I cant get this common cathode tri colour led to switch on

Discussion in 'The Projects Forum' started by RodneyB, Mar 11, 2013.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I am trying to switch on a common anode LED as per the inserted diagram. Having no luck and cant see what I am doing wrong


    I would also really like to know how to get an image to show in the message panel and not as an attachment
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Does it light when you bypass the transistors and base resistors, and just use the battery, current limiting resistor and LED?

    Even though your schematic isn't labeled, I presume that you are connecting the positive of the battery to the anode of the LED. Correct?
     
  3. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    It does light when I put the positive on the common Cathode ad the current limiting resitor to the Anode and the negative.

    I am connecting the positive of the battery to the anode of the LED

    The 3 LED's in my diagram are the tri colour LED with a common Cathode.
     
  4. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Sorry its common CATHODE

    I am connecting the positive to the common cathode and then from a resistor to the negative it lightds
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Oh. That explains it. Positive goes to the anode.

    You apparently are confusing the anode and cathode. The cathode is respresented by the cross bar at the point of the arrow.
     
    Last edited: Mar 11, 2013
  6. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I am really confusing EVERYTHING

    The TRI Colour LED is common Anode

    When I put the positive on the common Anode and then through a current limiting resistor to each of the cathodes the individual LED's light.

    But I can not get it to switch through the transistor
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    So, the battery is good.
    The LED is good.
    The schematic is correct.

    Possiblities are:
    1. bad transistors - unlikely
    2. wiring error - likely
    a. Looking at the front flat of the transistor, the legs are e, b, c.
    b. Switch could be wired wrong. Try eliminating the switch.

    Post what you find.
     
  8. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I now have a situation stranger than fiction! The circuit is working If I Use a 10K or 1 K resistor it switches the LED on. BUT, if i touch either side of the bsase resistor it switches on as well.and all that is happening is the touch of my finger is enough to get it to switch on
     
  9. edwardholmes91

    Member

    Feb 25, 2013
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    It sounds like the transistors are acting as amplifiers and making it almost touch sensitive. Also to answer your question regarding how to get images on posts. Click the image button and then type in the link where the image is located. You must host the image elsewhere for this option. Otherwise to host on the forum then you need to use the attach option I believe.
     
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Your touch is likely coupling AC to the base of the transistor and switching it on. You can see it on an oscilloscope when you touch the probe.

    Glad you got it working. What was the problem?
     
  11. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    There were a couple of things. Firstly I had a brake in the jumper cable on my Breadboard. And I had the transistor the wrong way. I am yet to buy an osciloscope, How does my touch couple AC and how can I stop it.
     
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You can't stop it without getting away from all AC wiring like the wires in your house and on the poles outside your house. It is radiated from the wires, and your body acts as an antenna. Then, when you touch some conductor (like the resistor lead,) the AC is connected to the conductor.

    I have built a lot of breadboards, and I like to think I am pretty good at it, but when I have a problem, it most often turns out to be a wiring error. Don't forget to repair that jumper wire, or throw it away; you don't want to fight that battle again.

    Good luck with your next project.
     
  13. edwardholmes91

    Member

    Feb 25, 2013
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    Thanks for the explanation, I never knew that and it's interesting to know! :)
     
  14. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    thank you very much for all the help and advice
     
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