# Need help with a solar garden lighting setup

Discussion in 'The Projects Forum' started by gledy2, Mar 17, 2014.

1. ### gledy2 Thread Starter New Member

Jun 11, 2013
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Needing a little help, as I have been researching and reading up about electronics for this project, and ended up confused!

My project is to do away with my solar garden lights that break/need repaired every year, and make my own setup. I plan to bury 40 glass bricks i have acquired around the edge of my garden and put run a white led to the bottom of each. I have bought some leds, but struggling to work out the size of 12 vsolar panel and 12v deep cycle battery i need. The relevantled specs are as follows, not sure what reverse current is yet! This is the only data i could get on them :

Forward Voltage Typ Forward Voltage Min Reverse Current Vr=5V
If=20mA If=20mA Max
3.4 3.0 10

I took it that each leds draw is 20ma.
I put this into a calculator and it suggests 10 runs of 4 leds in series with a 1 ohm resistor in each run. The results were:

• total power dissipated by the array is 2404 mW
• the array draws current of 200 mA from the source
I am guessing this would be the best way to run them when using a 12v battery, not all in series. I am in scotland and would like the setup to run for 8 hours in winter. I get a maximum of 8 hours of sunlight in winter.

I am not sure if my draw per hour is 200ma as results above, or 40 x 20ma which is 800ma?

Now i am getting lost as to what specs battery i need and solar panel. I guess the 200ma or 800ma draw is per hour. So i am thinking then i need a battery with either 1.6 a capacity or 6.4 a minimum capacity for 8 hours of running.

For the panel I am trying to work out what i need by using P=VI so power is 12v x 0.2= 2.4W or 12v x0.8=9.6W. I think this means the Circuit will use either amount per hour. So for 8 hours of use i will need 19.2Wh or 76.8Wh. I think i can calculate the panel from that, but could someone please let me know which figure i should use or where I have gone wrong, thanks, allan

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,394
497
What is LED forward voltage (V forward)?

3. ### gledy2 Thread Starter New Member

Jun 11, 2013
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Thanks shteil, it says typical 3.0v, max 3.4v, it didnt come out well when posted!

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,394
497
I am not sure 12 volt battery will be enough.

So far the basic circuit looks like this:

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5. ### wayneh Expert

Sep 9, 2010
12,145
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20mA is 0.02A, so you slipped a decimal. 40 LEDs burning off about 68mW each (3.4V at 20mA) is total of 2.72W. In reality, you'll want to run your LEDs at 10-15mA to increase their lifespan.

A rule of thumb for charging and discharging a battery system is that the overall losses will be ~50%, so you would need at least a 5W panel if you had equal time for charging and discharging. If you really want 8 good hours of light on a winter night following a couple cloudy days, you'll need a much larger panel. If it's OK to go dark occasionally, you can cut down on the panel.

You may want to look into DC-DC converters. These are inexpensive on e-bay and will allow you almost unlimited options when matching voltages of your panel, your battery, and your load (the LEDs). For instance, you could use a 6V panel to charge a 12v battery (you'd need a boost converter for this) and run your LEDs at 24V (again via another boost). One big benefit of using the converters is that they regulate voltage for you, and they are much more efficient than other options for achieving regulation.

But converters don't change the thermodynamics. You need to generate enough power with your panel to power your load and all other losses.

Last edited: Mar 17, 2014
6. ### wayneh Expert

Sep 9, 2010
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In the scenario kindly provided by shteii01, you'd need a PV panel capable of an open circuit voltage of ~20V. That would give some current all the way up to the battery's full charge (14-15V) plus the drop across the blocking diode (~0.7V). The panel and the battery need to be matched up a bit to ensure you don't overcharge the battery. And then there is the issue of preventing excessive discharge also.

You're seeing why there are commercial devices that manage all these functions; charge, discharge, on/off timing, brightness, etc.

7. ### burger2227 Member

Feb 3, 2014
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Most commercial devices use a lot less battery, smaller solar panels and less LED's. The normal garden lights have one battery, two LED's, a chip, a coil and a small solar panel. The 4 pin chip keeps the battery from full discharge and turns the LED on when solar charging voltage drops low. The chip also can run some number of 3 volt LED's up to 90 ma on one AA or AAA battery. With just 2 LED's at 15 ma, it will run about 8 hours.

My solar garden lights have lasted over 2 years with the original batteries. Naturally the LED's stay off longer in the winter time as there is less sun light.

Your circuit diagram does not have a way to turn the LED's off so far and no way to protect the battery.

Last edited: Mar 17, 2014
8. ### gledy2 Thread Starter New Member

Jun 11, 2013
10
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Thanks for the diagram shteii, that was the plan with a solar charge controller. As burger pointed out, i do need to introduce something that will turn them on and off at night, and to protect the battery. I have not learnt about that part yet, so didnt want to comment on it yet! Apologies for the mistake wayne, still learning! Thanks for clarifying about 12v being not enough, I was about to ask! Yeah i was trying to match the panel once i found out the battery specs, but now I see it is a little more complicated than keeping everything at 12V. I was going to use a solar charge controller to take care of the overcharging issue, will do some research on something to stop the battery discharging to far, i was just thinking a deep cycle battery would be ok, but after further reading just now it seems fully draining it is not so great. I was going to take a figure of 4 hours sunlight in winter to work from, so i am guessing that will be a 10w panel minimum. Had a look at dc converters and understand them now, happy to use them, happy to use any setup of panel or battery, and understaning the benefit of using the converts to regualte voltage. Thanks burger, my solar lights seem to die every year due to water ingress so i was keen on making my own system up, that i could replace individual parts if they fail. The AAA recharable batteries have lasted in them though. I have tried a few different ones, but not found any good ones yet. The (hopefully) finished product should look something like this with white leds, 20 glass bricks running down either side of a gravel driveway...

9. ### gledy2 Thread Starter New Member

Jun 11, 2013
10
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If i were to continue as per shteii's diagram, can someone point me in the right direction as to what the total current draw would be for 8 hours useage? Just having a think and I think if i can determine some specs for the battery, then i can then match a panel for it. Can anyone tell me if i was right with 200ma per hour or 800ma per hour? Wayneh, just realised that the 0.2a was correct, it was from this...

10. ### burger2227 Member

Feb 3, 2014
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Those 1 ohm resistors are not required. That site always adds them when none are required. Some feel safer using them.

Well now all you gotta do is figure out a way to turn them on and off. Maybe an Op Amp circuit to turn it off when the voltage drops to a certain value. Depends on the battery type.

11. ### Bernard AAC Fanatic!

Aug 7, 2008
4,173
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I do not believe that 1 ohm will serve any purpose; on 12.5V, suggest 3 LED's in series with 120 ohm, with 13 strings in parallel. Total current about .26 A, @ 8 h = 2.1 Ah. 7.5 Ah, SLA battery looks good as would a 15 W SP@ 20 V OC; might give about 5 Ah--even better in Tucson.

12. ### wayneh Expert

Sep 9, 2010
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Most of mine are a bit different. They use a CdS cell to switch the LED on/off, and they don't prevent full discharge. They run until dead.

Anyway, the OP could use a CdS sensor as well, to control the lighting. There are commercial light/dark switches you can get, and you could control the whole thing with one of those. I have some that turn on at dusk but then turn off a settable number of hours after coming on. I use that for my "gaslight", so that it's not on after midnight or so. It would be good for this application, I think, to save battery. One benefit of a detached sensor is that you can point it where you want, out from under a bush for instance.

13. ### wayneh Expert

Sep 9, 2010
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I tend to agree, but what about when the OP's battery is being fed 15V while charging? OK, it'll be light out and the LEDs should be off. Until a bug lands on the sensor and the LEDs come on in full sun. Doodoo happens.

14. ### wayneh Expert

Sep 9, 2010
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That should be a cool look. I have to admit I like the colors, though.

Bernard likes this.

Oct 29, 2009
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16. ### gledy2 Thread Starter New Member

Jun 11, 2013
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Thanks burger, will leave out the resistors if i go down the 12v route, been looking at batteries and there seems to be a lot more options in 12v batteries for deep cycle so will stick with that i think. Any idea on what my battery amperage should be as still lost on that from my original post. Will look into op amp circuits tomorrow and see what they do! A bit more of a headache than i thought, but not really anything for sale that is like solar powered recessed glass bricks with leds so will persevere!

17. ### wayneh Expert

Sep 9, 2010
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Op-amps are great fun but I'd ignore them for now. I don't think you need to build much of anything unless you want to. The off-the-shelf modules you need are available and it will take some creativity to put it all together.

18. ### burger2227 Member

Feb 3, 2014
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I have chips for sale that prevent discharge and can use a CDS or LDR or a solar panel. Pin 1 takes the solar voltage and sends it to the battery until it goes low. Then the LED's go on. The chip with the coil is a Joule thief.

http://www.ebay.com/itm/131143112992?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649

With the solar panel doing all of the charging and switching, there can be no charging with LED's on.

I have some Op Amp charging circuits that limit charging, not discharge, but it is a similar circuit. I'll post some tomorrow if nobody does tonight. You'll have to figure what kind of battery and solar panel to use.

Last edited: Mar 17, 2014
19. ### gledy2 Thread Starter New Member

Jun 11, 2013
10
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Thanks bernard for answering about the battery, i have a couple of questions that are probably dumb, but i havent learnt enough yet to understand.

1) What would the benefits be of 13 strings of 3 leds with an 120 ohm resistor over 10 strings of 4 lleds with no resistor?

2) Do i need a 7.5Ah battery if my total would only be 2.1Ah, I can understand some reserve would be good but that seems a lot? Not a bad thing for dark days i guess?

3) Would a 15w SP that gives about 5Ah charge the battery in 1.5 hours on a perfect day?

Wayne yeah looking at my old lights a CdS cell (after googling) seems like the best idea to work switch on. I would love to know what you used for your gaslight as i would love to switch them on at dusk and automatically quit after 8 hours, would be pefect for my needs. A detached sensor would be great as i plan on mounting the panel on a south facing garage sloped roof, and the battery inside, with the sensor in the garden out of sight somewhere.

Wayne I am guessing the answer to my last post is that the resistors in each string are there to protect the leds from situations like the bug landing on the sensor whilst charging.

Yeah the colous would be fun on a path, but on either side of the drive just going for white, boring but i like the idea of them! I will get a photo up once i eventually get done, still the main items to purchase first!

Thanks spinnaker, will look into buck regulators tomorrow evening, lots of homework now! Will get my charge controller last once i know the rating of the panel i am heading for, thanks for the heads up.

20. ### wayneh Expert

Sep 9, 2010
12,145
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My gaslight was originally installed with a controller like this one, . Here's a cheaper version. There are many similar devices and your choice depends on your installation.

I haven't been able to find the device I now use. It's similar to this one (which doesn't get good reviews). Also described a bit better here. There seem to be many more dusk-to-dawn sockets without a timer. But I consider the timer a very useful function. You may have to separate the light sensor function from the timer function. For instance have the light sensor control power to a 6-hour timer.

Found it. This is the exact unit I have. It's discontinued. Maybe this is still available somewhere at a more reasonable price?

Last edited: Mar 18, 2014