Connect with us

Simple One Shot circuit needed

Discussion in 'Electronic Design' started by [email protected], Sep 22, 2007.

Scroll to continue with content
  1. Guest

    I need a simple circuit that when triggered will send out a pulse that
    will last about a minute and then go off. The
    circuit will stay off until it is manually reset. The input will be a low
    voltage 5-12VDC that will flow thru a switch that is normally closed. When
    the switch is opened the circuit is activated and the pulse will be created.
    This circuit has to be extremely stable. There cannot be any false
    triggering. It would probably be best if it
    operated under battery power.

    Robert Barch
     
  2. Howhurley

    Howhurley Guest

    Robert,

    How accurate does the duration of the pulse need to be?

    When you say extremely stable, does it have any safety connotations to
    it?

    What voltage battery power?
     
  3. Hal Murray

    Hal Murray Guest

    I'd probably start by doing it in software - one of the tiny PICs or AVRs.

    They come with internal oscillators. If that's not accurate enough
    you can use an external crystal.
     
  4. Robert Baer

    Robert Baer Guest

    The simplist circuit is a one-transistor oneshot, but time stability
    for a miinute is not good.
    For reasonable stability, use a few 555s setting the first one at
    (say) one second, the next at (say) 10 seconds and the last at 60 seconds.
    The first one to be capacitivly coupled to the second, and the second
    to be capacitivly coupled to the third in a similar manner.
    Use a scope to adjust the timing so that the Nth pulse from the
    preceeding 555 "overrides" the RC timing.
    Division this way to 15 at most is reliable.
     
  5. Guest

    There are no safety connotations
    The voltage of the battery is open just as long as it will power the
    circuit and generate the pulse. You can select
    anything you want just make it easy
    The pulse really doesn't have to be very accurate. Lets make it variable
    with a pulse of one minute plus or minus five
    seconds either way.
    Robert
     
  6. ehsjr

    ehsjr Guest

    You can use a 555 monostable, like this:

    12V+ ------------------+------+---+
    | | |
    | ---------
    | | 8 4 |
    +---|7 |
    | | |
    [270K] | 555 |
    | | 3|---> Out
    +---|6 |
    o | | |
    In >---o-o---+---[.1uF]-- | --|2 |
    SW | | | 5 1 |
    | |+ ---------
    [10K] [220uF] | |
    | | [.1uF] |
    | | | |
    Gnd ---------+------------+------+---+

    When the switch is closed with + on the input, the output
    at pin 3 will be -. When the switch opens, the timer will
    run and pin 3 will go + for about 65 seconds, then return
    to minus. The ground for your input signal and the ground
    for the 555 must be connected together.

    Ed
     
  7. Fred Bloggs

    Fred Bloggs Guest

    Things get complicated when you ask for a one-time trigger with manual
    reset and immunity against false triggers. You can do something like
    shown below but I would recommend adding a POR (power on reset) IC with
    open drain output connected to RST node shown with '*'. Left 555 is
    latched OUT high when IN switch opens, and this is used to pull right
    555 out of RST while simultaneously triggering its output for specified
    duration. The PN3904 tied into manual RST momentary switch prevents
    false triggers by disabling RST action until input switch is re-closed.
    View in a fixed-width font such as Courier.

    ..
    ..
    .. V+
    .. --------------------+-----------+------------+----------+------------
    .. | | | |
    .. | | | |
    .. NC | .----------------------------------.
    .. IN-o->o-. | | | | | |
    .. SW | +---------555 | [1K] [RT] +---------555 |
    .. | | VCC | | | | | VCC | |
    .. 10K | | | | | | | |
    .. | | | | | | | | |
    .. +-----|TRIG OUT|--+--|--[10K]-+--------|TRIG OUT|---->
    .. | | | | | | | | |
    .. 10K .-|DIS | | | +-1K-|DIS | |
    .. PN3904| | | | | | | | | |
    .. __ | +-|THRESH RST|-----+ * | +----|THRESH RST|-'
    .. | \| | | | | | | | | |
    .. | |-+ | | GND | | 0.1 |CT | GND |
    .. | <| | | +-----------+ | === === +-----------+
    .. | | 10K | | | U| | |
    .. ----+---+---+-------+--------------------+---+----------+-------------
    .. | | | |
    .. | RST --- MOM | ---
    .. '------------o o--------------' ///
    .. SW NO
    .. OUTPUT PULSE WIDTH
    ..
    .. 1.1 X RT X CT
    ..
     
  8. MooseFET

    MooseFET Guest


    If you are going for a minute, you are likely to want something like a
    CD4060. Making time constants a minute long is not an easy matter if
    you want any sort of accuracy in the timing.

    The CD4060 will fairly easily make a pulse that goes high one minute
    after the reset is removed.

    The handling of the switch debouncing can also be done with CMOS
    circuits. You will need to put a resistor in series to protect the
    CMOS against any voltage spikes.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-