4001 latch circuit

Discussion in 'General Electronics Chat' started by fred_uk, May 25, 2009.

  1. fred_uk

    Thread Starter Active Member

    Jan 3, 2009
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    0
    [​IMG]

    hi, today i tried to construct the above circuit. I wanted a simple latch circuit to provide a digital input to a second board. I couldn't get it to work ive got a 4001 quad nor gate, what can i do next?

    do i need an alternative circuit design? i am operating at 6v

    thanks for reading...
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You need to make some changes to the inputs. Use 20K resistors, and arrange then as pullups - they connect to the +6 rail and to both inputs.

    Use a single switch, but SPDT (single pole double pole) instead of SP NO. The idea is to have one input pulled to ground through the switch, while the other input is pulled up with a resistor.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your circuit should work fine the way you show it. When the input is high then the output of a NOR gate is low. The switches should be momentary.

    But the ordinary Cmos gates have a very low output current when the supply is only 6V. The LEDs will be dim and might prevent the output voltage from being a valid logic high.
    Use the other two gates in the package or two transistors to buffer the outputs of the gates from the LEDs.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    As has been said, CMOS doesn't drive even light loads well. Use this to buffer the LEDs and the gates...

    [​IMG]
     
  5. fred_uk

    Thread Starter Active Member

    Jan 3, 2009
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    thanks for everyones replys

    i wont be needing the led's i just need a single output to connect to my digital input on my i/o board

    would i need to use a buffer or should i increase the resistors to 20k, i will try later this morning and post the results. Essentially i just need the circuit to latch. I am using a reed switch (normally closed) for the alarm, and a microswitch (can be normally open or closed) for the reset

    thanks again!
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Like Audioguru said, the 10k resistors are fine.

    Tell us more about the reed switch being normally closed.
    Is it opened momentarily when the alarm sounds?
    Or is it held open when the alarm sounds?
    Or is it held open until the alarm sounds, then closed momentarily?
    Or is it held open until the alarm sounds, then closed until the alarm is turned off?

    It's going to take some different wiring to hook it up, depending upon how the reed switch is used.

    You'll need to use the same voltage level for your board as the I/O board requires.
     
  7. fred_uk

    Thread Starter Active Member

    Jan 3, 2009
    67
    0
    The read switch is normally closed and breaks (goes normally open) monentarily on alarm. The reset switch is a micro siwtch with normally open and closed contact and can be arranged to suit.

    Both the latch and third party interface operate at 6v
     
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