Hi there,
I'm looking to build a solar-powered sign with LED's but I would like the solar panels to be long and narrow (less than 1" wide). I need a total solar panel length of about 20-24 inches but any other size will be considered. I found thin solar cells like this but they're 1.5" wide, a bit too wide for me.
Also, what's a good way to attach these solar panels, especially if they don't have an adhesive backing? Is there a good type of glue that can be used for that? I think what they do with products like solar powered garden lights is route the wire to the inside of the enclosure under the solar panel and epoxy the wire up to waterproof it, but how do they actually attach the panel to the surface?
To summarize, here are some specs that I need:
- Less than 1" wide preferably
- At least 12" long, but up to about 22" is OK
- I'd rather not have to put multiple cells in parallel. I'm thinking I need 4.5-6V.
- I'm driving a bunch of RGB LED's, about 50 of them when it gets dark outside. Wonder if that's even possible because they draw about 20mA on average, so that's a heck of a lot of power for a 1W or less solar panel
- What's the best battery to use? I was originally thinking standard 3.7V Lipo batteries but I'm also considering Ni-MH or Ni-Cd batteries. However, I heard Ni-MH have bad discharge drainage.
- I need to attach the solar panel in a way that's waterproof
Let me know if you have any ideas, thanks!
You need to calculate the power use and plan your solar panel/battery storage accordingly. For example, 50 3.2V LEDs at 20 mA each will consume about 3.2 Watts.
LiPo batteries are extremely problematic and will be completely destroyed in the very first charge/discharge cycle, if you do not follow the rules carefully. You need special circuitry for both charge and discharge control.
Yes, I understand, I would be using a Lipo battery charger IC chip like the MCP73831. Are there RGB LED's with much lower current rating? From what I've seen, some 20mA LED's are way brighter than other 20mA LED's. It seems there's no direct correlation between current and brightness so I'm wondering if there are some low-current ones that are decently bright as well.
You should make some calculations/estimates, but these are only estimates... You'll have to experiment.
My gut feeling is you're going to need more solar cells (or fewer LEDs).
- I'd rather not have to put multiple cells in parallel.
The one you linked to is rated at 2V so you'd need to wire them in series for more voltage. But the wattage will also increase proportionally with the number of cells.
Don't forget the time-factor. Are you going to run the LEDs all night? It's a matter of Watt-Hours. You need to collect and store the required total energy. And, you're not going to get 1W from a 1W solar cell all day long... I assume they are optimistically rated so you can probably get 1W on the equator at noon during the summer... A 1W solar cell isn't going to run a 1W LED all night because you're not going to collect 1W all day long.
There will also be some energy loss related to charging/discharging the battery... You are going to get less energy out of the battery than you feed in.
The "normal" way of driving a "normal" 20mA LED with a current-limiting resistor is about 50% efficient with power lost in the resistor. That's "normally" not an issue with small low-power LEDs but in your application with 50 (actually 150, since they are RGB's) driven with "limited" power, the lost energy could be an issue. And, it's not easy to make an efficient/switching driver for many RGB LEDs because you can't wire RGB's in series.
[quite] From what I've seen, some 20mA LED's are way brighter than other 20mA LED's. It seems there's no direct correlation between current and brightness so I'm wondering if there are some low-current ones that are decently bright as well.[/quote]There is lots of variation, so look at the 'mcd' rating. But I wouldn't say "no correlation", as there are no 50mW LEDs that are as bright as a 1W LED.
You don't have to run an LED at it's maximum rated current, and there are LEDs that are as bright (or brighter) at 5mA than others LED at 20mA. (But, I don't know if they make "super bright" RGB LEDs.)
Thanks for the responses guys!
So since the resistor wastes a lot of energy, is it possible to just directly PWM the LED's with a MOSFET or transistor without using a resistor in series with the LED? The only negative consequence would be accidentally using 100% PWM and burning out the LED's. (Assuming something like 3.7-4.2V for a lipo batt).
Also, I did a little sniffing around in Digikey and found this blue LED that draws only 2mA but gives out 1600mcd!!! Insane. I'm now looking for an RGB LED that mixes 3 LED's of similar specs inside. 
is it possible to just directly PWM the LED's with a MOSFET or transistor without using a resistor in series with the LED?
NO.
PWM sets the time average voltage, not the instantaneous voltage or current. If you leave out the resistors you will burn out the port pin, the LED or both.
The rough estimate I gave above, 3.2 Watts for 50 LEDs, did not take into account anything else. 50 LEDs at 20 mA each would consume 5 Watts from a 5V power supply.
OK I see, that makes sense.