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Discussion in 'General Electronics Discussion' started by SolJoy, Feb 26, 2012.

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  1. SolJoy

    SolJoy

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    Feb 26, 2012
    Hi all, I just joined today :D , hehe I need some help with a little project I am trying to build. I got a power source (lets just say 3 volts) but it has very low current (high impedance?) and wont light a led when it is connected, so what I have been doing is using it to charge a capacitor (4700uf) which takes a couple mins to charge.which I then discharge to light the led for a few secs, but i have been doing that by hand. so what I am asking here is if someone could help me with like a switch that say allows current to flow from the cap to the led when the voltage on the cap is 3 volts lighting the led and when the voltage drops to say 2.99 it will turn off letting the cap charge to 3 volts again, thus creating a blinking led. Thanks again all. sorry for the newbie question :eek:
     
  2. bma1984

    bma1984

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    Feb 28, 2012
    What are the specs on the led? Most small leds require very low power to operate. Given your standard red led, it has a voltage drop of around 1.8v and a modest current draw of 20ma would light it up fine. If you have a 3v wall adapter, I find it hard to believe that it cannot deliver the load. I would grab any old wall adapter (I'm looking at a cell phone charger now that is rated at 5v 550ma which PLENTY of juice to power multiple leds) and use good ole Ohm's Law to find an appropriate valued resistor to limit the current for the given supply.

    Use my 5v adapter for example.

    5v - 1.8v (voltage drop of the led) = 3.2v
    3.2v would be the voltage drop needed across the resistor.

    Divide that by the current draw of the led (20ma typical) 3.2v/.02A=160ohm
    Now just grab a 160ohm resistor with a power dissipation rating of (5v*.02A=0.1Watts)
    So your little 1/8watt 160ohm resistor would work fine.

    Now just plug in your numbers for whatever supply you find and you are all set.
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
  4. SolJoy

    SolJoy

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    Feb 26, 2012
    OP

    Aye Harald Kapp thats what I am experimenting with; Earth Batteries and Joule Thiefs ^_o The Earth Battery by itself is very low current but it can charge a cap making it a high current (low impedance) for a short while, so if i can find a device to switch on and off like i said in the above post we would have a nice pulsing DC power supply, and we could always just make another one to cover the missing pulse of the first, thus we would have a nice DC power supply, I tried avalanching a transistor, but getting a earth battery to 9 volts in my living room isn't the easiest thing. I tested it once and was working kinda screwy long long recharge time on 9 volts like an hour, but I imagine if i made the earth battery 18 volts it would prolly charge alot faster. but ya to many experiments and not enough alligator clips :(
     
  5. SolJoy

    SolJoy

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    Feb 26, 2012
    hmmm i wanna try a 1000 volt earth battery thats 2,000 screws... very do-Able IMHO ^_^
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Are you using dissimilar metals, or are you trying to tap telluric currents?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Google "energy harvesting". That might just be what you're looking for.

    Harald
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    FYI an interesting video covering two Linear Technology devices for energy harvesting from thermoelectric and solar cell sources:
     
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