 240Ω Where To Get The EM-408 The EM-408 GPS Engine module is available in Australia from Altronics for $99.00 (Cat. K-1131). Another source for the EM-408 is SparkFun Electronics of Boulder, Colorado, USA. Their website is at www.sparkfun.com and payment can be made using most popular credit cards. At the time of writing, they were offering the EM-408 GPS module for US$64.95 plus US$3.40 for handling and shipping to Sydney (check prices to other cities). ie, it supplies the NMEA data stream at 4800bps and also supplies the $GPRMC sentence we need to extract the time. June 2009  37 4004 D1 1k 10k 1k 1k 100nF DB9F VR1 5k OPTIONAL LINK 430 1 5 GLOBALSAT EM-408 GPS RECEIVER MODULE (ATTACH WITH DOUBLE-SIDED TAPE) CON2 NMEA OUT SET 3.3V 1 V 3. 3 + (PATCH ANT) CON1 4004 D4 D3 4004 4004 1000 F 240 REG1 LM317T D2 12V DC 100 F This close-up view shows how the EM-408 is connected to the PC board via the 5-way interface cable supplied with the module – see text. Q1 BC338 19060170 9002 © REVIE CER E MIT SP G wired directly to the display board). The optional link shown just to the right of trimpot VR1 can also be left out, as it’s not needed for this particular project. Begin the assembly by installing the resistors, followed by trimpot VR1 and the 100nF monolithic capacitor. Table 1 shows the resistor colour codes but it’s also a good idea to check each one using a multimeter before installing it. Note the 10kΩ resistor that’s second to the left from transistor Q1 – be sure to install it in its correct location. The two electrolytic capacitors can now go in, taking care to fit them with the correct orientation. Follow these with the four diodes (D1-D4), then install transistor Q1 and regulator REG1. As shown, the latter is installed with its leads bent down at right angles about 6mm from its body, so that they go through their matching holes in the board. Secure REG1’s metal tab to the board using an M3 x 6mm screw and nut before soldering its leads. Don’t solder its leads first. If you do, the solder joints could be stressed as its tab is bolted down and this could lift (or crack) the board tracks. Check that the diodes, transistor Q1 and the regulator are all installed with the correct orientation. Fig.2: install the parts on the PC board as shown here. Note that CON1 (the DC socket) and CON2 (the DBF9 connector) are both left out if you intend installing this board in the same case as the display board (see Fig.3). This view shows the completed assembly. The GlobalSat EM-408 GPS module is attached using double-sided adhesive foam tape. The NMEA data stream emerges from the EM-408 at its Tx output (pin 4) and is then fed to a simple inverting buffer stage based on transistor Q1. The inverted signal appearing at Q1’s collector is then fed to the serial input of the display board (or to the serial port of a PC), either directly or via a DB9F socket and serial cable. Board assembly The assembly is straightforward with all parts, including the EM-408 GPS module, installed on a small PC board coded 07106091 and measuring 122 x 57mm. This board has cut-outs in each corner so that it can be housed in a standard UB3-size (130 x 68 x 44mm) utility box, if you want to build it as a separate unit. Fig.2 shows the assembly details. Note that there’s provision to mount both a 2.5mm DC input socket (CON1) and a DB9F socket (CON2) on the board. However, these are fitted only if you intend building an external unit. Leave these parts out if the module is to be mounted in the clock case (it’s Fitting the EM-408 The EM-408 GPS engine module Table 1: Resistor Colour Codes o o o o o No. 1 3 1 1 38  Silicon Chip Value 10kΩ 1kΩ 430Ω 240Ω 4-Band Code (1%) brown black orange brown brown black red brown yellow orange brown brown red yellow brown brown 5-Band Code (1%) brown black black red brown brown black black brown brown yellow orange black black brown red yellow black black brown siliconchip.com.au GPS TIME RECEIVER Fig.3: just three leads are required to connect the GPS driver board to the display board. Note that the connections are actually made at the back of the display board (not at the front, as shown here for clarity). © 2009 07106091 (5) 1 (2) +12V 4004 4004 4004 4004 (GPS DRIVER PC BOARD) 88 8 (MAIN DISPLAY BOARD) V21+ a b c a a f f b g e e c d dP d g e c e a d b g c f e e b f b d c d dP 19050140 9002 © TU ODAER E MIT SP G c d dP 1 (5) + (2) 4004 + +12V CON2 12V DC IN is next on the list. This is attached to the top of the PC board using a strip of double-sided adhesive foam tape and must be orientated as shown in Fig.2. However, before fitting it in place, you have to make the interconnections siliconchip.com.au between it and the PC board. As supplied, the EM-408 comes with a matching 5-way interface cable. This is about 25mm long and is fitted at each end with a mini 5-way SIL plug, one of which is plugged into a matching socket on the GPS module itself. For this application, you have to cut the cable in half and then use one half to make the connections between the module and the PC board. Remove about 4mm of insulation from the five June 2009  39 HOLES A ARE 3.0mm DIAMETER, COUNTERSUNK A A (UPPER REAR OF ENCLOSURE) SIDE OF LOWER SECTION OF ENCLOSURE 3.0 A 50 A 19 29 1.0 10 5.0 RADIUS ALL DIMENSIONS IN MILLIMETRES 98 94 (REAR) 2.5 RADIUS 5 15.5 15.25 20.5 15.5 SIDE OF UPPER (CLEAR) SECTION OF ENCLOSURE (FRONT) Fig.4: follow this drilling diagram to mount both the GPS driver module and the display board in the same case. The semi-circular notches along one edge of the lid and along the top of the base can be made using rat-tail files of the correct diameter. (Note: if you intend mounting just the display board in the case, using the drilling diagram published in Pt.1 last month). 40  Silicon Chip siliconchip.com.au leads and tin them before soldering them to the PC board. Be sure to feed the leads through the board holes in the correct sequence and note that the wire with the grey insulation goes into the uppermost hole (marked “1” on Fig.2). After they have all been soldered, plug the end of the cable into the matching socket on the end of the EM-408 module. This is done with the module orientated socket-end-down and roughly vertical with respect to the board. Take care to ensure that the plug and socket mate correctly – they’re very small and are polarised. Once the connection is made, fit the strip of double-sided adhesive foam to the underside of the EM-408. That done, remove the protective tape from the outer surface of the adhesive foam and carefully swing the module down so that it rests on the top of the PC board. During this process, be sure to leave a small amount of slack in the cable so that the plug isn’t pulled out of its socket. Once the module is in the correct position (see photo), press it down gently to ensure that the adhesive foam “grabs”. Finally, if you intend installing the board into a separate case, fit the DC socket (CON1) and the DBF9 socket (CON2). Conversely, leave these parts out if the module is going to be installed in the same case as the display board. That’s it – the module is now complete. then apply power from an external 12V DC source. If you’re not using the DC socket, simply connect the supply’s positive to D1’s anode. The negative lead goes to the outside copper earth pattern. Next, use your DMM (set to a suitable DC voltage range) to monitor the output from REG1 (eg, at its metal tab or at the lower end of any of the 1kΩ resistors). With VR1 set to mid-range, this should be close to 3.3V but may be either slightly lower or higher than this figure. Adjust VR1 to set the voltage from REG1 as close as possible to the correct 3.3V. The driver board assembly is now finished and can be fitted into either the clock display enclosure or a separate UB3 jiffy box. Setting up Drilling the case This simply involves adjusting trimpot VR1 to set the output of regulator REG1 to 3.3V to give the correct supply voltage for the EM-408 GPS module. To do this, first set VR1 to mid-range We’ll assume here that you want to fit the GPS driver module into the same case as the display board. If so, the first step is to connect a 200mm length of 3-way ribbon cable to the 30mm x M3 SCREW LED2 LED1 LED3 Fig.5: here’s how the two PC boards are mounted inside the case. (DISP1) 25mm UNTAPPED SPACER (DISP2) DISPLAY PC BOARD FILET OF EPOXY CEMENT TO ATTACH SPACER TO MOULDED PILLAR GPS RECEIVER MODULE PC BOARD ATTACHED TO REAR OF ENCLOSURE (UPPER CENTRE) VIA 4 x 10mm LONG CSK HEAD M3 SCREWS & 8 x NUTS DISPLAY BOARD SCREWS MATE WITH THREADED INSERTS IN PILLARS (LOWER PART OF ENCLOSURE) module’s external wiring points – see Fig.3. The other end of this cable goes to the main display board but leave this end disconnected for the time being. Having attached the cable, the driver module and its associated display board can be installed in the case. Fig.4 gives the case drilling details. As shown, four holes must be drilled in the base (towards the top) and these are used to mount the driver module. They should be all be countersunk (ie, on the outside of the case) using an over-size drill. In addition, you have to cut notches along the mating edges of the top and bottom halves of the enclosure (these provide access to the DC socket and the switches on the display board). These notches are best made using rat-tail files of the correct diameter, although it may be also possible to drill them if the two halves of the case are secured together. Note that all these holes are in quite different positions from those shown in Fig.4 last month (ie, for the into VIDEO/TV/RF? 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Two useful programs are GPS Clock from Time Tools (freeware) and GPS Time And Test from BrigSoft (shareware but free to try for 30 days). Download them from http://www.timetools.co.uk/atomic- “display only” enclosure). Note also that you don’t need to make a cutout to provide access to the DB9F socket in this version, since the driver board cable is wired directly to the back of the main board. Final assembly Fig.5 shows the final assembly details. The new GPS driver board assembly is attached to the base of the enclosure using four M3 x 10mm countersunk-head machine screws, with four M3 nuts used as short spacers and another four nuts used to hold the board in place. To allow plenty of “breathing space” between the driver board and the main display board (especially to provide some clear space above the EM-408’s patch antenna), in this version the main board is mounted much further forward than in the “display only” version. This is achieved by mounting 42  Silicon Chip Fig.7: “GPS Time And Test” synchronises your PC’s clock to the GPS signal at preset intervals down to as low as one minute. Both programs also show the incoming NMEA data sentences. clock/fw/gps-clock.htm and from http:// www.abstime.com Each does a slight different job. For example, GPS Clock (Fig.6) shows the UTC time, along with the date, your longitude and latitude and the NMEA sentences. By contrast, GPS Time And Test (see Fig.7) synchronises your PC’s existing clock to the correct local time (ie, the clock is locked to the GPS time signal it on 25mm-long untapped spacers. These sit on the existing moulded mounting pillars in the case and the assembly secured using M3 x 30mm machine screws. Unfortunately, it’s quite tricky to fit the main board into the enclosure with the 25mm spacers simply sitting on the moulded pillars. However, there is an easy way around this and that is to glue the spacers to the tops of the pillars using 5-minute epoxy cement – see Fig.5. This is done by first “clamping” each spacer to its pillar using a 30mm screw and flat washer. That done, you can apply a small “fillet” of epoxy around the bottom of each spacer to hold it in position. Leave the assembly for a few hours to allow the epoxy to set reasonably well before removing the screws and flat washers. Once the cement attaching the spacers has set, the three leads from but displays local time). Other information displayed includes the time before and after synchronisation plus the NMEA sentence containing both the time and the co-ordinates for latitude and longitude. You can also set the synchronisation interval and set various COM port properties. There’s a lot more GPS software (both freeware and shareware) out there on the Internet. Check it out for yourself. the GPS driver board can be connected to the main display board. Note that these leads should be soldered to the rear of the main board, rather than to the front of the board as shown (for the sake of clarity) in Fig.3. If you prefer, you can fit PC stakes to the three wiring points (from the copper side) to accept the lead terminations. The assembly can now be completed by securing the display board in position using the M3 x 30mm screws and then fitting the clear lid to the case. Applying power You will need a 12V DC 300mA plugpack to power the unit. When this is connected, the displays should light up immediately and initially show “0000”. After a short time (anywhere up to about 40s), the EM-408 GPS engine should begin sending the NMEA 0183 data sentences to the clock display. siliconchip.com.au This view shows the GPS driver board mounted in the base of the case. The four 25mm untapped spacers which support the display board are also shown glued to their respective integral mounting posts (see text). The DC power socket and pushbutton switches on the display board are accessed via holes in the side of the case (note: the DB9M connector is not necessary on the main board if mounting the GPS driver board internally). The unit will then display either UTC time, local standard time for eastern Australia or daylight saving time for Eastern Australia, as selected by switches S1-S3. If you live in a different time zone to eastern Australia, then it’s a simple matter to program in a different offset from UTC (the default offset is +10 siliconchip.com.au hours for eastern Australia). This is done by pressing buttons S4 (hours increment) and S5 (minutes increment), as described last month in Pt.1. The clock will then show the correct local time for your location. Note that any changes you make to the offset from UTC time are stored in the micro’s on-board memory and are retained even if the power is interrupted. Note also that if the power is interrupted, the clock will automatically start displaying the correct time within 30-40s when power is subsequently reapplied. It all depends how quickly the EM-408 GPS module begins receiving SC data from a GPS satellite. June 2009  43