NEED HELP with switches in a circuit

Discussion in 'General Electronics Chat' started by bck1990, Apr 20, 2011.

  1. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    Hi everyone,

    I am building a circuit with a row of 8 alphanumerical 16 led segment displays. Say for example I want it to show 'HAPPY' for 1 second, pause for 0.5 sec, 'BIRTHDAY' for 1 second, pause for 0.5 sec, '<NAME OF PERSON>' for one second, pause for 0.5 sec and repeat the cycle, all these are shown on the same row of the led segment displays.

    I plan to keep it extremely simple. So I will set up one circuit to light up HAPPY, one to light up BIRTHDAY and so on.

    But how do I get it to flash one word and pause before flashing the second word as described above?
    Like switch on circuit 1, then pause, then switch on 2 and so on. Do I need a timer IC or something?

    Any suggestions guys? Thank you very much!
     
  2. Wendy

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    Mar 24, 2008
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    You could do this with a sequencer, but it is not the best way.

    The best way is with a µC. You could do this with a minimum of parts using a µC, reprograming it for new messages, and spend a lot less money. The downside is there is a serious learning curve. Two popular brand names (there a many more) for this kind of chip is a PIC and an Arduino.

    However, there is a 3rd option. You can buy them from Fry's electronics (look for electronic name tags) and many other sources premade. They are common, and cost around $20 give or take.

    Let us know how you want to do this.
     
    Last edited: Apr 20, 2011
  3. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    Hi, thank you so much. I will read up on the various alternatives you've suggested. Appreciate your help!
     
  4. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    Hi Bill, I was thinking of using the 555 timer ic and the cd4017 decade counter ic together. But the output from the cd4017 definitely does not have enough current to drive 8 16-segment displays right? So i was thinking of using a 3 BC547 transistors (each of their bases connected to each output of the cd4017) as an amplifier. Do you think that's a good idea?
     
  5. Wendy

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    Mar 24, 2008
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    If you go that route, yes. You can also use small MOSFETs, I think a good part is 2N7000.

    The sequencer is simple enough, but heavy on parts. Display1, blank, Display2, blank, Display3,blank. That is 6 steps. Simple diode steering will do the rest. You may want to increase the number of steps to adjust the timing, but the principle is the same.

    Fry's and other places have electronic badges online. Are you set on building this yourself, or would you care if you could buy it online?

    The sequencer option is not very programmable, but a lot easier in skull sweat. The µC is extremely cheap, but heavy on the skull sweat, and easily reprogrammable to a different message.

    What is the size of the display (physical dimensions) you are aiming for? You have already mentioned it will be 8X16 LEDs. Not all the LEDs will be needed if it is a fixed message. Basically diagram which LEDs are on, then eliminate the ones that never turn on.
     
  6. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    I guess it will be fine without the blanks too. I would love to build it myself as a gift. No need for programming too since I just want it to show the same message. So the person switches the circuit on and the message starts to flash in sequence, that's all. I am using 8 of the 16-segment alphanumerical display chips (common anode/cathode). 8 of those chips. Not single LEDs.

    Thank you very much! Once again, will now read up about your options.
     
  7. Wendy

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    Mar 24, 2008
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    Think of sequencing in terms of animation (and the blanks are incredibly easy). Each picture follows the next. I use Paint, but you can use text if you want. Do you have the displays picked out? Part numbers please.

    The name you want to use would be nice too. I am a pretty fair draftsman, I do a lot of artwork and schematics around here.
     
  8. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    A fifteen segment display (part no. LTP588G) could be used as well. I can get these easily from an electronics shop, I will use BC547 as well as some capacitors, some resistors, and 555 timer IC and 4017 IC.

    I attached my schematic below (pardon my poor handwriting and drawing). Basically the row of LEDs represents each and every LED on the LTP588Gs so there are many of them.

    The 12V power source is not confirmed. Most likely I will be using cells for the power supply so the device is portable and could be switched on anywhere. =)
     
  9. Wendy

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    Mar 24, 2008
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    You have the basics, but you need to think how you are going to share some of the segments. This is where the steering diodes come in.

    I have looked very hard for the datasheet for that display (LTP588G) with no joy. Do you have a reference?

    Datasheets are your friend, always. If you can't find it on the web then you need to get one from your vendor.

    I just noticed this thread,

    http://forum.allaboutcircuits.com/showthread.php?t=53311

    I suspect you need to review what you think about LEDs. Chapter one of LEDs, 555s, Flashers, and Light Chasers is a tutorial about LEDs, you probably should review it. It pretty much explained what you asked and more.
     
    Last edited: Apr 20, 2011
  10. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    ok maybe a different one. this
    http://www.lumex.com/specs/LDS-E3904RI.pdf
    since i can get it locally as well.

    Yea i really do not know much about LEDs. I know from physics how it works ideally and stuff, but in reality I am really quite clueless. hmm in that datasheet it shows that the forward voltage is 2V, does that mean the forward voltage of all the LEDs are the same?

    what I wanted to do is to just connect the LEDs i need for them to light up. for example, the first segment display shows H (for happy) in the first circuit, the same display will be used to show B (for birthday) for the second circuit. hence there will be just so many wires everywhere, more than 20 wires from the first display alone.

    thank you for sharing the link. I really need to do some reading for now.
     
  11. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    Do you know if the max steady current of 30mA as stated in the datasheet means 30mA for each and every LED in the chip or 30mA when all 14 LEDs in it are working simultaneously? Meaning 30/14 mA for each LED?
     
  12. Wendy

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    30ma per LED. Sure wish I had a datasheet...
     
  13. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    the datasheet is in my previous post.
     
  14. Wendy

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    Thanks, missed it...

    You need to draw a diagram with the words highlighted.

    Unless the name is long you will need 8 of the displays for the word "BIRTHDAY", or is that "Birthday"?
     
  15. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    yes 8 of them. its BIRTHDAY.
     
  16. Wendy

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    Here is a template of the display LEDs. I think you have a problem. I'll be interested in what your solution is. It is why I kept saying you need to diagram the words first.

    Use M/S Paint to manipulate it.

    [​IMG]
     
  17. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    Think it would be better like that. The problem with these displays is that they are already connected in parallel. And adding a resistor in series to each and every LED would be crazy.
     
  18. Wendy

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    Why? It is the way it is done. Resistors cost 2¢ each if bought in bulk, and with LEDs it isn't an option. Sometimes there aren't shortcuts. When I design I look for minimum components, but you can only go so far.

    Regardless, you won't use the majority of segments. This means the numbers of resistors isn't nearly so high as you think it is.
     
  19. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    Do I need a stronger cell/power supply when I use so many resistors since they will all be drawing current? Or do they just limit the current and not draw them? Money isn't an issue, its the soldering haha but I understand I cannot go for shortcuts or the LEDs will pop one by one.
     
  20. Wendy

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    Problem with displays if one LED pops the entire display is basically worthless. If you need help finding resistors I can help (see my post in the new swap forum). I would think in terms of 4XAAA or 4XAA batteries, and even then they won't last long, but long enough. 9V batteries have very little capacity.
     
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