555 bistable (LTspice)

Thread Starter

TO2

Joined Jun 19, 2009
26

Kermit2

Joined Feb 5, 2010
4,163
Try it with a small value capacitor on pin 6 and ground. a single resistor from pin 6 to 7. another resistor from pin 7 to +5 V. Finally tie the trigger pin(3) to pin 6 and fire it up.
 

SgtWookie

Joined Jul 17, 2007
22,210
Keep in mind that the 555 timer that comes with LTSpice is an "idealized" timer that you can't buy. I have a more realistic approximation of a bjt (transistorized) LM555 timer at the house; I've uploaded it here before.
 

Thread Starter

TO2

Joined Jun 19, 2009
26
Keep in mind that the 555 timer that comes with LTSpice is an "idealized" timer that you can't buy. I have a more realistic approximation of a bjt (transistorized) LM555 timer at the house; I've uploaded it here before.
Yes, I also think it's because of the model.

But the model posted by you doesn't work for me as well.
(If you mean the model from this post: http://forum.allaboutcircuits.com/showpost.php?p=229723&postcount=14)

I have used the LTSpice example NE555.asc (in the example folder) and have modified the inputs accordingly to the way that I want to have.

And it works,
Reset high, Trigger high -> Output low.
Output low is in the mV range, in the previous cases the output is above 3 Volt, that's not low (using 5V for Vcc).
 

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Ron H

Joined Apr 14, 2005
7,014
With the connections you have, the output can be in either state. It may come up in a consistent state when you power it up, though.
I ran a sim with the NE555 model provided by LTspice. It originally came up with the output high. When I added Startup (see attachment) to the .tran directive, the output came up low.
 

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Thread Starter

TO2

Joined Jun 19, 2009
26
Thank you. It also works for me.

So I should always use the Startup option of the .tran directive then?
 

SgtWookie

Joined Jul 17, 2007
22,210
The Startup option is a good one to use, particularly in simulations where ambiguity may otherwise result from having voltages applied from time zero.
 
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