555 Multisim Help Needed!

Thread Starter

ro169

Joined Oct 10, 2014
62
Hi guys.

I seem to be having issues with simulating a 555 in astable mode on multi-sim 13. I need a period of approximately 1 second.

I used f = 1.44/(R1+2R2)(C)
Duty cycle is approx. 53%.

This circuit is just a rough first attempt and resistor and cap values will be tweeked later to more common values.

However when I simulated this circuit the scope displayed a straight line function which has left me quite puzzled. Since then I've checked my connections and calculations several times and as well as built the simulation circuit from scratch about 3 more times but it doesn't seem to change anything!

any suggestions??

//2 attachments are the circuit and the scope reading.
 

Attachments

Thread Starter

ro169

Joined Oct 10, 2014
62
Hi John, Thanks for your response.
I've just tried that.. still the same response from the scope.
 

jpanhalt

Joined Jan 18, 2008
11,087
I haven't checked your calculations and have never used multisim. However, if your period is 1 second, and I were using a real oscilloscope, I would increase the time/division. Of course, with a real scope you would still see the output bob up and down.

Sorry can't be more help. Your circuit looks fine for simulation.

John
 

bertus

Joined Apr 5, 2008
22,266
Hello,

What happens when you set the timescale of the scope much higher?
You now only have 10 mS/div.

Bertus
 

crutschow

Joined Mar 14, 2008
34,201
Sometimes oscillators will not start in a simulator without a small kick-start (generated by circuit noise in a real circuit). This can be done by adding the Use Initial Conditions command (UIC) or ramping the power up from zero.
 

Thread Starter

ro169

Joined Oct 10, 2014
62
I've just realized that the simulation isn't running at real time speed. I actually expected it to as I've used 555 timer projects before on the software and it did. so that problem seems to be solved!

However, I seemed to have encountered another 1, after the 1st pulse the output voltage from the 555 seems to exponentially decrease. Is there anything specific that could be causing this?
 

jpanhalt

Joined Jan 18, 2008
11,087
I checked several Multisim simulations for the 555. They had a load (say 1 KΩ) to ground. Have you tried adding a load to your output?

John
 

crutschow

Joined Mar 14, 2008
34,201
.............
However, I seemed to have encountered another 1, after the 1st pulse the output voltage from the 555 seems to exponentially decrease. Is there anything specific that could be causing this?
Is the virtual oscilloscope set for AC (coupled) input? It should be DC.
 

Thread Starter

ro169

Joined Oct 10, 2014
62
Have you tried adding a load to your output?

John
Yes I have tried adding a load.

Is the virtual oscilloscope set for AC (coupled) input? It should be DC.
You can check the oscilloscope settings in the screenshot i uploaded earlier.

Basically from the oscilloscope reading (with a 9V input to the 555) the first peak in the square wave represents 9V, thereafter the successive peaks are at approx 900mV
 

MikeML

Joined Oct 2, 2009
5,444
I had just posted this on another forum, so I put in your values and resimed it. Note the correct start-up behavior, and the timing.

47b.gif
 

Thread Starter

ro169

Joined Oct 10, 2014
62
Hi,
Thanks for all responses.

I have attached the oscilloscope readings from the simulation showing different times as you can note under "Channel A".
Also; if anyone has worked with multisim before, is there any reason/setting that makes the scope displays only about 1.3 seconds of data at a time. (if you look at the picture attached you'll see i had to use 2 screenshots to show times before and after approximately 1.3s as the scope only displays a certain period at a time.

scope.png
 

Thread Starter

ro169

Joined Oct 10, 2014
62
Hi all.

I'm still not sure what was causing the problem on the simulation, but I have altered the circuit and changed nothing on the simulator.
The results seem to be fine now.

Thanks For The Input.

mult.png
 
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