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Solar Powered Radio Controlled Clock

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Hi Everyone,

I am new here, so please don't shoot me down for asking dumb questions. I have had a good hunt around on here and online before asking this, so its not laziness that has brought me here.

I have 2 growing passions (the time and boats), and this project pulls them both together, so hopefully knowledge learnt on this project will help me in both areas. I have built automatic watches and learnt to sail, but I want a yacht (with solar and engine charged 12V systems), and I want what this project is about..... a solar powered radio controlled clock. Automatic watches are great, but not accurate, radio controlled watches and clocks are okay but use batteries which run out. So I want to build a solar powered RC clock.

The RC clock bit is easy, the 1.5V battery powered movements are readily available for about $15-20, and I can get hands, dials, etc really easily. But having a charging circuit for the rechargeable battery is not so easy. It needs to charge the battery, even inside, during a bleak UK winter, but not toast the battery in the summer. So I need some sort of charge controller to ensure that the batteries only charge when they need it, don't overcharge, and don't discharge through the solar cells when not lit.

I can solder and understand circuit diagrams but would appreciate the advice of the experts here as to choice of battery, size of cells, circuit diagram and components so I can build the charge controller. I am thinking I could mount it all on a board with voltage and current readouts and warning LEDs so I can track how the system is performing.

Okay, maybe that's just getting too complicated, but it would be nice to build one for me and then maybe more of them for family and friends so they all have something that always tells the right time and never needs a new battery.

I actually have an EcoDrive watch and I know they use capacitors, not batteries, so actually eliminating the battery would be even cooler. But again, I'm not an electronics expert and would need advice on how to make this work
Buy the clock, build the yacht.

Seriously, a radio controlled clock from scratch involves a radio, decoder (microcontroller), an output device, and of course your solar cell, battery, and battery charge circuit. From your description, it seems you want to buy kits for each element. Such kits do (sort of) exist, but then you have the problem of writing the code for the MCU.

Here is a ready made version:
That's list price at LaCrosse. Much cheaper on other sites.

To get you started on building your own, I got a receiver about 2 years ago from PV Electronics in the UK (**broken link removed** ). All of the sources for the CMMR-6 version, including Sparkfun no longer carry it. There are also plans for receivers on the Internet.

Oops, expired link. This should work:

There are also GPS-disciplined modules available for your clock:

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Hi, sorry, I think I didn't make myself plain.
I don't need any help on the RC clock as there are movements to replace quartz clock movements readily available, I have looked into them and am happy I can make the clock.
BUT I want it to run on solar. One guy did an 'instructable' on using a cheap solar garden light cell to charge the rechargeable 1,5V AA cell but admitted that the battery ran down. No good!
So its the 1.5V NiCad or NiMH battery charging circuit that I need, with enough solar to do it on a British Winters day (I have the lights on at noon) but not fry the batteries in summer.
The link I gave was solar powered. As is the one (about $5 on eBay) that sits on my desk. What happens to the battery depends on the type of battery and your charging circuit, as I mentioned.
I also have an LCD RC clock that I use to check the accuracy of the automatic watches I build. When they go out of the door they will run +/- 1 second a day. However, I'd like a nice mantel clock with hands for the lounge (den), I could just convert the clock we have in there at the moment, but learning about the application of solar will help me when it comes to boat applications as well.
Any idiot can buy a product off the shelf then wonder why it doesn't work in their application, I want to understand the technology by making the application a little bit testing. We really have very little sun here in the UK!
Then, when I'm stuck in the middle of the Atlantic, my motor has expired and I'm trying to work out how much juice I'm gonna get out of my solar panel to run my night time running lights till I get to Nassau to get the motor fixed, I'll know enough to get by!
Have you considered that in the middle of the Atlantic you may be too far away from the transmitter that the clock is designed to receive. The one in the UK is in Cumbria 60 Khz) and there is one in Germany near Frankfurt (77.5 Khz) I have some clocks bought from Aldi and Lidl that use the German transmitter but they sometimes loose the signal. I have found that the alkaline AA cell that powers them lasts much longer than a year.

the alkaline AA cell that powers them lasts much longer than a year.
..... so, unless you're planning being at sea for over a year, carrying one spare AA cell would seem a lot easier/cheaper than using a solar cell ;)
Also consider non-rechargeable lithium AA or AAA cells.

I built a temperature and humidity datalogger with a real time clock, to be installed inside fridges (above freezing).
I was very careful with the design to minimize idling current, much like you clock movement will be, and the lithium AAA batteries lasted over three years.
I'm a boater too.
Loran is now broadcast once more and can be picked up a long long way off the transmitter, but its a little too complex for a clock project like this.
There was a clock going back to the 60's that did just as you stated.
The thing to look for is a solar panel that will charge the battery but not overcharge it when its full, ie 0.5 or 1ma continuous charge might be Ok for certain batteries even when full, you'd have to do some research and look at battery specs.
Another possibility is a supercapacitor, so long as you dont exceed its rated voltage which could be managed with a clamp circuit they cannot be overcharged.
A standard 1AA clock movement may well work on a supercap, you could lash something up & test it out.
Any battery that is rechargeable can be trickle-charged with a solar cell. Trickle-charging works for all battery types, and will not damage a battery.
The only question is how much power is needed for the clock (voltage & current), and how long might it go without solar charging?
Given that we can choose or approve of a battery type and size.

Now that we know the battery, we can think about re-charging it.
The only problem is making sure the power out of the source isn't too much.
That can be handled with a current regulator, which for a small low-current device can be both simple and cheap and reliable.
Not enough power (when dark) will handle itself.

Once we know the battery requirements, we can determine if the solar cell is enough, and what voltage and current it can deliver.
Then we can determine if we need a voltage booster (hopefully not).
Last - we figure out the current regulator and build & test it.

But it all starts with knowing the clocks requirement for voltage and current, and how long you want to go between sunny days...
Just some ball park figures.

Clock movement runs at say approx 150uA average (maybe a bit less without the second hand).
Over 24 hours thats 3.6mAH.
So we have to charge a battery this much during the worst sun conditions, say sunlight from 9am till 3pm or 6 hours, thats a current of 600uA, add a bit for losses say 1ma.
And guess that a solar cell will make 1/10 its rated power on a dull day so that would need a 10mA cell, so a 2v 10mA cell or so would just about make enough power to run the clock, going over this a little but not so much to kill the battery would be a sensible target.
All this is very approximate and off the top of my head but should give you some idea.
If you went for electronic charge regulation the circuitry for that would porbably use more than the clock and would also need to be accounted for.
This will give you an idea of physical size for the 'cell:
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10mA x 2V. That's a start.

The only question is how many days might this device go without some daylight?
It could be designed to work on a cloudy day or even a week of lousy weather, but if navigating close to or above the Artic or Antartic circle, it could be 6 months!

(Not trying to be a smart-ass, I truly have no idea where something like this is going to be used, and I don't want to assume anything.)
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