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A timer question

Discussion in 'Electronic Basics' started by [email protected], Jul 8, 2005.

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  1. Guest


    I need help to get a one-shot pulse generator with
    both positive and negative trigger input. 555 timer
    only takes negative trigger input. 74LS123 can take
    either positive or negative but not both.

    Thanks a lot for help!

  2. I am assuming you want a pulse to be generated any time a signal
    changes state. If you connect the signal to an exclusive or gate, and
    also connect a delayed version of the same signal, the gate will
    output a negative pulse equal in duration to the signal delay. You
    can use the three extra gates in the pack to produce the delay. If
    you add an RC to the delay, you can lengthen the pulse as needed,
    possibly eliminating the 555. If you reverse the polarity of one of
    the delaying gates (by switching the unused input from a low to a
    high) the circuit will output a positive pulse.

    I suggest either a 74HC86 or a 4030 or 4070.
  3. Chris

    Chris Guest

    Hi, Tom. I like the CMOS 4538. It has two one-shots on one 16-pin IC
    Each has separate input pins for positive edge triggering and negative
    edge triggering. Each also has a reset input, and positive and
    inverted outputs. Output pulse widths extend from less than 100us to
    over one second. Timing is set with one R and one C per one shot.

    For pulses less than 100us, use the 4528, which has the same pinout.

    Both operate on +3V to +15V supplies, like the 555. The only negative
    is that the output is made to drive CMOS logic gate loads instead of
    the high load a 555 can drive. However, by adding a transistor and two
    resistors, you can make the 4538 drive as much or more current than a

    Also, the 4538 is set up to be retriggerable (another pulse will
    restart the time). However, it's trivial to wire it up to be
    non-retriggerable, like a 555.

    Good luck
  4. aman

    aman Guest

    Have you also considered using a microcontroller like PIC ? It is a
    little investment on the programmer but is a good long term investment.
  5. John Fields

    John Fields Guest

  6. ehsjr

    ehsjr Guest

    You can trigger your 555 with the negative pulse, through
    a diode. You can run the positive pulse through an NPN
    transistor inverter, and run the output of that through
    another diode to the 555.

    Input o----+---|<---+----------------------o to 555
    | D1 |
    | | R2
    | +---->|---+---/\/\/\---o Vcc
    | D2 |
    | |
    | c/
    +-----/\/\/\-----| Q1
    R1 e\

    R1 = 1K; R2 = 470; Q1 = 2N3904

    Hope this helps.
  7. Guest


    Thanks a lot for all your help! I really appreciate.

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