PICAXE as timer for GPS Bottle

[Robin Lovelock]

Hi Folks. I expect this thread will be a lot shorter than my old one for the robot boat work - still active. That's http://www.picaxeforum.co.uk/showthread.php?t=13039

This is for something almost completely different - I'm thinking of using a PICAXE as a low power electronic timer, for some GPS Bottles I'm putting together for the BBC. Background is on www.gpss.co.uk/bottle.htm

In brief, several GPS bottles will be thrown into the sea off the south coast of UK in early July, and will be tracked on a web site. This is similar to what we did several years ago (that bottle page above) but with more recent GPS tracker products, and hopefully a lower cost and simpler design.

Each glass bottle will include a GPS tracker that consumes about 50mA current (fluctuates between 40mA and 60mA max) from a NiMHd battery pack, probably of about 12v. The battery is trickle charged from a small solar panel, all inside the bottle. The solar panel will only provide 100mA to 200mA when the sun is bright, so the average power consumed needs to be a lot less than the trackers 50mA if the bottle is to continue working for many weeks, months, or indefinitely.

I'm thinking of putting a timer switch between battery and GPS tracker, as we did years ago, but this time based on a PICAXE board. The software should be pretty simple, since I did similar things before for the robot boat, but I'm wondering if I can get away with switching the 50mA (60mA max) directly from a PICAXE pin, instead of adding a transistor or chip.

Also, I'll need to check what maximum voltage I can put into the PICAXE, or add stuff to give it the required voltage from my 12v battery.

Some of this will be in your documentation, which I'll go through soon, but I'm guessing you may have some good ideas to reduce my work here. Aim is to have the simplest possible electronic switch, that can switch on the GPS tracker for perhaps 2 minutes every 20 minutes, thereby drastically reducing the average power consumed and allowing small solar panels and batteries inside the bottle.

Best Wishes to all
Robin
www.gpss.co.uk

 

[srnet]

Well NiMh are not a good choice for trickle charging, here is some information from Wiki;

Trickle charging

Some equipment manufacturers consider that NiMH cells can be safely charged in simple fixed, low-current chargers with or without timers, and that permanent overcharging is permissible with currents up to 0.1 C (where C is the current equivalent to the capacity of the battery divided by one hour). According to the Panasonic NiMH charging manual, extensive trickle charging can cause battery deterioration due to overcharging, and it is the least preferred charging method concerning battery performance. If it is used, the trickle charge rate should be limited to between 0.033 C and 0.05 C for a maximum of 20 hours to avoid damaging the batteries.[9]

 

[srnet]

I'm wondering if I can get away with switching the 50mA (60mA max) directly from a PICAXE pin, instead of adding a transistor or chip.

Maximum O/P curent of a PIC pin is normally assumed to be 25ma, so I would be using a transistor, if its the power you are switching.

But then some GPS modules have a shutdown pin, so you dont have to switch the power. The MediaTek MT3329 for instance, has a rated standby current of 15uA.

 

[MartinM57]

Sounds like the "tracker" is a 12V unit? So "using a transistor" needs a bit of care and attention. High side switch driving with PICAXE voltages vs low side switching that creates a false ground for the tracker blah blah blah

More details needed Robin....

 

[srnet]

Sounds like the "tracker" is a 12V unit?

Fair point.

Indeed more details ........

In the Modeling world, there is quite a bit of enthusiasm for LiFeP04 batteries, long lasting, robust, easy to charge ........

 

[RexLan]

Miniature latching relay for the switching ... zero power consumption.

 

[Robin Lovelock]

Thanks for the suggestions folks - for those who wanted more detail, there was quite a bit on what we did 7 years ago, on that www.gpss.co.uk/bottle.htm page.

Remember that a GPS tracker contains a GPS, mobile 'phone electronics, and a processor to give it intelligence. The Falcom units we used 7 years ago consumed about 200mA, but the units I will probably use - from my business partners and contacts in the far east - will draw nearer 50mA - if I powered them continuously, without a switch. Typical examples are the Concox GT02 or Globalsat TR-102 under "Components" on my www.gpss.co.uk/chase.htm - for those who are interested.

I'm happy with using NiMHd cells since they worked for several months on that last GPS bottle project 7 years ago, and have been used on numerous other projects - both hobby and professional tracker applications.

I'm a great believer in KISS (Keep It Simple, Stupid) and am looking for the very simplest switch solution. The PICAXE with a "transister" (or relay chip) could be it. My early robot boat PICAXE servo controller (on the other thread) did include a switch, in an earlier prototype - but as I say, KISS ;-)

7 years ago we used digital timers taken from mains timer units, but these only allowed 20 switchings each day: I'm planning every 18 minutes for 2 minutes. i.e. 24x3 =72. Accurate timing (rather than an old RC multivibrator) means it could be set to switch on at predictable times. e.g. on the hour, 20 past, and twenty to. This might be useful for other options like looking for a bottle at night - if it has an LED that comes on - or sending it an SMS just before it switches on.

So far the most useful comment for me seems to be the low limit of just a few mA on each pin. Hence the probable need for a "transistor" - even if inside a chip.

Robin
www.gpss.co.uk

p.s. if limit is 25mA per pin, maybe I could wire three output pins in parallel on a PICAXE servo controller, to give a limit of 3x25mA = 75mA = well above my expected max of 60mA.

p.p.s. have just looked at specs and see that the PICAXE will need a supply limited to 6v which could be met by something like a UBEC (Universal Battery Eliminator Circuit) or simpler components, but this seems to rule out a very simple PICAXE solution if it is to be powered from 12v and switch the tracker needing 12v. The UBEC would consume it's own power, in addition to the PICAXE, but I can check this since I use a UBEC in my robot boat to supply many of the 5v components like iPAQ computer, GPS, etc.

I guess I may now be looking around for non-PICAXE solutions, which could end up being simpler and more reliable, but any comments or suggestions will be welcome from you. Many Thanks.

 

[srnet]

I'm happy with using NiMHd cells since they worked for several months on that last GPS bottle project 7 years ago, and have been used on numerous other projects - both hobby and professional tracker applications

Well maybe, but you did suggest you wanted the unit to run 'indefinitely'.

For long term use I would be thinking Lithium cells of some type.

 

[MartinM57]

If I was sketching out a design with a PICAXE I would be looking at:
- a high side switch such as http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=1739397 - happily switches 12v with a 5v signal
- a low quiescent current regulator 5v regulator for the PICAXE e.g. the 2mA of a http://uk.farnell.com/fairchild-semiconductor/ka78l05az/ic-v-reg-5-0v-78l05-to-92-3/dp/1014073 - there's probably better but the Farnell parametric search doesn'y allow search on Iq
- all the tricks of the trade to minimise power consumption in the PICAXE.

I would not go down the line of parallel'ing PICAXE output pins - it's a bodge and could end in tears...

 

[mh108]

The low pin mA may not be an issue if you consider using a "Reed Relay" (or even a "Latching Reed Relay"). They can be driven directly from a single picaxe pin, with versions requiring/sourcing 10mA or lower. Some also have a reversed-biased diode included in the package! Small and marvellous little things!

 

[fritz42_male]

Ciseco.co.uk have a nice little latching relay board that takes input direct from a picaxe pin. The site says out of stock at the moment but speak to Miles and he might be able to help.

 

[srnet]

As for low Iq regulators, I have a 18F25K22 on the bench here. Its using a 3V LDO regulator, and when the PIC is put to sleep, total standby current is around 50uA. This is not using the shutdown facility of the regulator.

I am using the PICs timers, running from an external 32Khz crystal, to wake it up. The max delay is however 16secs, so for longer (and accurate) wakeups I will be switching to a using the alarms from RTC IC to provede more flexible, and much longer sleep times.

 

[Robin Lovelock]

Thanks folks. I think I have a hardware solution, and now would be interested in hearing any thoughts related to accurate timing. But first let me describe the hardware solution ....

I've ordered some servo switches, used for radio control robotics, rated at 10A, so one will plug onto a PICAXE board, and can switch the main 10v power to the tracker. The PICAXE will have it's own 4 NiMHd cells, trickle charged from the main 10v supply (with solar panels) by a simple resistor. This is how those old bottles worked - the only difference now being use of a PICAXE instead of a commercial digital timer. If I wire the relay correctly, the timer can fail in an "on" state, but I'm sure it will work long enough anyway.

Now for some more advice from you: as always, your experience can save me time hunting through documentation, other postings, or doing tests here.

I'm interested in having the timer set up to work accurately, so that it can be started "on the hour", and will accurately switch on (for maybe 2 minutes), then switch off for exactly 18 minutes. i.e. even after days, and maybe weeks, we can predict within a few seconds, when a bottle is switched on. I shall probably add a bright flashing LED light, which could aid visual location of bottles at night. That will not cost too much power, if only switched on with the GPS tracker.

Right now I have a simple BTIMER.BAS test program, to test timing, and evolve into the program I need. Right now it attempts to twitch a relay for about 1 second, then pause 59 seconds. I use the second hand on my watch to measure drift, then tweak the numbers used for delay.

I'm guessing that there may be background activity in the picaxe, perhaps associated with servo control, that will effect the nominal accuracy of the timing. e.g. if "pause 30000" gives exactly 30 seconds delay. If not, how accurate, and what correction to the number to apply ?

My simple little BTIMER test program just uses SERVO, SERVOPOS and WAIT.

The next question is, after compensating for consistent accuracy in the timing, what accuracy I should expect: 1) when operating at a consistent ambient temperature. 2) if the temperature in the bottle changes a great deal. e.g. late at night vs middle of the day.

High accuracy is not essential, but will have a major benifit, after the bottles are thrown into the sea, for things like finding them visually (when not too sunny), and/or testing with SMS if the web tracking fails for some reason.

Many Thanks for your ideas thus far, and for any more.

Robin

p.s. I've added some more info about general progress on
www.gpss.co.uk/bottle.htm - hope you enjoy some of it ;-)

 

[eclectic]

A quick question.

What about using GPS time to
periodically check/update your clock?

e

 

[manuka]

Robin: I've been waiting for someone to ask about funding (was that the BBC ?) & bottle numbers! I've followed your initial site (including the NZ Chatham Islands trip!), but note ranges were only a few miles from drop off.

So is this new quest intended as a "Blitz Street" documentary or strict science? If the latter (checking the Gulf Stream perhaps?), then industrial strength equipment should be used = $$$$$$$ budgets.

Even if the former (which I personally found enthralling viewing),still consider a rather more robust approach & also factor in mischief. Wandering oceanic bottles may be the stuff of mystery, but all too many are simply smashed on rocks,blown about by the wind OR become spirited away by beach combers. Air travellers may re-drop them in the sea half a world away! No names you'll understand...

A major technical issue may relate to the weak GPS sourced transmitted location signal. Cellular will only work near land sites of course, although it may be worth considering an increasingly cheap 3G Android GPS equiped phone as your entire engine. Satellite links require licenced channels & a fair bit of transmitter power - OR directional antenna. Both may be needed when the bottle is at sea & bobbing on the waves. The bottle will usually be horizontal if cast on a beach & any antenna then will be almost earthed...

To me an important issue is that solar setups in wintery nautical locations may receive almost no sun for some months. The unit will hence go silent & flattened batteries may go belly up.

Extra (after reflection): Best you come clean Robin-this imaginative project seems to require more than a few choice wine bottles to be emptied while at the beach methinks. Twist my arm & I'll help out down here in the South Seas with my quota. Stan.

 

[Robin Lovelock]

Thanks Folks. I'm still looking forward to advice ref my question of timing.

Yes, I've used GPS time to sync computers for over 15 years. Lots of background on www.gpss.co.uk - but for this little project, KISS really is important. Each module (e.g. timer, tracker, battery, etc) needs to be fairly simple, so we can swap products according to what's available. This tracker does have the GPS data available when switched on, but some don't. The simpler things are, the less scope for failures and difficulties. i.e. KISS.

Funding ? All out of my own pocket I'm afraid. My "business overheads" - or rather "pocket money" - since there is no business case for BBC publicity, unlike some of the TV stuff 15 years ago on www.gpss.co.uk/asontv.htm
Like the robot boat stuff, this is simply overlapping "hobby" activity for me.

As it happens, BBC Radio Solent have been very helpful in the past in bringing attention to my little charity project over the past 8 years - something completely different - getting the Government and NHS to obey the law, and pay the costs of those in care homes. Details on www.nhscare.info - for those with old friends or relatives.

Today the wife and I popped out to Maplins and Homebase. I bought £5 worth of flashing LEDs. I also spent £7 of my pocket money on glue, to glue the ends back on the bottles after putting the electronics in, and £15 on a digital thermometer for tests on how hot the bottle will get in the sunlight. Good if we can solve that old problem from 7 years ago, where bottles need to remain wet - the electronics fail soon after they are washed up on a beach. One reason why "indefinitely" is just an aspiration. I see a flashing LED may also be an alternative to the charging resistor.

Please don';t forget my outstanding question about accurate timing.

Robin
www.gpss.co.uk/bottle.htm

 

[premelec]

@Robin Please state concisely what accuracy and precision you need for timing. We all are familiar with 'quartz' clocks on our wrists and walls - is that good enough?

A CMOS 4060 with a crystal oscillator would draw very little power.

BTW massive amounts of current can be switched with a MOSFET with virtually zero current to control it once on [just gate capacitor to charge to a few volts].

 

[Robin Lovelock]

Thanks for that. My approach here is to find the limitations of suitable products (e.g. the PICAXE) and work within the limitation of these products. Not to try and build a bespoke solution to meet what may seem a good design aim.

e.g. if I can make the PICAXE based timer (without adding extra components) accurate to within perhaps 1 second drift, over maybe 4 weeks,
in typical variations of temperature found in the bottle (yet to be measured, but maybe 20c to 40c), I will be delighted. I could live with this being 10 seconds. If it was as much as 30 seconds, I could still live with it - and even that would not justify my extra time spent on adding more components - along with the expected lowering in reliability.

Sorry if I'm telling granny how to suck eggs ;-)

I'm hoping that someone will know the PICAXE accuracy - it may even be documented somewhere. If not documented (including effect of other processes used to control servos, and temperature) then maybe somebody knows the accuracy because they've had to do what I may end up doing anyway: change numbers and leave one running on my mantlepiece, flashing an LED every few minutes, for comparison with my wrist watch.

Maybe somebody can save me some of this effort ;-)

Robin
www.gpss.co.uk/bottle.htm

@Robin Please state concisely what accuracy and precision you need for timing. We all are familiar with 'quartz' clocks on our wrists and walls - is that good enough?

A CMOS 4060 with a crystal oscillator would draw very little power.

BTW massive amounts of current can be switched with a MOSFET with virtually zero current to control it once on [just gate capacitor to charge to a few volts].

 

[srnet]

accurate to within perhaps 1 second drift, over maybe 4 weeks, in typical variations of temperature found in the bottle (yet to be measured, but maybe 20c to 40c), I will be delighted. I could live with this being 10 seconds. If it was as much as 30 seconds, I could still live with it - and even that would not justify my extra time spent on adding more components - along with the expected lowering in reliability.

Well 1 second in 4 weeks is about 3pm, 30seconds is 100ppm.

With the internal PICAXE RC timers, no chance of getting anywhere near that.

However a carefully selected crystal used with the external resonator option, could get you around 100pm, maybe even lower than 50ppm.

 

[eclectic]

See here:
http://www.picaxeforum.co.uk/showthread.php?t=18536&highlight=clock+accuracy

and other posts for example.

It looks like a crystal/resonator or external timer is essential.

e

Cross post with srnet.

 

[srnet]

Cross post with srnet.

Well, I have been pondering this problem myself.

I have been looking at the RTC option, ingnoring temp stability for a moment, some of the RTC chips allow you to dynamically pull the external Xtal +/- about 1ppm.

Now if you could characterise the crystal, temp versus frequecy, then you could dynamically adjust the crystal accordingly, by measuring the temperature.

Of course you either need a very accurate referance, or do the testing over a longe period using something like a GPS or Rugby clock ......

 

[eclectic]

Well, I have been pondering this problem myself.

I have been looking at the RTC option, ingnoring temp stability for a moment, some of the RTC chips allow you to dynamically pull the external Xtal +/- about 1ppm.

Now if you could characterise the crystal, temp versus frequecy, then you could dynamically adjust the crystal accordingly, by measuring the temperature.

Of course you either need a very accurate referance, or do the testing over a longe period using something like a GPS or Rugby clock ......

Please see post#14

NMEA GPRMC / GPGGA would provide a nice "timer"

e

 

[srnet]

Please see post#14

NMEA GPRMC / GPGGA would provide a nice "timer"

e

Yes, as a referance for setting the time, although the poster did not seem keen on this.

In terms of setting a crystal osc freqeuncy to within a fraction a Hz, not very useful.

 

[eclectic]

Yes, as a referance for setting the time, although the poster did not seem keen on this.

In terms of setting a crystal osc freqeuncy to within a fraction a Hz, not very useful.

I was assuming a DS1307 or similar.
My fault for the lack of specific info. :-(

e

 

[eclectic]

Thinking aloud.

Set a "nominal" hour, using the (resonator controlled) Picaxe.
(No need for a clock.)
Wake up, and the GPS provides position
and exact time.

Log those values.

Over weeks/months, it might even out.

Awaiting Robin's comments.

e

 

[srnet]

I was assuming a DS1307 or similar.
My fault for the lack of specific info. :-(

e

Dont matter which RTC really.

If you want to a adjust a crystal osc (be 32khz for RTC or 8/16Mhz on a CPU) to get it beyond 100ppm you need to be able to measure it to a fairly high degree of accuracy.

Long term, over a week or more, a GPS or Rugby clock may be enough, just.

Otherwise you need a frequency counter with enough digits.

For 32khz crystals, you would get a good comparison if you used a frequency counter (with enough digits) to measure the Rugby carrier frequency which is transmitted with an accuracy of, wait for it, 0.000002 ppm, which ought to be good enough for most purposes.

 

[manuka]

Am I missing something here gentlemen? A bobbing bottle at sea level has such a poor RF "take off" that any low power transmitter within would give landlubbers pathetic reception. I've had quite a bit to do with ~160MHz marine AIS,which gives ~20km range for the well elevated mast antenna & several Watt transmitters. However AIS signals received shoreside from most ships are soon blocked by the earth's curvature. GSM cellular signals would experience similar woes, & they're additionally restricted to ~35km due to the nature of the technology.

"Find & report" schemes, involving 1000s of such passive drifters as Nike footware & bathtub toys, indicate at best only a 10% find rate from normal floatsam & jetsam. Although appealing,GPS trackers can be quite costly ($$$) and need retrieving to let you know where they've been. Given the likely 10% return rate this may mean perhaps a dozen ($$$$) need to be tossed in the sea for even a single return... With such wastage hence a live tracker is applauded, but it'd need to be of significant size to house a powerful transmitter,beefy batteries, good sized solar panel & decent transmitting antenna.

Flashing LEDs may have mileage close in,and even my weary eyes can easily see traffic lights 10km away right across Wellington harbour. However these are elevated,directed my way,are very bright and the clear NZ air facilitates distant viewing. With Channel chop waves you'd be lucky to spot sea surface flashing LEDs at night at even a few hundred metres I'd say. Flashing bottles may attract undue attention from boaties, rascals and small boys testing their stone throwing skills as well.

THOUGHT:In contrast to geostationary satellites at ~40,000 km,tracking reception could be by LEO (Low Earth Orbital) types that scud by only ~300km up. Although appealing (and long popular with radio hams as OSCARs),use of such flying mailbox store & forward schemes may not be free. Stan.

 

[MFB]

Manuka. In pre-GPS days I development a drift buoy system that received and retransmitted (at 32 MHz) Decca Navigator signals for tracking from a shore station. Even if these old navigation aids were still available, they could not compete in accuracy or cost with GPS. However, a reception technique from that programme that offered good results involved surface radio waves. Its hard to remember the details but I did spend a lot of time laying chicken wire on the shore line and the buoy transmitter was less than a wave length above the sea. It might be worth investigating surface-wave radio wave reception for this bottle project.

 

[Robin Lovelock]

Thanks Gentlemen - your postings are very illuminating, but please note that I am seeking the relevant specifications of PICAXE and follow KISS.

That thread URL was useful, but note that the last posting on it said "The PLL built into the picaxe should keep the freq very stable."
http://www.picaxeforum.co.uk/showthread.php?t=18536&highlight=clock+accuracy

It seems one or two of the recent posters may not have followed links to see what was done 7 years ago, or the other GPS tracking related to www.gpss.co.uk - this grandad is very used to sucking eggs

But please keep the chatter going, it "sometimes comes up with the goods" and this Forum has certainly done that for me in the past

Robin
www.gpss.co.uk/bottle.htm

p.s. I'd love to know the age and experience of some of you guys: I'm 64 and my history is on www.gpss.co.uk/history.htm - but still learning
If any of you are shy, you can contact me direct on gpss@compuserve.com Whatever country you are in, I probably know someone within walking distance of you - maybe to buy you a beer ;-)

 

[srnet]

Thanks Gentlemen - your postings are very illuminating, but please note that I am seeking the relevant specifications of PICAXE and follow KISS

The KISS point was noted, but with the internal RC based OSC of a PIC quoted at around +/- 2%, its a complete no brainer for accurate timing.

If you want accurate then you need an external crystal or similar.

An external crystal is adding 3 components, and exteral RTC is 4.

 

[Robin Lovelock]

Thanks: you say "internal RC based OSC of a PIC quoted at around +/- 2%"
Where did you see this ? Then there are following questions which would need answering - maybe this -2% to 2% accuracy is how close an individual PICAXE keeps to it's nominal timing, and not how much it might drift, one way or the other. It's just possible that, after testing the timing, each PICAXE does not drift very much, and could be calibrated with an appropriate number in it's program. This stuff might obviously be determined by tests here, but I'm hoping it IS documented properly, or at least someone has already got some reliable test data.
Robin
www.gpss.co.uk/bbcbot.htm

The KISS point was noted, but with the internal RC based OSC of a PIC quoted at around +/- 2%, its a complete no brainer for accurate timing.

If you want accurate then you need an external crystal or similar.

An external crystal is adding 3 components, and exteral RTC is 4.

 

[MartinM57]

Thanks: you say "internal RC based OSC of a PIC quoted at around +/- 2%"
Where did you see this ?

In the data sheets from the underlying PICs that the PICAXEs are based on.

There are also graphs in the data sheets of OSC tolerance vs supply and temperature - the accuracy of the internal OSC changes with both. I would strongly advise an external crystal and two caps, or a three terminal resonator if you want accurate timing from the PICAXE itself...both are a lot more KISS than testing individual devices and trying to work out calibration factors for a) inherent inaccuracy b) drift with supply voltage c) drift with temperature

 

[manuka]

Robin: Repeat after us-"time keeping on the internal oscillator PICAXEs is normally only within a second a minute, AND it further drifts with both supply voltage and temperature changes!" CALIBFREQ +/-30 ( POKE $90 ) conveniently tweaks the internal oscillator frequency,and is very handy for on the fly adjustments with DTMF tones & SM-T Hellschreiber. However these tweaks are generally only needed to perhaps ±1%,with small subsequent drifts tolerable.

MFB: An interesting one- I'm from the DECCA & LORAN-(A) era myself so can relate to the pre GPS technology.

Irrespective of how the floater's location is obtained,when beyond close in coastal waters significant "grunt" will be needed for data to be sent wirelessly from a sea level sender to shore. Given Robin's GPSS & technical background I'm sure he appreciates this, but no mention seems yet to have been made!

FWIW-the GPS based APRS (Automatic Packet Reporting System),popular since the late '90s with radio hams,now typically uses 2-10 Watt ~144 MHz VHF transmitters (licenced) and hill top digi-repeaters. See a couple of modern APRS tracking setups below (valued US$$$). Note the 12V 7.2Ah Gel Cell, which was kept charged by a 5 W solar panel. Stan ( ZL2APS since 1966)

 

[MFB]

The oceanographic drift buoys sent audio tones (containing the Decca phase information) using a 500mW transmitter. The base of the transmitting antenna was less than a metre from sea level and the base station was only a couple of hundred metres high, but usable signals were obtain over a range of more than 60Km. Although initial tests using surface-wave techniques indicated that these would provide even better performance, this was not used as standard because of the difficulty of setting up directional shore-line antennas. Its difficult to remember more detail because this was a mid-70's project and I was too young to bother about good documentation!

 

[BillyGreen1973]

Well, for my 2 pence worth...

I would be tempted to 'use what ya got'.
I would set the picaxe to do the 18+2mins timing using internal OSC. Then every 12 or 24 hours use the GPS time to 'reset' the timebase in the picaxe. This will give almost no drift over extended periods, and doesn't add anything the the component count (or cost) as it's all done in software.
Even something simple like using the 18M2's 'Time' command to count seconds from startup and reseting would be easy to do and give acceptable results.

While I don't have test data on the effects of Temp on the PIC's internal OSC, over 12 hours I would guess there would only be a couple of seconds dift at most.

In know Robin mentioned that some of the GPS units have different specs and he wants them to be interchangable, but I would think all GPS units can give UTC as part of the 'GGA-Global Positioning System Fixed Data' sentence.
The 'RMC-Recommended Minimum Specific GNSS Data' sentence has everything you could need for this project, time, date, lat/long, speed in knots, heading. A 'one-stop-shop' of data

 

[manuka]

MFB:I was about to say - " at ~32 MHz the earth's curvature will limit ranges", but noted your base station was up hundreds of metres! Yes - that indeed may just give 60km.

During 1940 considerable debate emerged regarding the Lorenz VHF bombing beams, which were thought to be also too range limited for Blitz use. However the elevated bombers naturally extended the usable range beyond the normal horizon, much like a plane aloft being able to still see the evening sun after it's set at ground level. Stan.

 

[Robin Lovelock]

Thanks again folks. I'm always pleasantly surprised how many postings there are within an hour or two. Perhaps I should have mentioned something else: I'm hoping to do all this work, including tests and building three timers, within no more than perhaps 4 hours of my time spread over the next week or two. The timer is just part of the job, and other parts - particularly related to the trackers - may take much more time than other things. This includes using this forum of course

I still feel at least one of you have not checked out the links given earlier, including www.gpss.co.uk/bottle.htm That was over 7 years ago, and those GPS bottles actually worked - out to perhaps 5 miles off the UK coast, then we picked them up again, off the French coast, and tracked one up to Den Helder in Holland. This was over a period of several months - I remember the excitement when I received a text from the first bottle appearing 5 miles off the French coast - in our hotel room in Italy !

But a lot of things have moved on since then, not least the much lower cost of GPS trackers. Even with it's limitations, I think the PICAXE solution will still cost me less effort, with a better result, than the commercial timers we used last time. They only switched on for 20x a day, and I'm planning 72x for the new bottles. i.e. every 20 minutes.

I've got a first version of the BTIMER.BAS program running, and it will certainly need some debugging and tweaking of those delay numbers. The r/c relays I ordered should arrive before the end of the week, and plug directly in.

Please don't forget my requirement said very early on: I can live with timers that are not particularly accurate. However, results so far are encouraging. I'll report more in due course.

Many Thanks again
Robin
www.gpss.co.uk

 

[premelec]

I'm not entirely sure where this thread is but mention that some small portable loggers use a method which may be applicable IF your PICAXE oscillator is reasonably stable over the period between reports- and you have constant monitoring of transmitted signal - you use your local clock [computer or whatever] to simply apportion times in accord with actual times to received times if reported time is T1 and actual time is T2 then correct readings by T1/T2.

If you actually need a local accurate time then there are small TXCOs [temperature compensated crystal oscillator] that would give you the accuracy you wish -

 

[eclectic]

snipped
I still feel at least one of you have not checked out the links given earlier, including www.gpss.co.uk/bottle.htm [/QUOTE] A genuine comment, there are a LOT of words to read on those pages. Could you provide a precis, so that folks just see the relevant parts? e

 

[Robin Lovelock]

I guess you want a precis of what is on www.gpss.co.uk/bottle.htm ?

About 7 years ago, a number of GPS bottles were thrown into the sea off Ramsgate, and over the next few months, were tracked on a web site and large screen display in the Margate Art exhibition building. These bottles contained GPS trackers, batteries, solar panels and an electronic timer, to switch on the electronics for several minutes, every hour or so.

They all worked and some were tracked along the French, Belgium and Dutch coasts. GSM range was suprisingly good at 5 miles from coasts.

Now we are assembing some new GPS bottles based on more recent components including the GPS trackers, solar cells and PICAXE based timer.

The timer is a standard PICAXE kit and the switch will be a standard servo-relay used for radio control. The PICAXE is programmed (BTIMER.BAS) to switch on for 2 minutes, then switch off for 18 minutes.

I am making it as accurate as possible to allow options of knowing when bottles are switched on. Bright LEDs will be used, to help with the process.

Things like GPS based timing has already been used on my earlier PICAXE tests, but this solution is being made as simple as is possible.

I started this thread hoping I might quickly find specs and/or test results indicating the accuracy I might expect to achieve. So far people are speaking of 1 second per minute, but my test today already seem to give better than 1 second per hour - and I have not fully calibrated the numbers in the program. However, this could obviously be far worse when I test under typical conditions such as change of temperature inside the bottle (20 to 40C ?) and supply voltage (not yet known. Hoped to be +-0.3v on 10v.)

I guess that also gives a summary of all this thread

Robin
www.gpss.co.uk

A genuine comment,
there are a LOT of words to read on those pages.
Could you provide a precis,
so that folks just see the relevant parts?

e