Help with thermometer style led display

Discussion in 'The Projects Forum' started by vstheworld, Jan 12, 2012.

  1. vstheworld

    Thread Starter New Member

    Jan 8, 2012
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    Hello all,
    I'm very new to EE and am hoping someone can help me with a project I'd like to do.

    Basically, I want to hit a switch and light up a sequence of (5) LEDs one at a time and keep them powered on. The order would be (3mm LEDs) red, yellow, white, green, red. If it's not too difficult to also have the final three light up at the same time. I'm hoping to be able to fit this in a 3" by 3" enclosure and power it by battery.

    As far as I've been able to determine it seems that I could use resistors in between the LEDs to create a cascading effect or if I'm understanding it correctly I could use an LM3419 display driver.

    Anyone who could help me along, explain the steps involved, schematics or the like would be greatly appreciated. Thanks!
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I think you mean LM3914. If so, the attached datasheet should help.
     
  3. vstheworld

    Thread Starter New Member

    Jan 8, 2012
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    Indeed I did, sorry for the typo. Are there any benefits or drawbacks to either solution?

    And thanks for the datasheet!
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    So, do you want this?

    Press switch and first LED lights.
    Press switch and second LED lights while first LED stays lit.
    Press switch and third, fourth, and fifth LEDs light while first two LEDs stay lit.

    Or do you want to press the switch only once and the above sequence to take place? If so, how long between each step?
     
  5. vstheworld

    Thread Starter New Member

    Jan 8, 2012
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    I'd like to press the switch once and have this sequence take place (pressing the switch while the LEDs are on would turn them all off):

    LED red on, add LED yellow on, add LED white, green and another red on at the same time. All LEDs to stay lit after they turn on. Alternatively, if it would make it easier for me instead of the final three LEDs turning on at the same time they could turn on in the order listed (on R, on Y, on W, on G, on R,).

    As for the time between each sequence, there doesn't need to be much/any pause. I was thinking the entire sequence would take about 1 second or so...
     
  6. Markd77

    Senior Member

    Sep 7, 2009
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  7. BillB3857

    Senior Member

    Feb 28, 2009
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    You could use an alternate action power button for power and use an R/C time constant to ramp the voltage input to the LM3914. A discharge resistor across the cap would reset it for the next usage.
     
  8. vstheworld

    Thread Starter New Member

    Jan 8, 2012
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    Thanks for the info guys! Working this weekend but I'm going to sit down and look at it tonight.
     
  9. vstheworld

    Thread Starter New Member

    Jan 8, 2012
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    Status update, I used the LM3914 and have everything working correctly. I just have to recalculate the resistor to speed up the sequence. So thanks for pointing me in the right direction! One more question though, if I wire up a SPST switch it powers up all lights at once instead of in sequence. I'm pretty sure I'm missing a very simple step here... Any thoughts?
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That's confusing to me. First you say you need to speed up the sequence, and then you say they all come on at once. This apparently has something to do with your switch. Can you post your schematic?
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    If you are using the R/C time constant to ramp voltage into the LM3914, you need to provide a discharge path when power is off or the voltage will just stay there for a long time. That's what I meant by providing a discharge resistor in my original post. Again, it would be of great help if you could post your schematic.
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A diode across the charging resistor, cathode to +V, will provide the discharge path without affecting charging.
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    Great idea! Wish I had thought of it:)
     
  14. vstheworld

    Thread Starter New Member

    Jan 8, 2012
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    To try and clarify:
    If I plug a 9v battery into the breadboard the LEDs light up in sequence but slower than I want so I think that I need to lower the resistor.

    My problem that I haven't solved yet is separate from that, if I wire the battery to an SPST switch and wire the switch to the board all the LEDs turn on at the same time.

    The schematic I've attached is from another electronics forum that I found from a while ago. Also, the switch isn't on the schematic I followed. Thanks!

    Whoops! Just noticed that this thread went to 2 pages, looking at your schematic and info now.
     
    Last edited: Jan 19, 2012
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