how to generate a single pulse from a switch closure

Basically, by with a capacitor on the trigger input so that the 555 is sensitive to edges, not levels. Google for "555 edge trigger" for examples.

There are software packages (e.g., ) that could help as well.

v+---------100k-------+------switch------+-------to 555 | | | | | | 0.1 uF 1K | | | | | | | | gnd gnd

John

And if you have an SPDT switch, it can be even simpler.

With a hazard.

John

And that pulse is going to which pin on the 555?

JF

On Fri, 16 Jan 2009 08:42:44 -0800, John Larkin wrote:

--- Don't you mean:

gnd---------100k------+------switch------+-------to 555 pin 2 | | | | | | 0.1 uF 1K | | | | | | | | +v +v

Version 4 SHEET 1 1156 680 WIRE 112 -240 -80 -240 WIRE 384 -240 112 -240 WIRE 704 -240 384 -240 WIRE 1024 -240 704 -240 WIRE 1088 -240 1024 -240 WIRE 112 -192 112 -240 WIRE 384 -192 384 -240 WIRE 1088 -192 1088 -240 WIRE 752 -128 512 -128 WIRE 1024 -128 1024 -240 WIRE 1024 -128 976 -128 WIRE 112 -64 112 -128 WIRE 208 -64 112 -64 WIRE 384 -64 384 -112 WIRE 384 -64 288 -64 WIRE 752 -64 384 -64 WIRE 1088 -64 1088 -112 WIRE 1088 -64 976 -64 WIRE 752 0 592 0 WIRE 1088 0 1088 -64 WIRE 1088 0 976 0 WIRE 112 64 112 -64 WIRE 592 64 592 0 WIRE 704 64 704 -240 WIRE 752 64 704 64 WIRE 1088 96 1088 0 WIRE -80 112 -80 -240 WIRE 224 112 224 -16 WIRE -80 256 -80 192 WIRE 112 256 112 144 WIRE 112 256 -80 256 WIRE 224 256 224 192 WIRE 224 256 112 256 WIRE 272 256 272 -16 WIRE 272 256 224 256 WIRE 512 256 512 -128 WIRE 512 256 272 256 WIRE 592 256 592 144 WIRE 592 256 512 256 WIRE 1088 256 1088 160 WIRE 1088 256 592 256 WIRE -80 336 -80 256 FLAG -80 336 0 SYMBOL Misc\\\\NE555 864 -32 R0 SYMATTR InstName U1 SYMBOL voltage -80 96 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 12 SYMBOL voltage 224 96 R0 WINDOW 0 -53 5 Left 0 WINDOW 3 -242 110 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value PULSE(0 1 1 1E-6 1E-6) SYMBOL sw 304 -64 M270 WINDOW 0 32 15 Left 0 WINDOW 3 32 44 Left 0 SYMATTR InstName S1 SYMBOL res 128 160 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R1 SYMATTR Value 100k SYMBOL cap 96 -192 R0 SYMATTR InstName C1 SYMATTR Value 1e-7 SYMBOL res 368 -208 R0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res 576 48 R0 SYMATTR InstName R3 SYMATTR Value 1000 SYMBOL res 1072 -208 R0 SYMATTR InstName R4 SYMATTR Value 1e6 SYMBOL cap 1072 96 R0 SYMATTR InstName C2 SYMATTR Value 1e-6 TEXT -72 280 Left 0 !.model SW SW(Ron=1 Roff=10Meg Vt=0.5Vh=0) TEXT -114 360 Left 0 !.tran 5

JF

Does a 555 need a negative trigger pulse? I never knew that.

You're the 555 expert.

John

Yes it does. Trigger is a PNP and threshold a NPN.

Oh. I only used a 555 once, free-running, to charge-pump some +12 into

-10 or some such. I took the RC feedback from the output pin, instead of all that complicated charge-discharge stuff, which seemed more obvious than the multi-resistor stuff on the data sheet.

But that was a long time ago, and I doubt I'll ever use one again.

John

If you hold pin2 low, the output goes high and stays there. It won't time out. Pin 2 has to be ac coupled. bg