Small Scale Solar Sign Help

System · January 2010

I'm trying to create a solar sign to light up a real estate lawn sign. I've take a Solar PV course, but unfortunately it focused mostly on grid tied residential PV systems, so I don't have much knowledge on small scale PV systems, especially ones operating with batteries. I was thinking of following the design of solar lawn lights (ie photoresistor, LED, battery, circuit, solar panel), and was just wondering if I could get some help with sizing and pricing. I am thinking of using 8 seperate 1/2 watt LEDs, and was wondering how large of a solar panel I would need (and cost) to be able to power these lights for 12 hours at night. Also, would 2 AA rechargeable batteries be significant for storing the power for these lights. Any help would be appreciated! Thanks.

GreenerPower · January 2010

Re: Small Scale Solar Sign Help

Would this solar LED spot light from HomeDept fit your need ? (the stake is removeable).
Well, how do you plan to secure it from being stolen.
GP

niel · January 2010

Re: Small Scale Solar Sign Help

it is feasible to use leds and make a sign out of them. i would assume the message to be permanent for simplicity. driving the leds properly is your problem as you didn't go into electronics very far with your course. the voltage and current must be prevented from exceeding the ratings of the leds used and if a constant current isn't available then, using resistors, one could keep the current from being exceeded. due to a type of thermal runaway the current increases on them as they operate so i try to keep that below 1/2 of the current maximum. i know this does not answer your question, but i don't know if i can directly say what you can do.
a possibility may be to buy an led sign capable of being run with 12vdc and then feed that with solar in conjunction with a small controller that has lighting ability and a battery to run it with at night.

BB. · January 2010

Re: Small Scale Solar Sign Help

A pair of 2xAA NiMH batteries would be

~2,000 mAH * 1.2v * 2 series battery = 4,800 mWatt*Hours = 4.8 Amp*Hours

4.8 Amp*Hours / 12 hours = 0.4 amps average

Realistically, I would count on about 1/2 that amount of energy (depending on a whole bunch of variables)--so that you have about 0.2 Amps (200 mAmps) for 12 hours a night for 2x AA NiMH batteries).

P=0.2 amps * 3 volts = 0.6 watts

Or, at best, one of those 1/2 watt LEDs (you have to limit current to an LED--so you either need a ballast resistor--which wastes power--or electronic current limit circuit) per 2x AA battery.

Note: your eyes are not linear but logarithmic in how they see light. If you cut the power by 1/2 -- the human eye will barely see the difference if you did an A:B comparison (first full power, then 1/2 power). So--you could reduce power usage by 1/2 for less battery / less solar power and still have pretty much the same results.

For our eyes, a factor of 10 change in power levels is a "night vs day" difference. A 4x change in power change is easily seen.

How big a solar panel... Given all of the inefficiencies of small rechargeable batteries and solar panels--I would start with an assumption of 1/4 or 25% efficiency. So:

0.2 amp LED circuit * 12 hours * 1/0.25 * 1/2 hours a day of sun (winter) = 4.8 amp solar panel

4.8 amps * 6 volt = 30 watt solar panel assuming "simple" pwm charger

That is a pretty pessimistic set of assumptions... You could probably cut the panel to 15 watts or less and get OK performance. In any case if you want to build one test circuit, the above should easily meet your requirements and you can try scaling back to make something less costly.

If you use smaller LEDs (i.e., 0.05 watt or less) then everything becomes 1/10th the size.

Also, changing the requirements so that the LED's are small and flashing to attract attention--and using a reflective sign for the text/ad would allow the use of a much smaller power supply (LED's are not lighting the text which requires a lot more power).

-Bill

mike95490 · January 2010

Re: Small Scale Solar Sign Help

Biggest problem will be keeping the sign anchored overnight.

You could also, have small T3/4 LEDS make up the letters, they make rectangular shape LEDs, and you spell out words. Much less power than back-lighting translucent plastic or spotlighting a sign.

System2 · January 2010

Re: Small Scale Solar Sign Help

Hey guys thanks for the responses. I'm trying create something like this for my friends real estate business (http://www.siliconsolar.com/solar-real-estate-lighting-p-16142.html), and instead of just buying 40 of theses signs online, I was interested in seeing if I could manufacture it myself for less money. Based on Bill's response, could anyone help me price out a system of this sort, primarily, the cost of the solar panel. Thanks.

Brett

Solar Guppy · January 2010

Re: Small Scale Solar Sign Help

You can't build that yourself for less, you can approach the business as ask for a discount for buying 40 units, that probably your best option

BB. · January 2010

Re: Small Scale Solar Sign Help

Other things to think about...

White LED's have a pretty poor history for life... The small traditional LED's are probably useless after 500 hours or so... I don't know about the better ones (Cree, etc.)--but those may start to dim in 2,000 hours (I would test before spending a lot of money).

Regarding the sign light---You would probably get 80-95% of the attention if you only powered it 4 hours p;er night vs 12 hours per night...

Lamps will last longer and battery/supply only has to be 1/3 the size/cost.

-Bill

System2 · January 2010

Re: Small Scale Solar Sign Help

Great point Bill...I didn't even consider that I would probably only need to light the sign until about 10-11 pm.

Solar Guppy, what part would be the main driver as to why I couldn't produce the solar sign for less than $45. I'm guessing its the panel itself...how much would it cost. Thanks.

niel · January 2010

Re: Small Scale Solar Sign Help

seeing as how the signs will be placed on various vacant properties, you may have a different problem in going with solar/led signs in that these are apt to be stolen. maybe try some florescent paint or something like it that a thief won't think it worthwhile to take and would be cheaper for you too.

Solar Guppy · January 2010

Re: Small Scale Solar Sign Help

DenverBrett wrote: »

Great point Bill...I didn't even consider that I would probably only need to light the sign until about 10-11 pm.

Solar Guppy, what part would be the main driver as to why I couldn't produce the solar sign for less than $45. I'm guessing its the panel itself...how much would it cost. Thanks.

Go try and purchase 40 solar panels, battery packs 500 white leds, design the circuit board for the current limiter for the LEDS, charging circuit for the batteries and have the PCB'd fabbed, stuffed and tested, a custom injection mold for the housing/cover and then figure your labor costs to assemble the product.

I'd guess your looking at $500+ unit when you start looking at the tooling ( typically 5K and up ) and US labor for a small run like this

I'd assume you have never built a product before if your asking the question. These things get made in China were labor rates are 1/100th of the US and they handle all everything for you, all you have to do is commit to a build of 10K

If this doesn't seem to convince you, go to home depot and get one of the solar spot lights and take it all apart, then you will have in front of you most of the hardware it takes to make a product like this.

System2 · January 2010

Re: Small Scale Solar Sign Help

I definitely understand that fabrication, tooling and labor costs would make producing something like this very expensive and not economical. I believe my friend just wanted to try producing his own, and then if it was successful, have the design outsourced and mass produced. I am in complete agreement that it more than likely will be uneconomical, however, I'm meeting with him tomorrow and he was interested in what the cost for parts would be to produce this. For this reason, could you humor me and give me a rough estimate on the cost of just the parts to build one of these solar signs, not including fabrication, labor, PCB design etc. Thanks again for everyone's help.

Small Scale Solar Sign Help

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