one time pulse with a 555 timer

Hello

I am trying to keep it as simple as possible. I have scoured loads of sites etc, an have even found the circuit I want, or should I say it is described as the one I want.

I have found a simple circuit which I replaced R with a potentiometer to make the output variable, but the aim is to make this a one time circuit. I want to push a current through the circuit it fire for a variable amount of time (via the potentiometer), then stop even though the trigger has not.

I have been told that this is done with the use of and extra capacitor, resistor, LED and Transistor (used as a not gate) to get this to work. So far all of the layouts I have been given have not worked.

I seen the post at thread240-151528 (sorry could not find how to add it as a link). Seemed to be asking a similar question and woundered if that was the way to go? Any ideas would be great.

Thanks
Scott

You should go to google and then search for "555 one-shot".

You will find a lot of circuits to do what you want.

Here's the second one on the list:
http://www.uoguelph.ca/~antoon/gadgets/555/555.html

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

I have tried that I think that I mite be mis-understanding or expalining it wrong. I have a trigger which will send power but does not stop. From that trigger I want a short pulse. All of the one shot monostable circuits I can find seem to need a trigger which supplies a voltage/current for less time than the actual output.

Sorry if I am sounding dumb here.

Use one timer to latch on the trigger pulses. Use a second timer to generate the one shot.
respectfully

Here is an example of what waross described:
http://people.consolidated.net/fsmyth/pages/555apps.html#555apps4

is the trigger continuously running? what is its frequency?
You want a __--__ pulse at the beginning of the trigger
pulses? How long ?

Plesae read FAQ240-1032
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OP: "...a trigger which will send power but does not stop. From that trigger I want a short pulse."

Sounds like a 'Power On Reset' circuit. Apply power to the circuit, and get a nice clean reset pulse. There are a million solutions including application-specific 3-terminal devices. Most POR emit a low-going reset pulse (ground seeking) - but inverters are easy.

What's your insistence on using something that doesn't work for the application?  Why not use a 74121 http://www.datasheetarchive.com/search.php?q=74121&;sType=part&ExactDS=Starts which was designed to be edge triggered?  Or any of the many other one-shots?

TTFN

I have a motor which gets switched, but want to put a one shot  pulse on the on the motor. The idea is the switch is thrown then the motor will power for a short while then stop, even though the switch has not been turned to the off position. I would like to make the time the pulse runs variable also. It runs off 12 volts and the time variation should be between 0.5 of a second to 10 seconds.

I have read through all of the material given and it seems similar to what I have tried. Most of them continued to pulse the motor as the input was still high.

Simply differentiate the switch closure:
Let be R1=1 Mohm, R2=1kohm, C1=.1 microF. Sw1 = Normally open switch. (each line is one node, each part has
two pins, 1. and 2. )

V+ --- R1.1
R1.2---C1.1---Sw1.1
C1.2---GND
Sw1.2 ---R2.1--- 555.trigger
R2.2---GND

When the SW is open the C1 charges up to V+ through
R1 in about .1 sec. When Sw is closed, C1 discharges
through R2, generating a single positive pulse of about
.0001 sec. Use this to trigger the 555

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

Are you reading the same datasheets I'm reading?  There's a reason why these are called "EDGE-TRIGGERED" one-shots.  The 74121 can be programmed for a pulsewidth up to 28 seconds

TTFN

IR:
Even if he uses 121, the trigger must be debounced or otherwise it may trigger on both edges.

Plesae read FAQ240-1032
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the 555 can work with a wide range of V+, the 121 needs regulated supply.

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

If the shortest pulse is 0.5 seconds, then the switch effectively is debounced; the pulsewidth simply stretches a few milliseconds.

TTFN

IR:
The leading edge is ok, but the trailing edge may bounce, too.

By the way, the 555 has higher current and can either sink or source.

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>