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One shot Multivibrator

Discussion in 'Electronic Design' started by Tanty, Nov 23, 2003.

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  1. Tanty

    Tanty Guest

    I have a motion sensor that knocks an active low signal for about one second
    when motion is sensed. I would like it to trigger an alarm for five minutes.
    I thought of using a typical one-shot multivibrator (74ls121) but its
    maximum output pulse is 28 seconds. how do i extend the pulse time or is
    there a multivibrator which supports longer pulse times.
    Thanx
    Tanty
     
  2. mdp

    mdp Guest

    You could use the one shot to start a bank of counters. Build as many
    stages as you need. Use the last Carry Out (or Borrow if downcounting) as a
    reset.
     
  3. Fred Bloggs

    Fred Bloggs Guest

    Use a 555, route the TTL into the trigger and the timeout is 1.1RC so 5
    minutes is 300 seconds and for R=1Meg gives C=270uF. I tlooks like this:
    Please view in a fixed-width font such as Courier.



    5V
    --------+---------+------------+------------------
    | | |
    / | +---------+------+
    1M | | VCC |
    / | | | 5 min
    \ +--+RST | +-------------+
    | 1K | | | |
    +---/\/\-----+DIS | | |
    | | 555 | | |
    | | | ----+ +--
    | | OUT+--->
    | | |
    +------------+THRESH |
    | | |
    | +---+TRIG |
    | | | |
    === | | GND |
    270u | +---------+------+
    | | |
    ------+----------------------+-------------------
    |
    |
    |
    TTL IN >---------+

    --+ +--
    | |
    +--+
     
  4. Fred Bloggs

    Fred Bloggs Guest

    Something like this:

    Please view in a fixed-width font such as Courier.


    100k
    +----+--/\/\---Vcc
    | |
    | +--||---+
    | 0.1u |
    | -+-
    1/2 | gnd
    74HCT74 | 5 MINS
    +---o----+ +--------+
    | PR | | |
    | | | |
    +--------|D Q|---------+---> | |
    | | | | | |
    | | | | ---+ +--
    | +----|CK | |
    | | | | |
    | | | CLR | |
    | | +----o---+ |
    MOTION | | | 100k |
    DET >---||-------|---|---------+--/\/\-+-Vcc |
    0.001u | | | | |
    --..--- | | +--|>|--+ |
    || | | 1N4148 |
    || | | |
    ++ | | |
    | | |
    +---|----------------+ |
    | | |
    | | |
    | +-------+------+---+
    | | Q13 MR |
    +----||----+--------|CT |
    | 0.15u | |
    | - | |
    | /| | CD4060 |
    +--/\/\---/\/\------|RT |
    | 82k / 50k | |
    | | |
    | | |
    +-----/\/\----------|RS |
    220k +------------------+
     
  5. Fred Bloggs

    Fred Bloggs Guest

    Fred Bloggs wrote:

    ..... slight revision here:

    Please view in a fixed-width font such as Courier.


    100k
    +----+--/\/\---Vcc
    | |
    | +--||---+
    | 0.1u |
    | -+-
    1/2 | gnd
    74HCT74 |
    +---o----+
    | PR |
    | |
    +--------|D Q|---------+
    | | | | 5 MINS
    | | -| | +--------+
    | +----|CK Q|---------|----> | |
    | | | | | | |
    | | | CLR | | | |
    | | +----o---+ | | |
    MOTION | | | 100k | ---+ +--
    DET >---||-------|---|---------+--/\/\-+-Vcc |
    0.001u | | | | |
    --..--- | | +--|>|--+ |
    || | | 1N4148 |
    || | | |
    ++ | | |
    | | |
    +---|----------------+ |
    | | |
    | | |
    | +-------+------+---+
    | | Q13 MR |
    +----||----+--------|CT |
    | 0.15u | |
    | - | |
    | /| | CD4060 |
    +--/\/\---/\/\------|RT |
    | 82k / 50k | |
    | | |
    | | |
    +-----/\/\----------|RS |
    220k +------------------+
     
  6. CMOS equivalents (MC14xxx series, or 4xxx series) can do LONG time periods.


    --
    *
    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

    http://hamradio.cmcorp.com/inventory/Inventory.html
    http://www.harvardrepeater.org
     
  7. Fred Bloggs

    Fred Bloggs Guest

    Ordinarily yes- but I would use the Nichicon low-leakage capacitors
    available from Mouser-
     
  8. Luhan Monat

    Luhan Monat Guest

    Hmmm,

    Although the math works out, charging a 270 uf cap with a 1 meg resistor
    is what I call 'iffy'. If there is any leakage in the cap, it will
    never charge. Also, you can't really count on T=RC with this slow a
    charge on an electrolytic.
     
  9. I just built a lightswitch with a motion sensor that does this. It used a
    comparator along with a little RC circuit to do the timing. Using a pot to
    control the comparator voltages allowed me to control the time.

    The timing circuit was as follows:


    VCC
    |
    +-------+
    | |
    | |
    | |
    | |
    Input Pulse | |\|
    ->|----+-----|-----|-----|-\
    | | .-. | >-------------
    | | | |<---|+/
    --- .-. | | |/|
    220uF --- | | '-' |
    | | |1M | |
    | '-' | |
    | | | |
    | | | |
    | | | |
    | | | |
    +-----+-----+-------+
    GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    The output drove a wire low to power a MOC3012, which in turn powered a
    triac.

    Adjusting the pot adjusts the timeout. You can't get exact timings, but you
    can get close enough.

    Regards,
    Bob Monsen
     
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