# 555 bistable (LTspice)

#### TO2

Joined Jun 19, 2009
26
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):
Rich (BB code):
http://www.falstad.com/circuit/#\$+1+5.0E-6+10.20027730826997+46+5.0+43
v+816+576+816+304+0+0+40.0+5.0+0.0+0.0+0.5
165+656+336+672+336+2+0.0
w+720+304+816+304+0
r+848+368+912+368+0+10000.0
r+624+432+560+432+0+10000.0
w+624+432+656+432+0
w+912+368+912+304+0
w+912+304+816+304+0
w+560+432+560+304+0
w+560+304+720+304+0
w+848+368+848+352+0
w+816+576+560+576+0
w+560+576+560+528+0
w+816+576+976+576+0
w+976+576+976+352+0
w+656+464+656+576+0
w+656+576+560+576+0
r+848+400+896+400+0+100.0
w+784+400+848+400+0
w+896+400+944+400+0
w+784+368+848+368+0
s+944+320+976+320+0+1+true
s+560+496+608+496+0+1+true
w+560+528+560+496+0
w+624+432+624+496+0
w+624+496+608+496+0
w+976+352+976+320+0
w+944+400+944+576+0
w+944+576+976+576+0
w+848+352+848+320+0
w+848+320+944+320+0
o+20+64+0+39+5.0+9.765625E-5+0+-1
o+5+64+0+39+5.0+9.765625E-5+0+-1
o+18+64+0+39+7.62939453125E-5+9.765625E-5+0+-1

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

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#### 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.

#### TO2

Joined Jun 19, 2009
26
That's for a astable circuit. The bistable circuit should work like a flip-flop.

#### Kermit2

Joined Feb 5, 2010
4,163
Sorry, my mistake.

#### Kermit2

Joined Feb 5, 2010
4,163
TO2

#### Wendy

Joined Mar 24, 2008
21,914

#### 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.

TO2

#### 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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TO2

#### 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?

#### Ron H

Joined Apr 14, 2005
7,014
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.

#### TO2

Joined Jun 19, 2009
26
Oke. Thank you.
Also thanks to everyone for the help.

Problem solved.

#### 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.

TO2