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Trying to simulate a square wave. NOT WORKING

by omonoid
Tags: simulate, square, wave, working
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omonoid
#1
Jan20-11, 08:19 PM
P: 18
For my extra credit project i am building a simple square wave osc i am trying to simulate the schematics in Multism, a program i just learned today.

Here are the original schematics:


more info on the circuit is found here


These are my schematics suppositly replicatinmg the ones above. The ocilliscope just shoots up then remains blank.


Thanks
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Bob S
#2
Jan20-11, 09:28 PM
P: 4,663
You might try two 9-volt batteries, with the midpoint connected to the common ground.

Bob S
omonoid
#3
Jan21-11, 02:08 AM
P: 18
now i just get a straight line

skeptic2
#4
Jan21-11, 09:40 AM
P: 1,803
Trying to simulate a square wave. NOT WORKING

It seems to me there is a jumper where C1 should be. Did you intend that. If you replace it with a capacitor, does it work?
omonoid
#5
Jan22-11, 02:42 AM
P: 18
no that didnt work i aslo saw that as a possible problem. It is now a cap
Bob S
#6
Jan22-11, 12:52 PM
P: 4,663
It takes a long time to charge C1 before the multivibrator starts. Wait at least 0.25 sec before starting plot. See thumbnail.

Bob S
Attached Thumbnails
012211.jpg  
skeptic2
#7
Jan22-11, 02:47 PM
P: 1,803
It also looks to me like you have the inverting and non-inverting inputs reversed from the example.
skeptic2
#8
Jan22-11, 03:02 PM
P: 1,803
omonoid, I just created and ran your circuit and get a beautiful square wave with a frequency of about 210 Hz. I think if you reverse your inputs it will work.
yungman
#9
Jan22-11, 04:15 PM
P: 3,883
You build the circuit yet to proof it works? I agree with Bob S that you need to put two 9V battery and put the middle to ground. In your simulation, you don't have a ground reference anywhere. Even if you build the circuit as in the simulation, it won't work. It is different from you drawing with two battery.
skeptic2
#10
Jan22-11, 09:20 PM
P: 1,803
I meant to say I created it in SPICE and I did use two 9 V supplies. Afterward I changed the circuit as little as possible to work off of one supply. See attached.
Attached Files
File Type: doc SquareWave.doc (43.0 KB, 26 views)
Fish4Fun
#11
Jan23-11, 05:35 AM
P: 247
Took me a few, LOL. You DO need two batteries, give this a try.

Fish
Attached Thumbnails
OSC.jpg  
sophiecentaur
#12
Jan23-11, 11:45 AM
Sci Advisor
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P: 11,958
With respect, the initial diagram tells you that you need a + and - supply. The input biasing is taken to the Earth point, which is half way between the two supplies. How could anyone be surprised that the circuit didn't work when connected 'lop sided'? These devices have pretty good common mode rejection but give 'em a chance!
vk6kro
#13
Jan23-11, 04:48 PM
Sci Advisor
P: 4,016
This circuit works well in practice, but it does depend on the output either being positive or negative and a simulator can start up with the output being zero.

I tried this recently and had to deliberately unbalance the circuit by adding a large value resistor from one of the supply rails to the non inverting input.

omonoid
#14
Jan23-11, 05:34 PM
P: 18
thanks for all the feed back. I would like my final product to have a pot to vary the frequency and one to vary the amplitude. Would i just preplace one of the resistors with a pot to make the freq vary? Also for amplitude would i just wire in a pot before the output?
vk6kro
#15
Jan23-11, 06:02 PM
Sci Advisor
P: 4,016
First try the extra resistor shown in the previous post to see if you get oscillation.

If you do, then R1 and C control the frequency so it is easiest to make R1 variable.

The voltage at the non inverting input sets the amplitude of the square wave, however adjusting it may cause loss of oscillation.

You could just put a pot (maybe 10 K) from the output of the opamp to ground and take the output from the moving contact and ground. Try simulating it.
omonoid
#16
Jan23-11, 06:28 PM
P: 18
ugh i must suck at simulating. My result was a flat line on the oscillospope

I also noticed its a LM124 op amp. I only have a 741

here is my schemtaics for the simulation

vk6kro
#17
Jan23-11, 06:45 PM
Sci Advisor
P: 4,016
I get delays of a second or so before it starts oscillating.

Can you set your time so that you watch it for 5 seconds?

The actual opamp probably doesn't matter for this simulation.
omonoid
#18
Jan23-11, 06:53 PM
P: 18
Hmm i started at the oscope for like a minute and it was a flat line
Attached Thumbnails
f.png  


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