My 1st project - Solar powered led grid with leds and light sensor

[murmundone]

I'm a newbie in electronics, sometimes i do smaller projects, repair minor stuff. I would like to learn more about circuits, and calculating the structures needed. I study product design, so i'm interested in this stuff at every level. Any help appriciated.

We have a school project, we designed concrete covers for the bottom of traffic sign poles to protect them from cars running them over. The design features a lighting, powered by a solar panel and activated by a light sensor swich.

The questions i have: If i would like to power up 30 pcs of 3V 20 ma LED-s with a solar panel, and use a light sensor to activate and deactivate the lighting, what kind of/performance solar panel should i use, how big performance battery do I need to store the power, and what kind of light sensor do i need to complete the deal, finally: How do i put it together into a foolproof system, this will be cast into concrete.

I live in Europe, Hungary

Any help appriciated!

 

[Rich D.]

There is no such thing as a fool proof system. They will always be able to produce a bigger fool.

First, I would make sure it has a gasketed access cover. If you build more than a few, some WILL fail, and you would either have to throw the whole thing away or just accept that you didn't need it in the first place.

Given those two thoughts above, you want to power a 3V, 20mA LED - 30 of them. That is 1.8 watts of output, or somewhere in the range of < 1 amp. Assuming you have current limiting resistors, that could easily take another watt of output power for full brightness, so figure 2.8 watts OUTPUT. (Current regulators could help that a bit, but not linear current regulators.)

I don't know the exact efficiency of any battery system, but I do know it is not 100%, not even possible. Let's figure you get only 2/3 of the energy back. That means you'd need to deliver 3.73 watts, we'll round that up to 4 to keep it simple.

You must now specify how long you want these LEDs to remain lit. One hour? A long winter's night after a cloudy day? Take your number of hours multiplied by the 4 watts of output and that gives you some idea of how much energy storage you will need.
I'd guess to last half the night after a cloudy day you'd need about 40 watt-hours of energy stored. You'll need to adjust this to taste. The type of battery you use will determine the actual voltage stored and will likely influence the final design of the LED wiring.

If we for now figure we'd need 1 amp and maybe 10 hours of run-time, that would mean a battery rated at 10 Amp/hours. That's a fairly big battery that could cost $25 or more, and if we figured needing a full 10 hours of charging time from a solar source, the solar battery would have to be able to deliver at least a few volts above the battery voltage at 1, or more likely 2 amps of charging current to account for the inefficiencies and clouds. I'd GUESS 6 volts at 2 amps or a 12 watt solar battery at least. The desired number of hours of illumination greatly effect the size of the solar battery.

These are all estimates based on my guesses as to the performance desired for this design. If you were to provide specific numbers then the design could be specified more accurately.

In the end, it is far more economical to cover the thing in those cheap plastic retro-reflectors of assorted colors.

 

[ChrisP58]

The first thing I would suggest is to forget the idea that all LEDs need 20mA. Instead, choose a high efficiency LED that will put out the brightness you need on much less current (1ma or even less). You will pay a little more for the LED, but you will save even more in the smaller required battery and solar panel.

Next, run the LEDs is series, not parallel, and drive them with a current regulating boost converter. That way you have no wasted energy in all those current limiting resistors. This would be one candidate, but there are many to choose from.

**broken link removed**

 

[dr pepper]

Welcome to eto.

This is all ballpark, but it'll give you some direction.

Assuming you drive the led's at 10ma to give reasonable light, you have 30 of them, so thats 300ma at 3v.
If you use a lipo battery and a switching led driver, and suppose the lipo is a 3c 11.1v you'd need 11.1v/3v = 3.7, so 300ma/3.7 = 81ma from an 11.1v battery.
81ma x 12 = 972ma, so 3c 972mah, seeing as the switching driver will not be 100% efficient battery specs tend to be optimistic then a 2 ah battery would be a good start.

The the solar panel, assuming you have 10 hours of daylight and a 3v cell, then 972/10 = 97ma, however solar cells are way over rated these days so I'd triple that at least to 300ma, maybe 500ma.
You could use a higher voltage cell with a charger circuit, in fact with a 500ma cell you'd really need a charge controller.