need simple 5v ~50Hz AC ouput for LCD

Discussion in 'The Projects Forum' started by elRey, May 18, 2009.

  1. elRey

    Thread Starter Active Member

    Feb 23, 2009
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    Hello,

    I have an unknown 4 segment (clock) LCD that was driven by a PCF8576CT. I've located the four baseplates and now need to identify the common segment pins for the LCD. So I just need a simple circuit I can use the output to discovery, thru trial and error, the segment pins on the LCD.

    i.e. touching one lead to one baseplate and then touching the other lead to each of the other non-baseplate pins and noting which segments activate.

    Thanks,
    Rey
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    An LM555 free-running oscillator should give you the clock.
     
  3. elRey

    Thread Starter Active Member

    Feb 23, 2009
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    Thank you. That will give me a 0-5v wave. Now how do I get a -5v to +5v wave?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you have the +/- 5 volt lines, run the 555 with -5 to the ground pins and +5 to the positive ones. That should give you the +5 to -5 pulse.
     
  5. elRey

    Thread Starter Active Member

    Feb 23, 2009
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    Ah, I don't have the -5v line. I'm testing on a Electronic Lab with +1.5v, +3v, +4.5v, +6v, +7.5v, and +9v lines and gnd. How can I create a -5v line?

    Thanks again for you help.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Not to sound snippy, but you need -5 volts. One way to get there is with an ILC7660 charge pump. It outputs the negative equivalent of the positive voltage fed to it.

    Your LCD may run okay with +/- 4.5 volts, or +/- 6 volts.

    here's a link to a data sheet on the 7660 - http://www.intersil.com/cda/deviceinfo/0,0,ICL7660,0.html
     
  7. elRey

    Thread Starter Active Member

    Feb 23, 2009
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  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Just leave out C3
     
  9. elRey

    Thread Starter Active Member

    Feb 23, 2009
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    Thank you! I will try it tonight.
     
  10. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Is the display ground floating? If it is , might connect two resisters,around 1k, in series from 555 output to ground, and use center as ground, boost 555 operating V to 10V. Should then have +5 &-5 square waves refferenced to center tap.
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    Curious why?
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    With C3, approx. -5V DC out; wi th-out C3, -5V square wave.
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    OK, I must have misread something because I thought he wanted -5VDC.

    Thanks Bernard.
     
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