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y h c n d e Anal s i n o r o gu eC lo c G P S Ge of -S f a r G s ’ m ha k w it h l o n g b at t e ry e f li This GPS Clock Driver converts an ordinary wall clock into a highly accurate timepiece that will keep exact time (within seconds) for up to eight years using a pair of C cells. You need not touch the clock during that period; it will automatically adjust for daylight saving by adding and subtracting an hour exactly when needed. This is a clock you can rely on to tell you the correct time. 18 Practical Electronics | September | 2023 I t is amazing how useful it is to have at least one highly accurate clock in the house. At a glance, you know the correct time without having to remember if that clock is running slow or fast and by how much. Most people would be happy with a wall clock that was accurate to the minute, but with this project, it will be accurate within a few seconds. Even better, any inaccuracy will not accumulate – the clock will remain that accurate for the life of its battery. Adjusting for daylight saving is an annoyance with traditional quartz wall clocks. Twice a year, it forces you to get up on a chair or step stool to take down the clock and adjust its hands. Our GPS Driver automatically makes those adjustments for you. At 2am on the day specified for the start of daylight saving, the clock will begin running fast until it has added the required hour. Then, at 3am on the day specified as the end of daylight saving, the clock will run slow or stop until it has returned to the non-­ daylight saving time. This is accomplished using a GPS module to get the precise time from the network of GPS satellites and some clever software to control the clock’s hands. We have published similar designs many years ago (the last was in PE, February 2018), but they all had a relatively short battery life. By using ultra-low-power components and some extra tricks in the firmware, this design will run for about two years on a pair of AA cells and up to eight years with C cells. It will work with most wall clocks on the market. All that is needed is a modification to connect wires to the stepper motor in the clock’s movement. Luckily, that is usually easy. Stepping clocks There are two types of analogue wall clocks: stepping clocks, where the second hand steps once a second, and sweep clocks, where the second hand moves smoothly around the dial. Stepping clocks are more common than sweep types. They have a Lavet-type stepping motor consisting of a small magnet that rotates between a coil’s magnetic poles. The clock driver delivers alternating positive and negative pulses to this coil, and the rotor rotates 180° with each pulse. These pulses are about 40ms in duration, and one is delivered per second (as shown in Scope 1), causing the second hand to advance once per second. Stepping clocks vary considerably in quality and price. We purchased Practical Electronics | September | 2023 Scope 1: the output of the GPS Clock Driver for a stepping movement consists of alternating positive and negative pulses that drive the rotor in the clock’s motor to make a 180° step with each pulse. Each pulse is about 40ms in duration, and they are delivered once per second. Scope 2: the output driving a sweep movement; a continuous stream of positive and negative pulses at 8Hz. Each pulse is 31.25ms long with 31.25ms between pulses, resulting in 16 pulses per second. At low battery voltages, the clock driver lengthens the pulse time by 24% and reduces the idle time by the same amount, delivering more energy to the clock’s motor. an example for testing from a budget store for a couple of quid, and while it was not the best, it was also not the worst clock movement. Its accuracy was terrible but, as we are replacing its driving circuit with our own, that doesn’t matter. Typically, stepping clock movements have a coil resistance between 200W and 500W, with a higher resistance indicating a longer battery life (the budget special was 375W). Sweep clocks The alternative sweep clock movements, sometimes called ‘silent’ or ‘continuous’ movements, have a similar drive motor except that it is driven by a continuous stream of positive and negative pulses at 8Hz, as shown in Scope 2. This continuously spins the rotor, with its momentum keeping it moving between each pulse, so it does not make individual steps like the stepping type movement. As a result, the second hand moves continuously (sweeps) around the dial, and the clock is near silent. This contrasts with the stepping types, which make an audible tick sound every second. Each ‘sweep’ pulse is 31.25ms in duration with a dwell time of 31.25ms between pulses, resulting in 16 pulses per second. Because the motor is drawing current 50% of the time, you would expect the battery to be flattened in no time compared to a stepping clock. Sweep movements avoid this by using a coil with many more turns and a higher resistance (typically 5kW). 19 Einstein’s theory of relativity and GPS accuracy GPS satellites circle the Earth at an altitude of 20,000km and are used at ‘trilateration’ locations using precise onboard clocks. In their high-altitude orbits, the clocks experience a weaker gravitational field, so spacetime is warped differently for them compared to clocks on Earth. The effect is that the clocks speed up at a rate of 45μs/day. The satellites are also whizzing around at pretty high speeds (about 14,000km/h), and the time dilation predicted by Einstein’s special theory amounts to slowing the clocks by 7μs/day. Together, these effects amount to a net speeding up of 38μs/day. That doesn’t sound like much, but ignoring it would lead to a vast inaccuracy in the global positioning system within a few hours. Light travels over 10km in 38μs, and that sort of error in position per day would not make for accurate navigation. The solution is to slow the satellite clocks by a precise amount calculated using Einstein’s theory of relativity so that they match time measured on the Earth’s surface. This allows the system to work to accuracies of metres rather than kilometres. Edited excerpt from ‘Why does E=mc2’ by Brian Cox and Jeff Forshaw, ISBN 978-0-306-81758-8 Sweep clocks are more expensive, typically £30 to £80 (and up). However, we found an excellent example at IKEA (called ‘TJALLA’) for just £9, and it performed pretty well, rivalling a genuine Seiko sweep movement that we purchased for around £17.50. The only problem with the IKEA movement was that it was difficult to pull apart to modify– and even harder to reassemble. Keeping perfect time When the clock is running, the GPS Clock Driver will need to occasionally add or subtract a second to keep the hands accurate. This is easy for a stepping movement; the Driver delivers two pulses in one second to advance the clock by one second, or no pulses for a second to retard it by one second. With daylight saving, this is more noticeable. When daylight saving starts, the hands need to advance by one hour and to do this, the Driver generates two steps every second for an hour until the hands have reached the correct daylight saving time. At the end of daylight saving, the clock will stop stepping for an hour until the time catches up with the position of the hands. Sweep movements need a different approach because we must maintain the momentum of the spinning rotor; it cannot simply go twice as fast or stop/ start. So, the adjustment must be more subtle. To add or subtract a second, the movement is run 12.5% faster or slower for eight seconds. With daylight saving, this means that it will take eight hours to add the required hour and a similar time to retard by an hour. While this is a long time for the clock to be catching up, it only happens twice a year. Instead, you could disable daylight saving in the setup and manually adjust the hands when required. How the Clock Driver works Fig.1 shows the GPS Clock Driver block diagram. Microcontroller IC1 generates a sequence of positive and negative pulses that are buffered by op amp IC2. IC2 drives the motor in the clock movement. A crystal oscillator running at 32768Hz (215) drives a 16-bit counter/ timer in IC1 to generate the precise timing required. Importantly, this timer can operate while the microcontroller’s core is in sleep mode, so it only consumes a few microamps. The microcontroller spends most of its time in this low-power sleep mode. When it is time to generate an output pulse, the timer wakes the CPU to drive the output pin to start the pulse, and it resets the timer to wake again when the pulse is due to finish. When it wakes again, it terminates the output pulse, sets the timer for the next pulse and goes back to sleep. This continues forever, with the microcontroller jumping in and out of sleep and toggling the output pin to generate the pulse train for the clock’s motor. The CPU’s running time is short compared to the sleep time, so the average current drawn by the micro is very low. The sequence of pulses to the clock’s motor alternate between positive and negative, with a dwell time in between. This is achieved by switching the pin between high, low and high-­impedance. Op amp IC2 buffers this signal to drive the clock by bringing its output to the positive terminal of the upper cell or the negative terminal of the lower cell, or the junction for the dwell time between pulses. This divides the load between the cells, with each providing half the power for the clock motor. GPS synchronisation Not shown in Fig.1 is the boost voltage regulator that powers the GPS module. Occasionally, after delivering a pulse to the motor, the firmware will not put the CPU to sleep but will keep running and enable the boost regulator, which delivers a regulated 4V to the GPS module. It will then get an accurate time from the constellation of GPS satellites. Generally, it takes less than a minute for the GPS module to locate sufficient satellites and return the precise time. When the microcontroller has received this time, it shuts down the regulator Fig.1: the GPS Clock Driver uses a crystal oscillator running at 32768Hz and a 16-bit counter/ timer within microcontroller IC1 to generate the precise timing required to drive the clock motor. IC1 generates a sequence of positive and negative pulses that are buffered by op amp IC2 to drive the clock movement motor. IC1 spends most of its time in sleep mode to extend battery life. 20 Practical Electronics | September | 2023 and makes some calculations to determine any timekeeping errors. After this, it reverts to its regular strategy of sleeping until the next pulse is due. Initially, the time between GPS synchronisations is set to 12 hours, but over time the firmware will increase this to five days. The average battery power required for GPS synchronisation is minimal, so this process does not materially affect the battery life. The firmware keeps track of the position of the clock’s hands as the number of seconds since 1 January 2000. The GPS time is also converted to this format, so it is easy for the firmware to compare the two and calculate any correction that may be required. The difference between the two numbers represents the error in the 32768Hz crystal oscillator, which is used to keep the time between GPS synchronisations. By working out this error, the firmware can correct for it over the next period between GPS synchronisations by occasionally adding or skipping a second as needed. This will start working following the second GPS synchronisation and will keep the clock accurate regardless of any error in the crystal, including compensating for additional errors due to temperature and ageing of the crystal. The practical effect is that, apart from the first day, the clock’s hands will always be accurate within a few seconds between GPS synchronisations. Also, the next GPS synchronisation should not need a large correction; maybe only a second or two (or possibly none). When the boost regulator and the GPS module are initially powered, they can draw a lot of current, especially if the cell voltages are low. This cannot be sustained by a battery on its last legs, so the firmware measures the battery voltage when running the boost regulator. If it is below 2.25V (1.125V per cell), it will skip any subsequent GPS synchronisations. This will have little effect on the clock’s accuracy as it will only occur towards the end of the battery’s life, and by then, the firmware will have a good idea of any error in the crystal and will continue to compensate for it. Circuit details The full circuit, shown in Fig.2, is based around a Microchip PIC16LF1455 microcontroller. It is an extra-lowpower device that can operate with a supply voltage as low as 1.8V (0.9V per cell in this case). Most clock movements will stop running between 0.9V and 1.0V per cell, so the microcontroller will run for as long as the clock’s motor can keep going. Practical Electronics | September | 2023 This microcontroller also has USB support, so a mini Type-B socket is provided for configuration (CON4). When a host is connected or removed, the microcontroller will detect the USB +5V voltage on its pin 9. The 5V is dropped to 2V by the 10kW/6.8kW resistive divider, so it will not damage the microcontroller when the battery is at 1.8V. It will still be recognised as a high logic level when the microcontroller runs from a fresh battery (3.2V). Any change in the voltage on pin 9 will cause the microcontroller to restart. If, upon restarting, the USB voltage is present, the firmware will set the microcontroller’s clock speed to 16MHz and enable the USB interface. LED1 will flash three times to indicate that the firmware is in configuration mode. If the USB voltage is not detected on startup, the clock speed will be set to 4MHz, and the USB controller will be disabled (both to save power). The firmware will go through the usual clock startup routine, flashing LED1 twice. The PIC16LF1455 has an unusual feature: it can use the host’s USB signalling rate to fine-tune its internal clock. The USB specification requires a high accuracy in this timing and that generally requires a 12MHz, or similar, crystal oscillator. But the PIC16LF1455 does not need this, which frees up two pins and makes for an easy-to-implement USB interface. The microcontroller steps the clock movement by driving its pin 8 high for a negative pulse, low for a positive pulse or setting it to high impedance for the idle time between pulses. This controls op amp IC2 (MCP6041), which uses the centre point of the two cells as its reference and drives its output either positive or negative relative to that. The MCP6041 has several desirable characteristics: its output will swing rail-to-rail, which means that little of the precious battery voltage is lost within the op amp. It also has an extremely low quiescent current (less than a microamp), so the battery is conserved between pulses, and it will operate at a supply voltage well below 1.8V (0.9V per cell). Boost regulator When the microcontroller needs to get the GPS time, it drives its pin 7 high, enabling boost regulator IC3, The GPS Clock Driver on the back of an IKEA ‘TJALLA’ sweep clock. The movement has been modified to bring the connection to the clock motor’s coil out through a hole. The Driver PCB was designed to be small as there is often little space behind a wall clock. 21 GPS Analogue Clock Driver Fig.2: the Microchip PIC16LF1455 microcontroller (IC1) runs the show. It steps the clock movement by driving its pin 8 high for a negative pulse, low for a positive pulse or setting it to high impedance during the idle time between pulses. Op amp IC2 buffers this signal and uses the centre point of the two batteries as its reference to drive its output either positive or negative. When the microcontroller needs to get the GPS time, it drives its pin 7 high, causing the boost regulator (IC3) to start running and power the GPS module. a Microchip MCP16251. It generates about 4V at its pin 5. This is set by the ratio of the 2.2MW and 1MW resistors; 4V was chosen so that the regulator will have some headroom to regulate the output voltage with fresh cells. The MCP16251 disconnects its output when it is disabled by a low voltage on its pin 3. This is unusual in a boost regulator, and is an important characteristic as it prevents the GPS module from draining the battery when it is not being used. The output from the GPS module (VK2828U7G5LF) is a standard asynchronous serial stream at 9600 baud with TTL signalling voltages. To protect the microcontroller when the battery voltage is low, BAT85 diode D1 clips its output to just a little over the battery voltage. The module comes with a connector and colour-coded flying leads, as shown in Fig.2. It also has two indicator LEDs; the red LED, which indicates power, while the green LED will flash at one pulse per second. Battery life The main factors determining how long the batteries will last are the 22 current drawn by the clock’s motor and the quality of the cells used. Our budget stepping clock drew an average of 170µA while the IKEA sweep clock averaged 135µA (both with a drive signal of 1.5V peak-topeak), typical of these types of movements. Because the GPS Clock Driver powers the motor from both cells, the typical average current drawn from each is 70-85µA. The average current drawn by the microcontroller is about 18µA, which applies to both cells. The shutdown current of the boost regulator and a few other sources add about another 3µA per cell. Finally, there is the current consumed by the periodic operation of the GPS module. The peak current is up to 100mA, but it is only drawn for a short period every five days, so its long-term average is quite low at about 5µA. Adding all of this together means that a typical clock will draw about 100µA from each cell. To keep the clock running for longer on low battery voltages, the firmware changes the pulse train duty cycle if the battery voltage is less than 1.125V per cell. It lengthens the pulse time by 24% and reduces the idle period by the same amount. The waveform’s frequency is the same, so it does not affect the timekeeping accuracy, but it delivers more energy to prevent it from stalling. This allows a sweep clock to continue operating below 1V per cell, thereby using the last drop of energy in the cell and lengthening the running time. The effect on a stepping clock is not as significant, but most will last until 1V is reached. By the way, if you are testing the minimum running voltage for your clock, you need to mount it in a vertical position. The effort required to raise the second hand against gravity will cause the clock to stop early compared to if it is mounted horizontally. Also, if you are not concerned with having a second hand, you can remove it, and the clock should run for a few weeks longer because it does not have to put in that additional effort. Good-quality alkaline AA cells have a capacity of 2000mAh or more with light loads (terminating at 1.0V) so, with a total current draw of Practical Electronics | September | 2023 100µA, you could reasonably expect the battery to last about two years. Obviously, this can vary considerably depending on the quality of the movement and the cells, but it is a reasonable estimate. If there is room behind the clock, you could separately mount two C cells which have a capacity about four times that of the AA cell, so you could expect up to eight years of operation (see below). The limiting factor would be the quality of the cells and their rate of internal self-discharge. Programming and sourcing the parts You will need to program the microcontroller yourself. There are six solder pads on the PCB for mounting a pin header. This enables you to program the chip in-circuit – you just install the header and connect a PIC programmer such as a PICkit 3 or PICkit 4. The firmware is available from the September 2023 page of the PE website at: https://bit.ly/pe-downloads You can also access the firmware from: http://geoffg.net/gpsclockdriver.html It is worth checking http://geoffg.net for updates from time to time, as there is the possibility that a bug will be found and fixed. Besides the PCB and microcontroller, the other project components are standard and can be purchased from the usual suppliers. Do not substitute the BAT85 diode with another type. It is a schottky type for a low voltage drop, but it also has a low reverse leakage, which is needed to extend the battery life. We have specified the V.KEL VK2828U7G5LF GPS receiver, a great performer that is readily available at a good price. If you want to use another module, that will probably be OK. Just make sure it uses TTL signalling and not RS-232 levels. The project’s firmware will automatically try the typical communication speeds used by these modules (4800, 9600 or 19,200 baud). It uses the NMEA RMC message generated by the GPS module, which is standard across all manufacturers. When purchasing the clock, you could choose a clock design that is attractive but swap out the movement for something else. Most highend clock manufacturers have standardised the physical dimensions of the clock movement and its mounting arrangement. However, this does not apply to cheap clocks, which do not follow any standard. You can also buy movements online with a wide variety of matching hands. So, making your own clock with a unique clock face is also an option. Practical Electronics | September | 2023 Construction The GPS Clock Driver is built on a 97 × 55.5mm PCB, coded 19109221. It is available from the PE PCB Service and is shown in Fig.3. It was kept small as there is often little space behind a wall clock. You can cut off the end section of the Driver PCB containing two AA cell holders if you want to use separately mounted batteries. That results in a 64.5 × 55mm PCB that should fit almost anywhere. If cutting the board, do that before fitting any components. Use a metal ruler and a sharp craft knife to deeply score the PCB on both sides deeply, then snap the board apart and tidy up the edge with a file. The first component to solder is IC3, the MCP16251 in a 6-pin SOT23 package. It is quite small but not overly difficult with a steady hand. First, determine its orientation. It has a laser-etched dot on the top near pin 1, but it is faint, so it is easier to read the four letters engraved on the chip and use them for orientation. You will need a x10 or more magnifier to read these letters (some smartphone cameras will do it too). The first two should be ‘MB’, while the last two can be anything. Pin 1 is at lower left with the letters the right way up. To solder the chip, first coat the PCB pads with flux paste, then place a tiny solder bump on a corner pad. Position the chip and, while holding it down, apply the iron to that pad. With the first pin tack-soldered and the chip held in position, check and adjust the orientation of the other pins before soldering them. Always apply plenty of flux and use minimal solder on your iron. Next, fit the USB connector. This has two small plastic posts on the underside that go into two holes in the PCB to position it. Coat the pins and PCB pads with flux gel and, with a small amount of solder on your iron’s tip, slide it across the PCB pad to the connector’s pins. When the tip of the iron hits the pin, the solder should magically flow around it. With these small devices, it is easy to create solder bridges between the pins, but they can be removed using solder wick (braid). Finally, check all joints with a powerful magnifier (x10 or x20) to ensure that each joint is correctly soldered with no bridges. Don’t forget to solder the larger mounting tabs. The inductor is the last SMD. Start by placing a small solder bump on one PCB pad, and then, while holding the inductor in place, apply heat to that pad. That should secure it in place. Then, use rosin-cored solder wire to solder the other lug before refreshing the first solder joint. The remaining components are all through-hole types. Start with the low-profile items like resistors before moving on to higher-profile components such as the LED and cell holders. You can use IC sockets for IC1 and IC2 as these will make removing the device easy if you suspect it is faulty. A clock using separately mounted C cells for power. C cells have a capacity about four times that of AAs, so a lifetime of up to eight years is possible. However, that will depend on the cells’ quality and their internal selfdischarge rate. The PCB is much smaller without the onboard cell holders. 23 Fig.3: assembly of the GPS Clock Driver is pretty straightforward. Start by soldering the three SMDs (IC3, L1 and CON4) and check carefully that they all have good solder joints before fitting the through-hole parts. The cell holder polarity is critical, while the LED needs to have its longer anode lead inserted into the pad labelled ‘+’. The ICs and diode also need to be oriented correctly. Like the ICs, LED1 and D1 are polarised, so they must be oriented as shown in Fig.3. The GPS module can be secured to the PCB using double-sided adhesive foam tape. The ceramic antenna should be on top, with the module’s metal shield and label against the PCB. Typically, the antenna should be horizontal and facing the sky for the best sensitivity. If you have the space, you could separately mount the module with the antenna in this orientation. However, our tests showed that the module worked just as well when pointing to the horizon, mounted on the PCB and attached to the back of the clock. The GPS module is supplied with a connector and colour coded-leads which go to the solder pads on the right-hand side of the PCB. Trim the leads to length and solder them to the respective pads: ‘WH’ means white, ‘RE’ red, ‘BU’ blue and so on. If using external cells, wire them to the four ‘EXT BAT’ solder pads. These can be used for terminating soldered leads or a 0.1-inch 4-pin header and socket. Modifying the clock movement The idea is to disconnect the clock’s stepping motor coil from its control board and connect two flying leads to the coil. All clock movements are different, so we can only give you general guidance here. The process involves freeing the clock’s movement from the clock housing, dismantling it, making the modification and reassembling it. First, remove the housing holding the front glass of the clock. Generally, this is held in place with screws accessible from the back. Then remove the hands. Generally, the second hand is a friction-fit on a pin in the centre of the shaft, so a gentle pull on this should free it. Next is the minute hand; in most high-end clocks, it is held down with a circular threaded nut. However, in cheaper clocks, it is often a friction fit The fully populated Driver PCB. The tactile switch for adjusting the second hand is near the top edge, alongside the USB connector for configuring the firmware. On the far top right are the inductor and other components associated with the boost regulator that provides 4V for the GPS module. 24 on the minute hand shaft. The hour hand is likely a friction fit on the hour hand shaft and should be gently pulled free. With the hands removed, you will find that the movement is held onto the clock face with a hex nut on the threaded shaft. Remove the nut and it should come free. Some cheaper clocks do not use a securing nut; instead, the movement is held in place by plastic clips on the rear of the clock. Take photographs of the movement and the layout of the gears before you start dismantling it, then take additional photos as you progress. It is very easy for the gears to fall out while you are handling the movement, and it will then be tough to reassemble it without a guide. In most cases, the movement will have a top cover held on by clips to the base. You can lever off these clips to remove the cover and gain access to the motor and gears. Inside, you need to identify the motor’s coil (this will be obvious) and the wires from the coil, which will be soldered to the PCB with the control chip (normally under a blob of black epoxy). The wires are very fine, so the best method of disconnecting the control chip is to cut one of the tracks leading from the coil’s termination on the control PCB. You can then solder your flying leads to the coil’s termination points and feed these out of the movement – you will probably need to drill a hole in the top cover to do this. Finally, reassemble the clock and terminate the flying leads on a 2.54mm-pitch 2-pin crimp plug. If you have a stepping movement, you can test your work by connecting a 1.5V AA cell across the leads and reversing it. Every time you reverse the cell, the clock should step by one second. Practical Electronics | September | 2023 Configuring the Clock Driver By default, the Clock Driver is set up for a stepping-type movement with no daylight saving compensation. If that is all you need, you can just insert the cells and start the clock running (see ‘Powering it up’ below). Otherwise, you will need to configure the Driver. Plug the USB connector into a computer or laptop and insert the cells. The Clock Driver will connect to your computer as an asynchronous serial port over USB, and the LED will flash three times to indicate that the firmware is working in configuration mode. Ensure that fresh cells are installed; partially exhausted cells may not be able to deliver the correct USB signal levels, causing errors. The Driver imitates the Microchip MCP2200 USB/serial converter. Windows 10 and 11 are delivered with the correct driver installed, but for other operating systems, you may need to load a driver. You can find this on the Microchip website: www.microchip. com/wwwproducts/en/MCP2200 You will also need terminal emulator software to send your keystrokes to the clock driver and display anything sent back. For Windows, the sotware we recommended Tera Term is: http://tera-term.en.lo4d.com This is free to download and use. PuTTy is another popular emulator that will also work. The terminal emulator needs to know the number of the virtual serial port generated when the clock is connected. For Windows, you can find it using Device Manager. Other details such as the baud rate are unimportant and can be ignored. With everything set up, hit the Enter key on your keyboard, and you should see the configuration header as in Screen 1. The first question asked by the firmware is ‘Sweep (Y/N)’. If you type ‘Y’ then Enter, you will configure the clock driver for a sweep movement. If you enter ‘N’ instead, it will be configured for a stepping clock movement. The next question is ‘Use daylight saving (Y/N)’, and if you reply ‘N’, you do not have to do anything else; it will save the settings and you will be prompted to unplug the USB cable. If you replied ‘Y’, you will need to enter the specifications for the start and end of daylight saving. Configuring daylight saving The firmware can cope with the daylight saving requirements for most countries worldwide, although some are just too complicated or vague (for example, Iran’s). Table 1 shows the Practical Electronics | September | 2023 Parts List – New GPS-Synchronised Clock 1 double-sided PCB coded 19109221, 97 × 55.5mm available from the PE PCB Service 1 V.KEL VK2828U7G5LF GPS module or similar (MOD1) 1 32768Hz watch crystal (X1) 1 4.7μH 4.3A 6×6mm ferrite-cored SMD inductor (L1) [eg, EPCOS B82464-A4] 1 4-pin low-profile tactile pushbutton switch (S1) [Altronics S1120] 1 2-way 2.54mm polarised right-angle header with plug and pins (CON1) 1 SMD mini type-B USB socket (CON4) [Altronics P1308] 1 6-pin header (CON5; optional) 2 PCB-mounting single AA cell holders (BAT1, BAT2) [Altronics S5029] 1 14-pin DIL IC socket (optional) 1 8-pin DIL IC socket (optional) Semiconductors 1 PIC16LF1455-I/P microcontroller programmed with 1910922A.HEX, DIP-14 (IC1) 1 MCP6041-I/P 600nA rail-to-rail input/output op amp, DIP-8 (IC2) 1 MCP16251T-I/CH DC-DC boost converter with disconnect, SOT-23-6 (IC3) 1 5mm red LED (LED1) 1 BAT85 30V 200mA schottky diode (D1) Capacitors 2 10μF 16V X7R multi-layer radial ceramic [eg, TDK FK26X7R1C106M] 1 100nF 50V X7R multi-layer radial ceramic 2 22pF 50V C0G/NP0 radial ceramic Resistors (all 1/4W 5% or better) 1 2.2MW 2 1MW 1 820kW 3 10kW 1 6.8kW 1 1kW settings required for Australia and New Zealand. For both the start and the end of daylight saving, you will need to enter three numbers: 1) The month when daylight saving starts/ends (1 to 12 – where 1 represents January). 2) The day of the week when daylight saving starts/ends (with Sunday being day 1). 3) The week of the month it falls in, with 1 being the first week and 4 meaning the last week. Then you will be asked for your time zone. This should be entered as the number of hours before or after UTC. So, for example, Sydney and Melbourne are +10, Adelaide is +9.5 and Los Angeles is –7. When you press Enter after that, you will see ‘Configuration Saved, Disconnect USB’. Disconnect the USB cable and the clock driver will restart as if the battery has just been connected (ie, it will wait for the next precise half/full hour then start running, as described below). It will remember the settings you have entered, so you never have to re-enter them, even when replacing the cells. Daylight saving starts at 2:00am and ends at 3:00am. The one exception is the United Kingdom, where it needs to start/end one hour earlier. The firmware determines if the clock is running in the UK by checking the time zone offset, which is zero in the UK. Powering it up All you need to do is set the hands to the next half hour or full hour (whichever is closest) and insert the cells, then hang the clock back on the wall. The clock will wait until the next half/full hour is reached and automatically start running. From then on, it will keep precise time until the battery is exhausted. Do not put cells into the clock’s movement. The GPS Clock Driver wholly replaces the controller board inside the movement, so it does not need to be powered. The onboard LED informs you of the progress during the startup process. When the cells are inserted, the LED flashes twice to indicate that the microcontroller and firmware are running. The firmware then powers up the GPS module, flashing the LED briefly at 1Hz while it is searching for satellites. When the GPS module has a lock (ie, it has the accurate time), the LED will change to a long flash every second. Finally, when the clock starts running, the LED will turn off. With a new GPS module, it can take some time (up to 45 minutes) to find enough satellites. That delay might result in the clock starting at the wrong time. So, when you first use the clock, keep an eye on when it gets a satellite lock and readjust the hands if necessary. Once the GPS module has its first lock on the satellites, it is generally much faster, with 25 GPS Clock Driver v1.0 Sweep Clock (Y/N) ? Y Use Daylight Saving (Y/N) ? Y Start Daylight Saving Month (1-12) ? 10 Start Daylight Saving Day (1=Sun) ? 1 Start Daylight Saving Day in Month (1 to 4=Last) ? 1 End Daylight Saving Month (1-12) ? 4 End Daylight Saving Day (1=Sun) ? 1 End Daylight Saving Day in Month (1 to 4=Last) ? 1 Time Zone (-12.5 to +12.5) ? +10 Configuration Saved Unplug USB ❚ Screen 1: configuring the clock driver using the USB interface. In this case, sweep clock drive has been selected and daylight saving has been configured to suit NSW/Vic/Tas/ACT. These settings are remembered, so you never have to reenter the configuration details, even when replacing the battery. subsequent attempts typically taking under a minute. Adjusting the second hand All clock movements allow you to adjust the hour and minute hands, but the second hand will probably not be at the 12 o’clock position and will be stuck somewhere around the dial. To correct this, you can hold down the tactile switch on the PCB while the clock is waiting to start, and the firmware will drive the second hand around the dial. Release it when it reaches the 12 o’clock position. That way, the clock will start with the second hand indicating the correct second. A problem with some movements is that when the clock starts running, the movement might start driving the hands a few seconds early or late. While not a big deal, you can adjust for even this slight error while the clock is running. Hold down the tactile switch when the clock is running until the LED illuminates. If you then immediately release the button, the firmware will advance by one second. On the other hand, if you keep holding down the button until the LED goes off again before releasing it, the firmware will retard the hands by one second. Remember that a sweep clock will need eight seconds to gain or retard its hands by one second. So, if using a sweep movement, you should wait for a while to check the effect of the last adjustment before making another one. You can verify your clock is accurate using a time source such as www.time.gov which will give you the exact time to the second – even compensating for delays over the Internet. With this as your reference, you can use the tactile switch 26 to bring the second hand to an exact agreement with this source and compensate for any starting error. You should correct for any startup error immediately after the clock has started running. This is so that you do not inadvertently adjust for an error in the crystal’s frequency, which will be automatically corrected by the firmware after the first 12 hours of running, following the second GPS synchronisation. All clock movements use a type of stepping motor that is locked to the pulse train delivered by the microcontroller. So, once the hands are accurately set, they will never lose or gain a second unless the battery is exhausted or the movement is faulty. Therefore, in the normal scheme of things, you should never have to adjust the clock again after compensating for any initial startup error. Troubleshooting To test your clock, insert the cells and observe the LED sequence as described above. Hopefully, it will run through the starting sequence, and the clock will start running. If it does not work as expected, use the LED to help track down the problem. The LED should flash twice when the cells are inserted (and the USB is not connected). If that does not occur, the fault could lie with the cells, the microcontroller or the LED. Check that the LED is the right way around and that it works before looking for other causes. If you do not see the double-flash, check the voltage between pins 1 and 14 of the microcontroller. It should be the same as the battery voltage (3.2V with new cells). If that is OK, check the microcontroller. Is its orientation correct? Has it been properly programmed? If you used an IC socket, check that it is properly inserted, with no pins folded underneath. After the double flash, the firmware will power up the GPS module. Within a few seconds, you should see a brief flash every second on the LED indicating that data is being received from the module. If you do not see this flash, the problem could be with the boost voltage regulator or the GPS module. Check the voltage between ground and the red wire to the GPS (marked RE on the PCB). It should be about 4V; anything else indicates a problem with the regulator and its associated components. If the regulator is OK, the fault must be with the GPS module. Check that it is connected correctly and that it uses one of the supported serial communication speeds (4800, 9600 or 19,200 baud). GPS satellite lock After a while, the GPS module will get a lock on sufficient satellites to obtain an accurate time. When that happens, the boost regulator will shut down and the LED will change to a long flash every second. Usually this will be within a minute or two, but it could take some time. There might not be a strong enough signal to get a lock. Take the clock outside and place it so that the antenna is pointing directly at the sky, and leave it that way for at least an hour. Typically, if the GPS module could gain a lock when you inserted the Table 1 – DST rules for AU and NZ (not observed in Qld, NT and WA) NSW, Vic, Tas and ACT South Australia New Zealand Start month 10 10 9 Start day 1 1 1 Start day in month 1 1 4 End month 4 4 4 End day 1 1 1 End day in month 1 1 1 Time zone offset +10 +9.5 +12 Practical Electronics | September | 2023 cells, it should be able to get a lock on subsequent synchronisations. However, a marginal signal level or moving the clock might change that. When the cells are inserted, the firmware will wait forever to get a GPS signal. However, after that first time, the firmware will wait for just 30 minutes to get a signal and then, if unsuccessful, it will give up and retry in 24 hours. To indicate this, the LED will then flash briefly every second until a subsequent attempt is successful and an accurate time is obtained. If you find that your clock is inaccurate, check the LED. If it is flashing, that indicates there was an insufficient GPS signal to get the accurate time. If you find that you are getting a short battery life, check the voltage of the exhausted batteries when you replace them. Most movements will keep going down to 1.0V. If it stops at a voltage significantly higher than that (say 1.2V), the movement has too much friction and should be replaced. We experienced this with a cheap movement that failed after a few years, so it might be prudent to purchase a spare movement (or clock) as a backup in case you need to swap out an old movement. That way, you The inside of a typical wall clock movement modified for our GPS Clock Driver. The motor coil is at upper right while the blue control board is on the left, with a blob of black epoxy hiding the control chip. This has been disabled by cutting a PCB trace, and flying leads have been soldered to the motor coil termination points. are guaranteed a replacement that will fit your clock and accept the same hands. 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The why and how to build PIC-based projects Teach In 8 Cover.indd 1 04/04/2017 12:24 PRICE £8.99 Includes P&P to UK if ordered direct from us SOFTWARE The CD-ROM contains the software for both the Teach-In 8 and PICkit 3 series. ORDER YOUR COPY TODAY! JUST CALL 01202 880299 OR VISIT www.epemag.com Practical Electronics | September | 2023 27