# LM555 Timing and Pulse Width Modulation

Discussion in 'The Projects Forum' started by 1337Hendrix, Nov 28, 2009.

1. ### 1337Hendrix Thread Starter New Member

Nov 28, 2009
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I would like to use 555 timers for its precision timing as well as its pulse width modulation.

Ive looked at countless spec sheets and have gotten contradictory information regarding what constitutes delay time or pulse width. Sheet A gives the same operation for delay time that Sheet B gives for pulse width. Its getting confusing since I've looked at a lot...

Basically, I would like to generate a pulse that is delayed by 3ms relative to a rising edge on my 60 Hz clock signal. Also, I want this delayed pulse to be 20usec in duration.

I figure I could perhaps first generate my pulse, then delay my pulse by 3ms by cascading two timers. Relative to the clock, it would seem to be a 20us pulse delayed by 3ms...but it would be relative to the falling edge, unless maybe I inverted the clock signal so that the rising edge becomes the falling edge.

Then I could just take the output to be relative to the non-inverted clock.

Maybe I could use a 74LS123 to generate the pulse, and then cascade that to a 555 timer for the delay...

Could someone clarify the time delay operation and formula versus the pulse width modulation operation and formula?

Does my design sound plausible with being so confused about what the spec sheets and trying to say??

2. ### Wendy Moderator

Mar 24, 2008
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Actually, for a delayed pulse I thing there might be something better than a 555. How familiar with electronics are you?

This is one way to do it.

Hmmm, a 555 can form an inverting Schmitt Trigger, I don't know if you can use the signal inverted.

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3. ### Bernard AAC Fanatic!

Aug 7, 2008
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Use a 556, first half, Bill's schmitt trigger,with .1 uF cap & approx[pot] 33k resistor, capacitivly coupled to second half of 556 as 20 usec. one shot via .01 uF & 10 k input to supply. Output = 20 usec + pulse delayed 3 msec from + 0 axis crossing.

4. ### 1337Hendrix Thread Starter New Member

Nov 28, 2009
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0
It looks like I don't have a schmitt trigger available to me in IC form. Creating one seems to be more work than needed...

What if I wanted to just use a 556 or a 74LS123? I have those readily available and would really like to put them to use.

I'm thinking that, if I simply use the 555, the first one would actually create the delay and the second would create the pulse. Or am I wrong?

5. ### Wendy Moderator

Mar 24, 2008
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That was my thought, though with electronics there is always another way to do things.

6. ### 1337Hendrix Thread Starter New Member

Nov 28, 2009
4
0
That is true, but I'm doing this to illustrate a concept, not as something practical...that's why I cant just go out and buy fancy high speed ICs.

So, using what I said above, the 555 timer triggers on the falling edge, when I need triggers on the rising edge.

Does the 74LS123 trigger on the rising edge if I connect it in a similar fashion to the 555? Or does it also trigger on the falling edge?

Is there a simple way to trigger it on the rising edge? How about a positive edge triggered 555?

Or can I just trigger either on the falling edge, but make sure the first stage's C doesn't charge up until 3 ms after the rising edge of the clock...so it looks like it got triggered at the rising edge.

The second stage will be triggered by the falling edge of the output waveform of the first stage, thus making the output look like a 20us pulse triggered at a 3ms delay relative to the rising edge.

7. ### 1337Hendrix Thread Starter New Member

Nov 28, 2009
4
0
Okay, Ive got it now.

I can use the 74123 as a positive edge triggered device by connecting the clock to Pin B instead of Pin A.

Thanks!