LED circuit array (please help)

Discussion in 'The Projects Forum' started by grimxneko, Jan 17, 2010.

  1. grimxneko

    Thread Starter New Member

    Jan 17, 2010
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    first off, im new to this site and also pretty new to electronics and stuff. with that said, i have a some questons...
    ok, so the situation is: im planning on making a fairly large infrared light sorce to mount on my video camera for night shooting. it's going to be an array of 20 infrared LEDs consisting of two different kinds of LEDs (ten of these big bright ones (Here), and ten of these mediocre smaller ones (Here)). also, im planning to use a 9V battery for the power sorce, and somehow i want to make the birghtness adjustable (like using a dimmer or something) along with an on/off switch. now, i set up a prototype kinda thing to see if the circuit would work and just to get a visual idea of what i want the final product to look like (Here). keeping in mind im a complete begginer, i dont quite grasp the concept of using resistors. so when i wired this all up, i simply left out any resistors. also, since this was just a test, i didn't solder anything yet. i just twisted the leads together like a twist tie. oh, and take note that i dont have a multimeter so i dont know anything about the specs or details of the LEDs themselves besides whatever the pages that i purchased them from said (in the first two links). soooooo here's where the problems come in....
    -i first tried to wire all the LEDs up in a parallel circuit but i noticed each additional LED made the whole thing dimmer. everytime i added one, the whole thing would become less bright. i tried this with up to 5 LEDs and gave up, thinking that by the time i got to my goal of 20 the whole thing wouldn't light up.
    -so next i tried a series circuit. it worked with a small strand of 5 LEDs so i went on to connect them all. when that was all done i hooked it up to the battery and none lit up. i was (and still am) very confused by this. i tried many times after that; checking the connections, checking the battery, checking the + and - leads, even checking every light for a dead one. this all yeilded to nothing but confusion and frustration. the results (when there actually were any) were varried as you can see here.
    -also, i would like to keep the same formation i have in the 3rd link (four columns of five LEDs with the two middle columns in the middle).
    -and i mentioned before that (assuming i get all this working right) i would like to have the option to control the brightness/ dimness of this. i really dont know how to do that (a variable resistor maybe?? idk), so that'd be nice to know too.

    so, in summary, im simply asking HOW?? how exactly should i wire all this up?? any help would be greatly appreciated.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The LEDs draw much too much current to be powered by a little 9V battery for more than a few minutes. Your little 9V battery is almost dead now. That is why they got dimmer as you added more LEDs.

    LEDs need to have their current limited but it looks like you did not use anything to limit the current so you are lucky that they didn't burn out.

    Wire them like you were shown on the other website.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. Doug Bell

    New Member

    Jan 17, 2010
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    Both kinds of LEDs are rated at 1.5 - 1.6 V. When you connected them in parallel to the 9 V battery, you may very well have burned some of them out by applying way too much voltage. The big ones must be current limited at 200 mA and the small ones at 90 mA. If your 9 V battery couldn't supply more than this or you didn't connect them very long, then the LEDs could still be ok.

    When you connected 5 in series, the voltage requirements added: 5 x 1.5 = 7.5 V. This is much closer to the right match to the 9 V battery. When you connected all of them in series, that was way too much voltage: 20 x 1.5 = 30 V, so they didn't light at all at 9 V.

    I would suggest connecting four strings of 5 LEDs each in series, and connecting each string across the battery. Don't mix the big ones and the little ones in the strings - make two strings of big ones and two of little ones. For a brightness adjust, put a separate switch in series with each string. Each string should be current limited by its own resistor in series.

    To current limit the strings of big LEDs, R = (9 V - 7.5 V) / 140 mA = 10.7 Ohms, P = (9 V - 7.5 V) x 140 mA = 0.21 W, so a 10 Ohm, 1/4 Watt or larger resistor will work fine. To current limit the strings of little LEDs, R = (9 V - 7.5 V) / 60 mA = 25 Ohms, P = (9 V - 7.5 V) x 60 mA = 0.09 W, so a 25 Ohm, 1/8 Watt or larger resistor will work fine.

    If you want finer control of brightness and/or to be able to adjust the brightness to compensate for the battery wearing out and lowering voltage, you could use a rheostat for each string. A 50 Ohm, 1 Watt rheostat would work for each string of big ones; a 150 Ohm, 1 Watt rheostat would work for each string of little ones. You would use these instead of the switches and resistors mentioned above. I chose those resistances so you can turn the LEDs down to about 20% of full brightness at full battery voltage. You still need one on-off switch for all four strings. The rheostats need to be higher wattage than the resistors mentioned above because you'll be using only a small amount of the resistance when you turn them up to full brightness, so the resistance element needs to be able to handle the max LED current over its whole length: P = I x I x R.

    You need to be careful not to turn each rheostat up too high. You may need to buy or borrow a small multimeter to check this. Use the Ohms setting and connect the meter across each rheostat. Adjust the 50 Ohm ones for 10.7 Ohms and the 150 Ohm ones for 25 Ohms. Mark where this is somehow, for example, if you use knobs with pointers, mark the case. For a new battery, don't adjust each rheostat for less than this resistance. As the battery wears down, you can use less than this resistance to compensate. If you turn the rheostats to too small a resistance, they'll get hot and the LEDs will die and/or you'll wear out the battery too fast.

    You can use a potentiometer as a rheostat, just don't use the right-most terminal of the three when they're facing up with the shaft toward you. That way, clockwise will be brighter, counter-clockwise dimmer.

    If you knew electronics, you could use a PWM (pulse width modulation) circuit to safely adjust to any desired brightness, and that would make more efficient use of battery life, but that's too hard for you at this point.
     
  5. Doug Bell

    New Member

    Jan 17, 2010
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    Oops - I mentioned current limits of 200 mA and 90 mA - those should have been 140 mA and 60 mA, limiting the power to 200 mW and 90 mW.
     
  6. grimxneko

    Thread Starter New Member

    Jan 17, 2010
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    thanks for your response and help, it actually made things a bit clearer. i understand about connecting 5 (of the same) LEDs in series, and i also appreciate now knowing what resistors go with them (though i barely grasp the math involved, lol). but to be honest i got totally lost with the rheostat thing. like, i pretty much understand the concept and what a rheostat is (though is it the same as a potentiometer?), but i dont really grasp anythign else you were saying about it. sorry ^_^; like are you suggesting using multiple ones (like one on each string?) which in that case, i dont understand the reasoning in that at all.
    actually, let me sum this up: i basically want to know where everything goes. where should i put everything exactly? that's really the thing that's throwing me off.
    also, from what i gathered, i tried to put together what im picturing this would look like. i doubt that it's right and i never made a schematic before, but is this right? oh, and as for the variable resistor i used in it, someone else suggested it and i really dont know if the Ohmage is right (or even if you'd measure it in Ohms). haha, sorry im so new at all this. anyway, i guess just correct me and let me know what to change.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Looks good to me...

    [​IMG]

    According to my calculations (assuming 1.5V Vf of the diodes) you have 60ma, 150ma, 150ma, and 60ma.

    The resistors can be used to measure real current. Measure the voltage across each resistor, then divide that voltage by the resistance. If the resistor is dropping 1.5V, and it is 25Ω, then it has 60ma going through it.
     
  8. grimxneko

    Thread Starter New Member

    Jan 17, 2010
    3
    0
    oh, cool. thanks.
     
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