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Generate a single/one shot pulse and then nothing until reset

Discussion in 'Electronic Basics' started by [email protected], Jun 18, 2007.

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  1. Guest

    Hey all,

    Been googling for ages but couldn't find something that seems like it
    would do what I need. I'm looking for a design that would generate a
    single pulse when triggered but then nothing until reset even though
    the trigger will remain. I'm thinking of a 555 timer but couldn't work
    out how to use one in this way as from what I can tell it's neither
    monostable or bistable or astable operation that I want. The pulse
    doesn't have to be clean or that long (about 1-2 second should be
    enough. Basically, what I want to do is to sound a buzzer once only
    and then don't sound it again until reset.

    Cheers
     
  2. Guest

    Actually recosindering it doesn't have to be reset (although if it
    does, it doesn't matter). All that's important is that it generates
    only a single one shot several second or so pulse and then should drop
    to low, even though the trigger remains high. Once the trigger drops
    to low, it should be able to generate another pulse when triggered
    again. Alternatively, it could generate a single several second pulse
    and then nothing until reset. It doesn't matter which as long as it's
    capable of only generating one pulse while trigger remains high.

    Cheers
     
  3. Rich Grise

    Rich Grise Guest

    Use an R-S flip-flop, and trigger your one-shot with the Q output. With
    a 555, you'll have to differentiate the edge, so that it will actually
    time out. Actually, I think that should be "not Q", so that the 555
    gets a negative-going trigger pulse. Then no more triggers will set the
    flip-flop, since it's already set, and you'll have to reset it to start
    the cycle again.

    Good Luck!
    RIch
     
  4. John Fields

    John Fields Guest

    ---

    As long as your input pulse is longer than your output pulse, this
    will work, and your output pulsewidth will be:


    T = 1.4 Rt Ct


    View in Courier:

    +V
    |
    +-------+
    |8 |
    +---+---+ [1M]Rt
    2|_ Vcc |3 |
    IN>---+--[10K]--+--O|T OUT|---|----> OUT
    | | | |6 |
    | [1nF] | 555 TH|---+
    | | 4|_ _|7 |
    +---------|--O|R D|O--+
    | | GND | |
    | +---+---+ |+
    | |1 [2.2µF]Ct
    | | |
    +-------+-------+
    |
    GND

    Here's the LTSPICE circuit list, and if you don't have LTSPICE you
    can download it for free from:

    http://www.linear.com/designtools/software/


    Version 4
    SHEET 1 880 680
    WIRE 304 48 -512 48
    WIRE 416 48 304 48
    WIRE 416 112 416 48
    WIRE 32 176 -128 176
    WIRE 304 176 304 48
    WIRE 304 176 256 176
    WIRE -336 240 -384 240
    WIRE -224 240 -256 240
    WIRE 32 240 -224 240
    WIRE 320 240 256 240
    WIRE 416 240 416 192
    WIRE 416 240 320 240
    WIRE 32 304 -32 304
    WIRE 320 304 320 240
    WIRE 320 304 256 304
    WIRE 416 336 416 240
    WIRE -384 368 -384 240
    WIRE 32 368 -384 368
    WIRE -32 416 -32 304
    WIRE -512 432 -512 48
    WIRE -384 432 -384 368
    WIRE -224 432 -224 240
    WIRE -512 560 -512 512
    WIRE -384 560 -384 512
    WIRE -384 560 -512 560
    WIRE -224 560 -224 496
    WIRE -224 560 -384 560
    WIRE -128 560 -128 176
    WIRE -128 560 -224 560
    WIRE -32 560 -32 496
    WIRE -32 560 -128 560
    WIRE 416 560 416 400
    WIRE 416 560 -32 560
    WIRE -512 592 -512 560
    FLAG -512 592 0
    SYMBOL Misc\\NE555 144 272 R0
    SYMATTR InstName U1
    SYMBOL voltage -384 416 R0
    WINDOW 3 24 104 Invisible 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value PULSE(0 12 0 1e-6 1e-6 5 10 2)
    SYMBOL res 400 96 R0
    SYMATTR InstName R1
    SYMATTR Value 1e6
    SYMBOL cap 400 336 R0
    SYMATTR InstName C1
    SYMATTR Value 2e-6
    SYMBOL voltage -512 416 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V2
    SYMATTR Value 12
    SYMBOL res -48 400 R0
    SYMATTR InstName R2
    SYMATTR Value 1000
    SYMBOL cap -240 432 R0
    SYMATTR InstName C2
    SYMATTR Value 1e-9
    SYMBOL res -240 224 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R3
    SYMATTR Value 10k
    TEXT -464 592 Left 0 !.tran 20
     
  5. ehsjr

    ehsjr Guest

    Assuming a negative going trigger, use a series cap between the
    trigger pulse and the 555 pin 2, with pullup resistors on each
    side of the cap. If the trigger is + going, use an NPN inverter,
    then the cap.

    Ed
     
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