DC pulse generator

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Alex electrical student

Alex electrical student

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I already built couple different types of dc square wave pulse generators with a 555. For some reason i can't see a square wave on the oscilloscope. All is see is at a setting of 500-1 nano seconds/division is a straight line that jumps up and down a little and at a setting of 20 - 50 micro seconds/division i see four little pulses moving at about at an amplitude of 11 volts. Can any one help me out why i can't see a perfect square wave?

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What frequency do you expect?
What duty cycle?
What amplitude?

I'm sure you already checked these:
Scope input set to DC?
There is likely a test output on the scope. Have you checked that to see if it shows right?

carl
 
I am building variable dc square wave pulse generator. It has two potentiometers. One for the frequency (100 Hz. - 2,000 Hz.) and second one for duty cycle (10% - 90%). I tested with two different oscilloscopes. I set them on dc setting. I,ll try to get my hand on a function generator that can make square waves, and with it test the oscilloscopes and see what ill get out of it.
View attachment 1262
View attachment 1263
In the image above. I can see clearly the square wave on the oscilloscope. But but function generator is set on 20,000 Hz. I'm thinking that oscilloscope can't catch the frequencies i am running at because that were the sweep is started.

But then i run into another problem because oscilloscope should handle 60 Hz AC.

Help me out.
 
Alex electrical student said:
... I'm thinking that oscilloscope can't catch the frequencies i am running at because that were the sweep is started.

But then i run into another problem because oscilloscope should handle 60 Hz AC. ...
Click to expand...

Alex-
I'm sure that scope will display - if its working and set right.

1. We still don't know what amplitude you are expecting.

2. You don't know if the scope is working

3. You don't know if the generator is working.

Let's eliminate a couple:
Dig out your schematic and figure out the expected amplitude.

Get a transformer and with a similar output (5-12V). Set sweep to display a couple of cycles - say 50ms clear across. Likely in the 2 - 5 ms/div range. Connect up the scope. Set gain and trigger to see the couple of cycles. Now you know the scope works. Its tough to troubleshoot if you don't know your test equipment is working.

Go to your generator. Set the controls mid span - 1000hz and 50%. Change your sweep rate to display 1 cycle per 2 divisions - 500ms/div.

What do you see now?

carl

Whoops major edit: second sweep rate should have been 500us
 
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The output amplitude is the same as the input amplitude, which is 12 V.
Here is the schematic.
View attachment 1264
On the very top of the schematic is TB4 which is the terminals.
Terminal 4 is input negative (-V)
Terminal 3 is ouput negative (-output)
Terminal 2 is output positive (+V)
Terminal 1 is input positive (+V)

If you look at the diagram, terminal 3 and 4 are separated semiconductor relay. The relay turns on and of by the control of 555. Because it a relay that makes the circuit to pulse, it shouldn't change the amplitude. Input amplitude should be the same as output amplitude. I'm using 12 volt battery for the input so the output will be pulsating 12 volts.

I'll test the oscilloscope on 60 Hz AC on Monday. I did it before and it worked.
 
Once you figure your scope out check the output of the 555 and then the output of the op amp to see if they are working. That will narrow down where the problem is if you are not getting anything out of Q4.

-Hal
 
hbiss said:
I think you need to learn how to use an oscilloscope before you go any further. ...
Click to expand...
Pretty sharp opinion Hal, but I thnk alex already had that figured out.

carl
 
coulter said:
Pretty sharp opinion Hal, but I thnk alex already had that figured out. ...
Click to expand...
Or maybe not.

Alex -
I just read your post on the metal shear. Is this another of your quizes? Let us know.

No response will be considered an answer.

carl
 
I am getting an amplitude of 11-11.5 volts output when my setting is on
2V/division and 500 ns/division. That tells me that q4 lets the voltage through. I checked the q4 the with the continuity tester. It was ringing when i applied power to the input, and didn't ring when i took the power away. So the q4 seems to work properly.

Below is a photo of my circuit that i built. I found one problem with it. I found two resistors touching each other. I circled them in the photo.
I don't have my osciloscope with me right now to test it but ill give you the results Monday. The two resistor that were touching each other were R8 and R9
View attachment 1266
 
This is no quiz.

It's one little part of project that i'm building and i'm having some problems with it. If im going to finish my expirement with good results then it will make every bodies life easier. Right now i wont tell you what i'm building, but if ill get all the little bugs out of it. But when i do finish it it will save every buddy lots of $$$$$
5,000 dollars or more a year.

My friends are already impatient, waiting for me to finish my project. They want me to duplicate it for them so they would save money. If you would help me out with the little bugs that i have in the circuit then i'll build you one of these.

But thanks for now
 
Alex electrical student said:
I am getting an amplitude of 11-11.5 volts output when my setting is on
2V/division and 500 ns/division. ...
Click to expand...

Yikes! I'm an engineer - I don't use nanos

ns = nanosecond? = 10^(-9) sec?

If so, your sweep rate is about three orders of magnitude too fast. For a frequency of 1000Hz, you want a sweep rate in the 500us/div range. That would put 1 cycle over 2 divisions.

us = microsecond = 10^(-6) sec

carl
 
Ok, what I think you have here is a 555 operating as a sawtooth generator, and then a LM311 comparitor with a pot controlling the set point of the comparitor. What I think the cicuit is supposed to do is to modulate the pulse width that the big FET conducts at.

I think that the problem is that the comparitor reference voltage is changing and thus it's flipping on and off due to lack of hystresis. Stabilizing the supply around the 555 and the comparitor, or using a window comparitor arangement, or a flip flop, or indeed replace the whole timing element with a PIC would be where I'd look.
 
I got the same results as before. But i went online and found that some one had the same problems with this circuit, so they revised it and know sell there pulse width modulator in a nice box that has controls on the front for $200 dollars.
 
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