Will this work?

Discussion in 'The Projects Forum' started by Phaedo, Sep 4, 2013.

  1. Phaedo

    Thread Starter New Member

    Sep 4, 2013
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    It is my girlfriends birthday soon and I thought it would be fun to make her an electronics based present. My idea is a circuit embedded in a bangle. Basically it involves a BPW34 mini solar cell which at peak outputs 50mA, a 3F capacitor and a red led. There will be a two way switch which either charges the capacitor or discharges it to an led. The switch will be triggered to discharge by a nearby magnet which I will carry and therefore be able to light up the led with a strength based on how long we've been away.

    Here's a rough idea:

    [​IMG]

    Will this work? I've figured I've got at least a week before the capacitor goes over its 2.7V limit but ideally there would be some way of stopping the charging at some point. Is there any way to do this? Thanks for any help!

    Phaedo
     
  2. adam555

    Active Member

    Aug 17, 2013
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    Why do you expect the capacitor to reach 2.7V in a week?

    What about during the nights; won't the capacitor discharge itself when the solar cell is not charging it?

    I don't see that working. :\
     
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  3. Sensacell

    Well-Known Member

    Jun 19, 2012
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    The LED would be damaged by the uncontrolled current that would flow were it connected to a fully charged cap- you need a resistor to limit this current flow.

    In general, the circuit would be rather disappointing, the LED forward voltage (1.8 V approx for a red LED) is a large portion of the peak voltage, as soon as the cap voltage dropped below this point, the LED would go out- leaving most of the available charge unused. Note that other LED colors might not light at all at 2.7 V
     
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  4. sirch2

    Well-Known Member

    Jan 21, 2013
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    Are you really sure this is what your girlfriend would want most for her birthday?
     
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  5. Phaedo

    Thread Starter New Member

    Sep 4, 2013
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    True, I didn't think of it discharging at night. Is there any way to prevent that?

    Thanks, I'll be sure to add a resistor, should I also add a diode to protect the solar cell. This is the first time I've messed around with capacitors. I did choose red because I knew it has the longest wavelength/least energy to work. Ideally what would happen is that the led would emit a decreasing glow depending on how long it had been since the last discharge, it doesn't have to be spectacular. The capacitance can be changed.

    Thanks for the help,
    Phaedo
     
  6. adam555

    Active Member

    Aug 17, 2013
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    Maybe if you also add a one or two small button cells...
     
  7. Phaedo

    Thread Starter New Member

    Sep 4, 2013
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    What so that the solar cell charges the cells and they charge the capacitor? Surely I might as well just use 2V worth of button cells in series then and scrap the capacitor?

    Edit: Or just to provide a tiny current to keep the capacitor from discharging?
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    First of all, you've waited too long. Go buy flowers and dinner and work on this for the next time.

    Consider using the circuit from a solar landscape light. Replace the LED with the one you like, and use the capacitor as the battery. I think this will solve all the issues raised so far.
     
  9. Phaedo

    Thread Starter New Member

    Sep 4, 2013
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    Haha, maybe you are right! It was only going to be supplementary anyway, more for my amusement ;)! I am happy with a red LED. What do you mean 'use the capacitor as the battery'? Thanks though.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Exactly that. If you want to use the capacitor instead of a battery, go ahead and swap it in. Or just use the battery - it'll hold a lot more juice for a given size and weight.
     
  11. Phaedo

    Thread Starter New Member

    Sep 4, 2013
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    I started of with a capacitor in, but a battery is seeming like a better idea now. But I don't know of a battery that could power the led and be charged by the solar cell and is small enough!
     
  12. wayneh

    Expert

    Sep 9, 2010
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    It's just a matter of how small you need. See this example.
     
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  13. Phaedo

    Thread Starter New Member

    Sep 4, 2013
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    The solar cell only outputs 350mV max as far I can work out though and that battery needs 2.8V-3.3V. Good find though!
     
  14. adam555

    Active Member

    Aug 17, 2013
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    Don't know what type of LED you are using, but regular ones need around 1.5V; so just one solar cell won't be enough.

    I have a miniature torch that uses 3 button cells in series (in a tiny space) to light just one regular blue LED.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    The solar light circuits all use a boost or "joule thief" circuit, so that only a single battery is needed.
     
  16. Phaedo

    Thread Starter New Member

    Sep 4, 2013
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    Do you know of an example schematic?
     
  17. wayneh

    Expert

    Sep 9, 2010
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    Take a look at this thread, and the other things it links to. You can also google for "joule thief" and you'll be overwhelmed.
     
  18. Austin Clark

    Member

    Dec 28, 2011
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    It might be easier to use button cells, as mentioned before, and a 10M ohm or so resistor connected directly to the Capacitor, so it is always charging. Then, using the magnetically triggered switch you mentioned earlier, you can connect the LED (through a resistor, to regulate current) to the capacitor. The capacitor will always be charging, even when the LED is ON, but that's ok, as it'll be a very small trickle.

    Another big flaw, however, is that if you only ever turn on the LED when you want to see how long you've been away, the capacitor will never have a chance to fully deplete, and it'll always be bright when you turn it on.
     
  19. adam555

    Active Member

    Aug 17, 2013
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    Just saw a video that might help you in your project. It's basically what you want to build, but without the timer. It uses 10 solar cells, a choke, a supercapacitor and a joule thief.

    http://www.youtube.com/watch?v=ghB2irHIN8I
     
  20. Phaedo

    Thread Starter New Member

    Sep 4, 2013
    13
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    That's awesome thanks! That and a reed switch would basically do what I envisaged! I could use less solar cells to control how often you'd need to flip the switch from off to on.
     
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