Latching LED Alarm (help)

Discussion in 'The Projects Forum' started by DC_Kid, May 16, 2009.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    here's the general circuit functionality.

    i monitor some pulses which advances a 4 bit counter. when bit 1 (0,1,2,3) goes high i need to latch a LED on even if the counter continues and bit 1 goes low.

    there are 8 pulses i monitor, hence 8 counters and 8 LED's.

    currently i use bit 1 output to trigger the gate of a scr which means i have 8 scr's too (discrete components, argh). in some cases several LED's will need to latch on so i need a way to manually reset them to a off state (a reset of all is required). startup needs to be latched low, etc.

    so, is there a IC (logic) that may do this functionality better than using scr's ?
     
    Last edited: May 16, 2009
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    You can use a NC push button in series with the LED as to be able to switch the current off and thus unlatch the SCR.

    If you want you can use a RS latch IC. Use its S input to make its Q0 output high and then, when you want, its R input to make its Q0 output low. Also, you will need a transistor on the output of the RS latch as to be able to drive LEDs.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
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    I know at todays standards using a bunch of SCRs to latch LEDs is considered archaic but they can still be effective. Hey, sometimes people build their projects around available parts.
    About 30 years ago a private school contracted me to build a custom Alarm System and Panel. There were over 85 openings to be protected and monitored. Yes... 85 (TO92) SCRs was a lot of SCRs but I had over a thousand of them is stock! :D
     
    Last edited: May 16, 2009
  4. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    thanks for replies. the scr break-circuit will work, but one more problem to solve. i have a 9th LED and this one needs to latch 'on' when any of the other 8 go on. this 9th LED is a remotely mounted LED, if any one of the 8 trigger the the user sees the 9th LED which indicates a problem. user then goes to device to see which of the 8 LED's alarmed, etc.

    ok, i think i can work it with SCR's, any suggestions for a SCR IC so i can save some precious board space?
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    Parallel 8 transistors with their base driven by the same signal which activates each SCR or by the output of each SCR. The first SCR which will be activated will activate one of the transistors too and thus the 9th LED.

    Another solution is to use an OR gate and OR all the SCR control signals.
     
  6. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
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    ahhh, but adding more parts i cant afford. i was hoping for a single IC to do the job. i need to make some space on my board for a negative DC supply circuit (8pin dip, a couple of small caps, resistors, and diode).
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    The ULN2004 is a 16pin DIP that packages 7 power NPN Darlingtons. It includes integral kickback protection. I've been using this chip for many years, mostly for relay drivers.

    http://www.datasheet4u.com/share_search.php?sWord=ULN2004
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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  9. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
    9
    two 16pin dips along with transistor dip(s) will take up more space than the scr route.

    also, i dont think the logic in the 54279 fits my needs. the output of my counter may change so i cannot rely on it to be in one state or another. do you see a way to use 54279 to latch Q high when S1 S2 or R goes from low to high?
     
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