Tri-color LED circuit request

Discussion in 'General Electronics Chat' started by Galt1069, Oct 22, 2009.

  1. Galt1069

    Thread Starter New Member

    Oct 22, 2009
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    Hello! I must say I'm very happy to have found this forum, with such knowledgeable and helpful people. Generally I read in depth before posting a question but I'm under a bit of a time crunch and am hoping I can find help here.

    I'm trying to wire up a Linrose Red / Green / Amber LED (Part #B4361L1/5-LED - http://www.linrose.com/) with a switch so I can switch it to Only red, only green or amber. LED box says to power both red & green to get amber. I'm trying to do this with just the LED, some 0.25 Watt / 100 ohm resistors, and power from one AA battery (~1.5V). Does anyone have a quick and dirty solder circuit that might work for this?

    If you're wondering, this is to be used to light a pumpkin in lieu of a candle for a pumpkin contest at a party my kids are going to, which is tomorrow, hence the time crunch. Would've preferred some lead time, but coulda woulda shoulda.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    There is no real "datasheet" for that particular LED.
    It does not say whether it has a common anode, or a common cathode.

    If it has two long leads and one short lead, the short lead is likely a common cathode.

    If it has one long lead and two short leads, the long lead is a common likely a common anode.

    Which is it?

    There are two different forward voltages; one is 2.1v the other is 2.8v. The current was not specified. 20mA is likely to be a safe assumption.

    The size resistor you will need will depend upon your supply voltage. I suggest that 4.5v (three AA, C or D size batteries) would be a good bet. Just two batteries would not provide enough margin for the current limiting resistors.

    Sorry, I have to leave momentarily. Perhaps someone else will help you.
     
  3. Galt1069

    Thread Starter New Member

    Oct 22, 2009
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    The LED has a common cathode. I'd be happy with just being able to turn on the Red or Orange, but since orange is Red and Green I'm running into a bit of trouble with the circuit.

    I downloaded TINA (www.tina.com) but it doesn't have a component that I can find for a tri-color LED. Still, I attached the sort of thing I have so far. I'm treating the two LEDs 1 & 2 as the red and green anodes for the tri-color LED. It's been a long time since electronics class, so please feel free to offer suggestions or corrections.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK, here are a couple of possible ways to connect things up.

    [​IMG]

    The left schematic has two switches; that way you have independent control of the two colors in the LED. If they're both on, you get orange.

    On the right, a single switch turns both LEDs on.

    Note the value of the resistors attached to each LED anode.

    On the left, two 1.5v batteries are used; the red LED needs a 47 Ohm resistor, the green a 10 Ohm resistor.

    On the right, three 1.5v batteries are used; the red LED needs a 120 Ohm resistor, the green needs an 85 Ohm resistor.

    I recommend the use of the three batteries; the LEDs will have a much more uniform brightness over the life of the batteries, and will be less risky - particularly in the case of the green LED.

    To calculate a current limiting resistor for an LED:
    Rlimit >= (Vsupply - LED_Vf) / Desired_Current
    In the case on the right, the red LED has a Vf of 2.1, the green, 2.8 (+/-5%) and Vsupply = 4.5v; and the LED forward current is assumed to be 20mA. So:
    Rlimit >= (4.5v - 2.8v) / 20mA (green LED)
    Rlimit >= 1.7 / 0.02 A
    Rlimit >= 85 Ohms. 85 Ohms is a standard value of resistance. If it were not, you would choose the next larger value of resistance.
     
  5. Galt1069

    Thread Starter New Member

    Oct 22, 2009
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    Cool, thanks a mil! Sure hope you enjoy circuits because with all the help I've seen you give people here you deserve some serious kudos.

    Curiosity brings up some questions:
    This is essentially a stand-alone circuit, meant to sit in a pumpkin with no wires going to house wiring or the like. Yet I see many such circuits with the ground symbol on them, as you did in these. What physical components (if any) does that circuit symbol represent?

    On the single switch circuit why'd you put the switch after the LEDs instead of before it?

    I started to try to work up a schematic to do all three colors, but ran into a roadblock of how to do it with just the switches I have (#35-005 GC Electronics SPDT Switch, rated 5A, 125VAC, 28VDC, 2A 250V AC) but ran into an issue with trying to get three results out of two "On" states. I seem to remember something about using crossover wires to do it, but that was a long time ago, and was in housing wiring.

    Btw, that isn't a request to get another circuit out of you, unless you're just bored. It's more a note for me to research how to do it later when I have time. :)

    In the meantime I'll use the ones you posted, and thanks again for them! I'm sure the kids will be overjoyed to have light in their pumpkins despite the party's "no flame" rule.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Well, it's as much to refresh my memory as to give other folks a hand. Everybody wins. ;)

    Ground is simply a zero volt reference point. This is a requirement for running simulations using PSPICE.

    Because you have a common cathode LED. By disconnecting the common cathode from the battery, current through both the red and green LEDs is turned off.

    In house wiring, three-way switches (actually, SPDT) are used in circuits where you want two switches to be able to control a circuit.

    I just used two SPST switches. I could've used two SPDT switches, but using just the common terminal and either the NC or NO. One of the connections would have been unused. It would've looked basically the same.

    No need to, really. As I said above, it would be electrically equivalent to the switch configuration in the left schematic.

    You're welcome ;)
     
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