LED circuit indicator

Discussion in 'OnBoard Electronics & Controls' started by bcervelo, Sep 13, 2013.

  1. bcervelo
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    bcervelo Junior Member

    I would like to use a 2 colour LED as a backlight to illuminate a circuit label, i want the label to be red when the switch is open and green when closed.

    Can someone provide a circuit diagram of how this would be wired?
     
  2. BertKu
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    BertKu Senior Member

    Could you give us the manufacturer type number and if possible the make. We can help you then easier. Bert

    Red is normally 1.8 Volt and green approx 3 - 3,5 Volt. Thus to enable us to make a proposal, the type number will help.
     
  3. BertKu
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    BertKu Senior Member

    We do understand you correctly, you do not want 2 separate LED's, but a multicolor LED. Bert
     
  4. BertKu
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    BertKu Senior Member

    Here is the circuit diagram for 2 separate LED's. The devil is in the details. We don't know whether you can accommodate a small leakage current of 1,2 mA at 12 Volt, when the switch is in off position. Good luck. Bert
     

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    Last edited: Sep 14, 2013
  5. BertKu
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    BertKu Senior Member

    The working of the circuit.
    If the switch is in open position, the red led receives its current via the resistor. The transistor is in open position, because the switch is open. Thus the RED led is on.
    As soon the switch is made , the green LED gets its current via the switch and also the transistor closes over the red LED, The RED Led get short circuit and is then off, thereby your application is complied with.
    Bert
     
  6. bcervelo
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    bcervelo Junior Member

    1 person likes this.
  7. BertKu
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    BertKu Senior Member

    Bad news. Those LED's are only 5mcd, instead of 12.000 or 16.000 or 18.000 mcd. What it means is that you have to push more current through the LED, to get a reasonable minute light shining.

    Secondly, I could only see 3 wires, i.e. internally it is connected and that means , it is impossible in what you want to do nor that you could use the proposed circuit.

    If you have 6 weeks the time, I gladly post you per seapost 1 x Red LED 16.000 mcd, 1 x Green 13.000 mcd , 4 resistors, 1 x BC237B transistor and 2 chrome LED holders to a poste restante postal address. In that way you don't need to give your identity away. Just give me a code name and give me this information in a private mail to BertKu. With compliments of South Africa.

    Maybe one day you could do me favor back . One never knows.
    Bert
     
  8. jonr
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    jonr Senior Member

    You can make it work fine with a small SPDT relay. That will allow 12V to be applied to either the red or the green LED depending on if the primary side is powered.
     
  9. BertKu
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    BertKu Senior Member

    It maybe true, but it does not change the fact that the LED's at high currents has only 5mcd. A small relay takes at 12 Volt 25 mA, except maybe some expensive types. The devil is in the details. Question , how much current is it allowed to be wasted? Jonr, just show us your drawing for him. Don't forget that the LED's are internally connected. Bert
     
  10. BertKu
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    BertKu Senior Member

    There is another solution. If the load is with a low resistance or low impedance, one could have one LED with a resistor over the contact and one resistor after the contact in series with the second LED to earth. But again only with LED's with high mcd's and thus drawing very low currents. The influence of the low current via the load should be fine. However again the devil sits in the details. Can you allow to have a low leakage current flowing through your Load?
    Let say 0,5 mA. Bert
     
  11. BertKu
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    BertKu Senior Member

    Here are the various circuit diagrams. Also the one from JONR , I assume that JONR suggested a read relay. I personally feel that semiconductors has a longer faultless lifetime then relays, but JONR suggestion is not a bad one, as you can use your proposed type of LED holder. You may have to solder 2 resistors in series of the LED's. I can't figure out whether they are included in the holder.
    I would take the optocoupler or the transistor solution, but that is your choice.
     

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  12. BertKu
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    BertKu Senior Member

    Just a note to Thread No 11. In circuit C. Should the load fail, open circuit, you will be able to see it straight away. The RED LED's would not go on, when the switch is in off position. Therefore higher mcd LED's are recommended. A LED has a lifetime between 50.000 - 100.000 hours and what it means is that at the normally 20 mA current, the intensity may have dropped to 80% only. In your case, the LED is only drawing very low currents, 1 to 2 mA, or even lower as required and therefore should last forever. Bert
     
  13. bcervelo
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    bcervelo Junior Member

    Bert thanks again for taking the time to drw and explain various solutions.

    Would the circiut be simpler if i sourced a 12v LED?

    I want to but them on back of a small panel with 9 switches for differet loads on a RIB, the panel needs to be as small as possible so space on the back is tight.
     
  14. BertKu
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    BertKu Senior Member

    This is all what you need for space for 2 LED holders and 2 x 5 mm LED's. the resistor is a 1/4 watt , 5 % . I took a 56 kilo Ohm resistor and see what bright it is. i.e.0.17 milli Ampere I understood , you want to do back lighting. You cannot do that with 5 mcd only. You need at least more than 1000 mcd. I pay for resistors per 100 >> 0.0002 English pounds each. Your problem will be to experiment first with ONE Led and a resistor to see what brightness you need. But 0.17 milli ampere will not do anything to your load. Except for the green Led I would use maybe even a higher resistance and for the red led a lower one like 18 kiliOhm. The reason is, if your load fails, the red LED will go off when the switch is in open position and the load fails and does not draw a current sufficient to lighten up via the green LED.
    By the way, get 2 mm or 3.2 mm shrink sleeving. Cut the resistor at both ends 5 mm, cut the plus of the LED shorter (longest leg) and solder than the resistor and red and black wire as short to the resistor as possible and then put the shrink sleeve over the open wire and resistor and heat it up. Those two will be less space than what I saw on the Internet what you wanted to use. Do not go over the 10 milli Ampere if you like to keep it working for life. Good luck.Bert
     

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  15. BertKu
    Joined: May 2009
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    BertKu Senior Member

    The problem is, we know so little of your application and therefore we have difficulty in advising you an elegant solution. If it is back-light only, I am wondering why do you need led holders then? A piece of long raw pcb, drilling a few holes and sticking the LED's on it with the resistors and scraping a few copper places a way for insulation, maybe a better solution then all those wrong 12 Volt LED's in mounting with build in wrong resistors.
    Try to explain to us , what you like to do with lots of words and explanations. also if possible some photo's from your mobile phone like I do. Maybe we have even a better solution for you.
    Bert
     
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