I need help! Trying to use 555 timer to pulse but input will be steady state.

I am trying to figure out a cct which uses a 555 timer to send a pulse to imitate pressing of a button when I turn the ignition on. I want the pulse to be about one second long. The thing is, when I start the car the trigger will always be at a steady state, not a pulsed input.

I have tried this, but the output just stays low, if the trigger is steady state low.

I was thinking I could use the momentary position of the ignition switch which turns the motor and it would be like an trigger pulse, but I wish to use he same circuit more than once , and when the car is already running.

Anyone any ideas on this?

Thanks.

------------- Oh no..hope you're not making a car bomb :(

See 555 app notes.. You're looking for positive edge triggering.

Or...just skip that boring way and use a microcontroller :)

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tried what?

just one: (view with fixed font) ign | .--------------------+ | | | . . . .|. . . . | . VCC(8) . | . . +--------------RES(4) OUT(3)------ out | . 555 . +--[100K]--+---TH(6) DIS(7)-- |   | . . `--|

Here's one way:

Ignition Switch / +12---o o-----+------+ |+ | [100uF] | | [10K] [Rly] | |> | | | | | Gnd -----------+------+---o o----> To 555 pin 2

That will give you a trigger pulse to the 555, rather than a steady level. You'll likely get several pulses due to relay contact bounce, but the bouncing will be finished in less than .1 seconds, so it's irrelevant.

Ed

The datasheet for the standard 555 shows a circuit that capacitive couples the trigger in on pin 6 (I think that's it) of the mono stable. Cap is .01 uf and there's a 10 K pull-up resistor on either side of the cap.

The low trigger toggles the mono stable through one timing cycle and the input trigger on the cap can remain low thereafter.

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Thanks everyone, although, I cannot read those ccts even with a fixed font. I know it took a lot of work to type them in, and I am grateful.

Where do I find the standard 555 datasheet so I can look at this capacitative coupling cct. Thanks again.

National Semiconductor has their data and applications on searchable CDrom for free, and on line. Too lazy to look it up so I picked the first hit that matched my search terms

FYI - all this stuff is on line - goggle for it

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Thanks Ed, but that's rather painful with the relay...

jasen showed an attractive approach, using switched power. That's a good idea, and should be considered.

We usually use continuously-powered circuits, maybe because they're easier to think about. In the case of a 555 timer chip, we need to AC couple the trigger signal.

Here's an example:

battery | ,------+---+--------------+----------, | | | | 555 | | | | . . . . . . . . . | | | 100k . VCC(8) . | | | | .___ . | IGN | | +------.DIS(4) OUT(3).--| ---- out | 22k 22k | . ___ . | | | | +------.THRES(6) RES(4).--' | | 0.01 | | .____ . 22k +-||---+-- | -----.TRIG(2) CV(5).-- | | | . . | |/ _|_+ . GND(1) . '----| --- . . . . . . . . . |\\v | 10uF | | | | ---+----------+--------------+------- ground

The Ignition signal is positive going, but the 555 wants a negative trigger, so we have to add an inverter. This makes this approach less attractive than jasen's circuit.

ign |

.--------------------+ | | | . . . .|. . . . | . VCC(8) | . . +--------------RES(4) OUT(3)------ out | . 555 . +--[100K]--+---TH(6) DIS(7)- | | . . `--|

Yup, it's ugly. :-)

His stuff is always good, as is yours, of course.

Thanks, Ed

Is there any chance one of you guys could elaborate or draw a proper pic and put a link to the photo site where the picture would be located so I could look at it?

Thanks again.

You can easiiy see ASCII drawings if you tell your viewer or browser to use a fixed font like Courier. If that's too painful, you can copy the text into a simple editor like Notepad, and view it with a fixed font, and print it as well.

I've shortened the text above to just the three drawings.

You can easiiy see ASCII drawings if you tell your viewer or browser to use a fixed font like Courier. If that's too painful, you can copy the text into a simple editor like Notepad, and view it with a fixed font, and print it as well.

I've shortened the text above to just the three drawings.

Hey, that 2nd example works a treat.

Thanks very much.

-- Ways for Google Groupers to view ASCII graphics --