Momentary output with maintained input

I need a circuit that will take a maintained ground input and provide a momentary (less than one second) ground output.  It would need to reset and provide the output once the ground input goes away and comes back again.

This will be used to disarm a factory alarm system without unlocking the doors.  That is why it needs to be a short pulse.

Would like to build myself if possible/practical.

A pulse timer should work for this without a problem, there are several ways to build pulse timers.

https://home.cogeco.ca/~rpaisley4/LM555Basics.GIF

Taken from: https://home.cogeco.ca/~rpaisley4/LM555.html

You would want the last circuit shown, a one second one shot.

You want me to come install it for you, too?   :)

I didn't think ti would show up, the file neds in .GIF, and I've never gotten an uppercase extension to work on this site...

Oh, and you will need to add a transistor to the output of pin 3 above.  I would go with a 2n3904.  Connect a 220 ohm resistor between pin 3 above, and the base of the transistor.  Connect the emitter of the transistor directly to ground, connect one side of a diode to the collector, and connect the other side to 12vdc.  Make sure the stripe of the diode is on the 12vdc side.  Then, hook your relay in parallel with the diode (postivie to positive, negative to the collector side of the diode).

It may be a good idea to put a resistor in series with the diode to limit the current otuput of the transistor, but the resistance of your relay coil should be enough to keep things happy.

the12volt wrote:

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KPierson, the file extention was missing in your first post.

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Whenever I try to link to a file with all caps extension (i.ei picture.GIF) the extension is deleted after I hit the post button.  The image shows up in the preview box, but disapears as soon as I hit post.  I've even tried to go back in and edit the post, and add the .GIF again but it just got deleted again. 

So I hook 12V + to the top left marked +, my maintained negative goes to the top side of where the switch is shown (and take the switch out), (or does it go to the lower left where the O is?)  And my momentary negative is off of the upper right where the O is.  What goes to the lower right O?

I am just grounding a door unlock signal wire to the BCM.  Do you think this would be more than 25mA?  How would the transistor hook up, if I would need one?  Also, I see the formula on the schematic to change the pulse time.  What would I use to get a shorter than 1 sec pulse?  Say 1/2 sec or less.

Yes, the 12vdc will go where the (+) is marked.  I'm not 100% positive that a 555timer will operate above 12vdc, so you may want to look in to that (there should be a max voltage rating for the chip when you buy it). 

The (-) out of your alarm would go to the top side of where the switch would go, if you were using one, which you are not. 

The lower right O is the ground plane.  You can't have a circuit without a + and a -. 

I would use a transistor on the output, hook it up like I stated above (220 ohm resistor between the output of the 555 timer and the base of the 2n3904 transistor.  Tie the emitter to ground, and then if you just want a (-) output to drive a BCM I would hook a 47 ohm resistor to the collector of the transistor and the other side of the resistor will be a 100mA (-) output.

Ha, the forumla is listed right on the sheet, I'm sure you have a calculator handy, just plug in the numbers.  Pulse Time = 1.1 x R x C   .5=1.1xRxC    =     .5=1.1 x 100,000 x C     C = 4uF. 

Typically, you pick an availible capacitance and find the corresponding resistance.  There are no set values for either component, you'll just have to make sure that both values are availible. 

Is this correct?  It should give me a .5 sec momentary negative.

Sorry, I've been in Ft. Lauderdale for the past two weeks.

From my calculation your pulse would be 4.4 seconds (1.1 x 1000000 x 0.000004) = 4.4

I would try a resistor closer to 110Kohms.  This should give you a 0.484 second pulse.

No problem.  I don't come here often unless I have a question.

So the resistor that goes to pin 7 on the timer is the one I change to make the pulse different?  I guess that makes sense because that is the one with the formula by it (duh!)  Does it look like it will work if I change the 1meg to 110K?  I have the transistor and resistors like you described them?