Hello,

I'm trying to simulate the 555 timer bistable circuit on Multisim and LTSpice. I may have done something wrong, because I'm not getting the expected behavior.
When Reset and Trigger is pulled high I expect the output to be low, but it's not.

Schematic:

(LTSpice schematic file include below.)

This is the behavior that I expect (java applet):

http://www.falstad.com/circuit/#$+1...18+64+0+39+7.62939453125E-5+9.765625E-5+0+-1

Scope 1: Reset
Scope 2: Trigger
Scope 3: Output

Please shed some light

 

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.

 

That's for a astable circuit. The bistable circuit should work like a flip-flop.

 

Sorry, my mistake.

 

http://www.eleinmec.com/article.asp?5

 

Bill's Index

The 555 Projects

555 Bistable Multivibrator

 

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.

 

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).

 

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.

 

Thank you. It also works for me.

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

 

Thank you. It also works for me.

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

I almost never do. It all depends on the circuit you are simulating.

 

Oke. Thank you.
Also thanks to everyone for the help.

Problem solved.

 

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