Monostable Multivibrator

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VJR85
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Monostable Multivibrator

Post by VJR85 » Sun Nov 12, 2006 11:11 pm

Hi,

I'm using a one-shot to avoid switch bounce and allow for propagation delay. I haven't used one before and I can't find an example online. The datasheet is here:

http://electronics-diy.com/pdf/74121.pdf

I have an active-high input and need the 121 to provide an active low output to trigger a clock signal.

I'm guessing I hook the input to pin 5, ground 3&4, and the output from the inverted pin1? I am unsure about the RC hookup.

Thanks for any help.

bodgy
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Post by bodgy » Sun Nov 12, 2006 11:45 pm

Not quite - look carefully at the truth table.

Now is the 121 to provide the pulse or do you need it just to go low?

I think you'll actually need

Code: Select all

A1 = low,  A2 floating or high, low to high on B 
Output as you say on notQ

if you want just a low then

Code: Select all

 A1 =low, A2 floating or high and high on B
As far as R and C are concerned, do you know what pulse width you require, for the next part of the chain?

If you don't need a pulse, then I think you can do without the R and C just connect Rint to Vcc as it says in the data sheet.


Colin
On a clear disk you can seek forever.

VJR85
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Post by VJR85 » Mon Nov 13, 2006 12:50 am

Pulse is 20ms. I have the equation...just not sure about the connections.

I need it to supply the pulse and return high. I just need it to advance a counter with an active-low clock input.

Pins 3&4 enter an OR gate with active-low inputs, so if one is low, the output will be high regardless? Or does one need to be floating or high like in your first code?

Thanks for the help...sorry if I'm not clear. I'm new to this.

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philba
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Post by philba » Mon Nov 13, 2006 10:57 am

best document I've ever seen on debouncing is jack Ganssle's Guide to debouncing http://www.ganssle.com/debouncing.pdf

He suggests a Schmitt trigger with an RC network. He provides several formulas to compute debounce RC values. Makes a lot of sense. Not a multivibrator but sure seems simple to me.

By the way, I don't bother with debounce circuits when using a uC. Debounce in software is a lot fewer parts and easier, imho.

VJR85
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Post by VJR85 » Tue Nov 14, 2006 9:01 pm

Thanks for the link!

I'm kind of stuck with this components since it's for school and I was told what to use.

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