I’ve built myself a couple of Sun Jars using the electronics from cheap solar garden lights. No problem, except that the LED isn’t very bright. It’s a very small LED and putting a volt meter on it shows its only getting 1.1v (from the AA battery. The solar panel recharges that battery). Any suggestions on how to make this thing brighter? I have a handful of superbright LEDs, but they’re rated at 3.3v and when I hook one up it’s incredibly dim. Are there brighter 1.2v LEDs I can buy? Any other ideas?
use two or more solar cells to charge several batteries in series. This will drive your high-brightness LED.
You have to be fairly careful running LEDs from a voltage source. Their brightness increases fairly evenly with the current. The problem is that the current doesn’t increase evenly with the voltage (in electronic-speak, they are non-linear devices). So, as you increase the voltage, they go from dim to really bright to going POOF over a very narrow voltage range. Typically, an LED isn’t going to be driven directly by a battery. Usually there is some sort of current limiting device in there, such as a dropping resistor or a current regulator. If you directly drive one from a battery, you have to be fairly careful about matching up the voltage of the LED with the voltage output of the battery.
You can put two AA batteries in series which will double the voltage. Since you aren’t exceeding the 3.3 volts of the brighter LEDs, that should drive them a bit brighter. The problem you may run into though is that your solar cell doesn’t put out enough voltage to charge two AA’s in series. You may have to add another solar cell or change to a different solar cell.
Gotcha. I guess I was kind of hoping that the original manufacturer just didn’t use the best LED and I could replace it with another, brighter one. I guess the other option is, as you mention, using 2 batteries and possibly 2 solar cells. Can I just wire the solar cells in series to the same board, or is it more complicated than that?
Anyone know where to buy bright, low-voltage LEDs?
If you want to increase brightness without increasing voltage, use several of the LED in parallel.
Although that may end up increasing the voltage as well, if a plain old resistor is used to limit current.
all of the white LEDs are going to be at least 3v. You can get bright red LEDs that are 1.2v
What color is the LED?
True. But generally speaking, it’s better to operate matched LEDs in series vs. parallel. Of course, this means you need a higher voltage.
For maximum brightness and efficiency, multiple LEDs usually work better than one big LED – that’s why flashlights usually use multiple LEDs. Given the fact that most run-of-the-mill red LEDs have a forward voltage drop between 1.5 and 2.0 VDC, I’m wondering if the best solution is to simply use two AA batteries in series and two red LED in series. (Yes, there would be no current limiting component or regulator. But given the fact that the operating voltage drop is specificed to be at least 1.5 VDC, it will probably be O.K.)
This is probably way more complicated than what you want, but here’s something else to consider: PWM. I have seen a number of designs that pulse an LED using a driver with high frequency, low duty cycle, and high peak current. (One engineer in our lab powered a “20 mA LED” using a 3% duty cycle and a peak current of 1 amp.) By selecting the right frequency, peak current, and duty cycle, your eye won’t know the LED is flashing on-and-off. The upshot is that you’ll end up with high brightness *and * high efficiency.
You could try the joule thief circuit:
http://www.emanator.demon.co.uk/bigclive/joule.htm
Other sources claim this circuit is around 70% efficient, but at least it’s easy to fudge together.
You might find out what kind of LEDs Deep Creek Designs used for their FirstStar light?
The brightest easily available LEDs are from LumiLeds. They are the “Luxeon” series, and are stocked at Mouser.com. There are Luxeons available at up to 5W.
Thanks for the replies and ideas, guys. Remember, though that since this is a self contained solar project, increasing the battery voltage will need more solar cells. Can I just add more cells to the system (see post #5)?
Read over the in-depth technical detail section of the site I linked for possible ideas, perhaps you could brainstorm with the people who make those lights?
You can wire solar cells in series with no problems. Depending on the type of batteries you are using, you might be able to get away with a very simple charge circuit (like none). The devil is always in the details. We need to know how much brightness, for how long after sunset you need to recommend a battery/LED circuit.
Well, here are the basic details:
In the current configuration, there is a 1 sq. in solar panel and a single 600mah AA NiCd, which drives a very small amber LED (seems to be even smaller than a standard 3mm). During the day, the solar cell (trickle) charges the battery, and at night the LEDs runs off the battery. Not sure the current draw of the LED, but it’s low enough that the light will stay on all night (or at least 8 or 9 hours), and will fully recharge during the daylight hours.
I assume that if I add another AA battery to drive a much brighter 3.3v LED, the single solar panel won’t put out enough voltage to charge the 2 batteries. Also, the brighter LED would pull more amps, right? So without a higher capacity battery (2000mah NiMh) it would run dry a lot sooner and probably not stay on all night (not that it needs to be all night, but should be as long as possible). Of course using a higher capacity battery means that the solar panel(s) wouldn’t recharge it fully during the daylight hours.
Are you using a control circuit that came out of the garden light? If so, increasing the voltage and/or current may screw up its operation. You may need to modify or replace it.
Come to think of it, I have no idea what type of circuit is in those lights… I suppose something (transistor or op-amp) senses the current from the PV cell and switch the LED on/off?
The circuit is quite a simple one. There’s basically LED, diode, transistor, resistor, and (I think) a cadmium sulfide cell. Apparently the diode keeps the voltage from the batteries to going back into the solar cell during periods of darkness. The the cadmium sulfide is just the light sensor and it’s separate from the solar cell, though it’s mounted on top.
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