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

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

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

On Mon, 18 Jun 2007 05:47:02 -0700, snipped-for-privacy@gmail.com wrote:

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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:

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

-- JF

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