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555 timer help

Discussion in 'Electronic Basics' started by [email protected], Jun 17, 2005.

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  1. Guest

    I'm a relative newbie with electronics, and I'm trying to build a small
    555 based adjustable timing circuit.

    I have a 555 set up in monostable mode which needs to be triggered by a
    positive pulse that may be longer than the desired output pulse. The
    555 also needs to be immediately reset if the trigger pulse goes low.

    My current circuit

    * = junction
    ^ = bypass
    T = npn bjt transistor


    12v ----------*-------*
    | |
    1k R1 10k R2
    | |
    *--*-0.1uF-*----- pin 2 (trg)
    | C1
    IN --*--1k--T pin 3 (out)--------- OUT
    | R3 |
    *------^----------*----- pin 4 (rst)
    | |
    | 10k R4
    | |
    0v --------*----------*


    Current problems:

    1. With the RESET pin connected as in the diagram above, the timer
    output (OUT) does not pulse. It remains high as long as the input (IN)
    is high and goes low when IN is low.

    2. If the RESET pin is disconnected and left floating, the output will
    pulse when IN goes low to high, which is correct. However, it also
    pulses on the high to low edge if it occurs after the initial output
    pulse is complete.

    Any help is greatly appreciated.

    -Frank
     
  2. Fude

    Fude Guest

    Oops, try again.

    + = junction
    ^ = bypass
    T = transistor


    12v ----------+------------+
    | |
    1k R1 10k R2
    | |
    +--0.1uF--+----- pin 2 (trg)
    | C1
    IN --+--1k--T pin 3 (out) ------ OUT

    | R3 |
    +--------^-----------+------- pin 4 (rst)
    | |
    | 10k R4
    | |
    0v -------------------------+
     
  3. Bob Monsen

    Bob Monsen Guest

    The chip is probably still in reset when the trigger pulse comes in.
    This may be caused by improper power supply bypassing. If so, you need a
    0.1uF cap between vcc and gnd.
    If you have an AND gate laying around, you can use that to cut off the
    pulse. Instead of tying reset to IN, tie it to Vcc. Then, attach IN to
    one input of the AND gate, and the output of the 555 to the other. Now,
    when you bring up the input, the pulse should be seen by the chip, and
    the output will be gated by the input, just like you want.

    There may also be a way to fool the reset, but it may not be portable
    across different version of 555.
     
  4. Fude

    Fude Guest

    Thanks for the tips Bob. I'll give them a shot this weekend.

    -Frank
     
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