I am trying to build a circuit which can drive six LEDs using two/four super-capacitors. I came across Sparkfun's Energy Harvester board here. I also want to use LDR to switch the LED drive circuit. How should I proceed? Thank you.


Six White LEDs: 3.2 volts, 20 mA

LDR's switching voltage: 0-2 volts (ON), > 2 volts (OFF)

Super-capacitors: 6 volts, 10 F ( I'm not sure this will be enough or not )


closed as too broad by Olin Lathrop, ThreePhaseEel, Voltage Spike, Daniel Grillo, Dave Tweed Nov 29 '16 at 18:19

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ There is too much unknown. Six what kind of LEDs? What voltage drop? What current? Have you even looked at the total energy required? No, not gonna follow a link to get pertinent information to the question. Under what conditions should the drive circuitry be switched on or off due to input from this LDR? You really should be able to see for yourself that this "question" is way too vague. This site is for engineering, not hand waving. \$\endgroup\$ – Olin Lathrop Nov 25 '16 at 20:33
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    \$\begingroup\$ You're now starting to give a bit more data for you to answer your question. You can work out your LED power now, so what total energy will your LED's use in your as yet unspecified run time? What energy are your supercaps capable of storing at 6v and 10F (0.5CV^2 to save you looking it up) \$\endgroup\$ – Neil_UK Nov 25 '16 at 20:50
  • \$\begingroup\$ A circuit containing a battery would make it easier. \$\endgroup\$ – Spehro Pefhany Nov 25 '16 at 20:57
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    \$\begingroup\$ What kind of car engine can drive for 48 hours on a tank of fuel? Not to be rude, but EE stackexchange is more of a critique site, people put up their own designs and we give insight as to their effectiveness. If you don't already have a design, I'd recommend the eeVblog. Questions here follow the form "here's something that I've designed/built/simulated/tested, what problems does it have? Or why doesn't it do what it's supposed to? " \$\endgroup\$ – Sam Nov 25 '16 at 21:37

Start by calculating the basic energy required. There is no excuse for "not sure this will be enough". Don't guess. Do the math.

Six 3.2 V at 20 mA for 48 hours comes out to 66 kJ. 10 F at 6 V is 180 J. Clearly that's nowhere near enough. Checking this should have been the first and obvious thing to do, as I mentioned in the first comment to your question.


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