555 Wizard Bad? v10 Multisim

[Euler's_Identity]

Yes, as I just tried it.

You can save the link from my last post, change the suffix from .zip to .ms10, and then see the circuit doesn't work in MultiSim V10.

I see no reason why this circuit shouldn't work.

However, if anyone out there can get this circuit to work, I sure would like to see your solution.

In the meantime, I'm considering building my own 555, like I suggested for the PLL of a year ago.

[Euler's_Identity]

Oops!

I forgot something important about the 555.  Once triggered, the trigger must return high.  Hence, I added a cap and pullup to the trigger.  I'm thinking this blunder on my part is what screwed things up MultiSim-wise.

However, I'm still thinking the output should've remained low, as the clock was high.  Do you have anything there Nestor? 

Attached to this post is a working fix.

[Euler's_Identity]

It really looks like I blew it on the on-time though.  Yeesh!

I could've sworn it's t = RCln(2)

Oh well...

[Euler's_Identity]

As it turns out, it's ln(2) for the astable and 1.1 for the one shot.  So that should've been t = 1.1RC, not t = RCln(2).  So use 1.1 for the derivation and all should be well.

Incidentally, what I was actually remembering was the logic-based one-shot equation, the one for the 74x121 or 74x221 (the dual of), which is t = RCln(2).

It's easier to remember ln(2) than it is to remember 0.693.  And besides, ln(2) is more correct. 😉

I'm pretty sure MultiSim has the logic one-shots also, as I seem to remember a design of mine some time back using the dual one.  This was way back though.

[dpresley58]

The problem I'm having is that the pinouts for the generic timer model in the program don't match the actual 555 pinouts. While the model has the descriptions, they're on the "wrong" pins. Not sure which ones to honor, but I'm not getting predictable behaviour from the generic version either.