Super Basic Circuit Help

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Main Question or Discussion Point

OK i have to build a waveform generator with adgujtable amp and freq. I have lots of soldering expericne, but no schematic reading experience.

I found this helpful guide to achieve what i want
http://www.instructables.com/id/Build-a-Square-Wave-Oscillator-Part-1-of-DIY-Mod/"
Here is the shematics (hard to read kinda)

FG9MG8CGFRWMNFA.png


My basic questions so far are...

First, in the parts list it calls for 5x 1kΩ Resistors and no 10k ones, but the schematics show 10k ohm ones instead...I bought 5x 1kΩ ones so hopefully i can use them.

Second, I have plenty of 4.7k and 10k pots. It calls for 500k ones. I have 2 of these http://www.radioshack.com/product/index.jsp?productId=3012598, but they onoly have 2 pins instead of the standard 3 pins. Am i out of luck, or can i use a combonation of what i have?

Third can the capacitors go in any direction? How can you tell based on these schematics

Fourth DO the ground points all connect together basically

Last, how would you recommend soldering it all? I have a perfboard but none of the holes are connected. Should i use solder to make the leads between two things?

Thanks a ton in advance

Any suggestions help!!!
 
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Answers and Replies

  • #2
vk6kro
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It looks like the circuit would still work with 1 K resistors, however the circuit may draw nearly 10 times as much current from your battery.

We are very lucky to have access to very cheap electronic components, so I would suggest that you go back to the store and get the correct components.

Then next time you need a 1 K resistor, you will know where to find one.

The upper 500 K pot is used to vary the frequency. A smaller one will give you less variation.
So, you need one of those, too.

The pot on the output doesn't have to be 500 k so you could use your 10 K pot there, but put one of your 1 K resistors in series with it instead of the 1 Meg resistor.

Yes, the earth points do have to be joined together.
So do all the other connections in the circuit, so if your perfboard doesn't connect them, you will have to do it with bits of wire, not just sloder. The cut ends of resistors make good wire bits for connecting things.


The 1000 pF capacitor can go in either way. The 100 uF capacitor is not suitable for that position. It is too big and it will not have a DC voltage on it, so I would get a 0.1 uF capacitor.
 
  • #3
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ok its kinda irritating that it said get 5 1k when you need 10k...but oh well one more trip to radio shack wont hurt

Im glad i wont have to buy more pots. So the 500k one i have have two leads, a middle one and a right side one. Does the wire coming from pin 2 of the op amp go into the right lead and the other in the center or vice versa?
[PLAIN]http://rsk.imageg.net/graphics/product_images/pRS1C-4471472w345.jpg\ [Broken]

The 1000 pF capacitor can go in either way. The 100 uF capacitor is not suitable for that position. It is too big and it will not have a DC voltage on it, so I would get a 0.1 uF capacitor.
Im confused. Are you saying the 100uF cap should not be used as directed in the schematics?

Is there any other things you would do different if you built this?

Thanks a ton vk6kro
 
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  • #4
vk6kro
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It doesn't matter that one of the leads of the pot is missing as long as it isn't the middle one.
So, yes, just connect to the two connecttions it does have.

That 100 uF is a bad choice for that position. If it got a voltage on it from something you connected it to, it could take several minutes to discharge.

After you get it going, you could make the capacitor switchable to get other frequencies.
Like this:
[PLAIN]http://dl.dropbox.com/u/4222062/switched%20capacitors.PNG [Broken]

You could also consider getting a wall-wart power supply for it because the batteries will get expensive if you keep using them.
 
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  • #5
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Ok that all makes sense. In the future i may try to make it switch waveforms, maybe some sort of toggle between square, wave and sine, but for now ill keep it simple.

For the power supply i was planning on using a Dc 9v jack and just plug it into the wall

Thanks
 
  • #6
vk6kro
Science Advisor
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Don't get any resistors yet. There are some problems with the circuit.

[PLAIN]http://dl.dropbox.com/u/4222062/Square%20wave%20Osc.PNG [Broken]

The circuit above is looking more promising.
 
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  • #7
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So it looks like the only difference is some resistors and that added +4.5 volts. What is the Purpose of that?

Also i only need a total of 2 33k resistors right? The drawing shows 2 chanes with 2 on each chain, but you only wrote 1 33k on each chains. So im assuming i only need 2 total
 
  • #8
vk6kro
Science Advisor
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So it looks like the only difference is some resistors and that added +4.5 volts. What is the Purpose of that?

Also i only need a total of 2 33k resistors right? The drawing shows 2 chanes with 2 on each chain, but you only wrote 1 33k on each chains. So im assuming i only need 2 total
The 4.5 volts is the same one that goes to pin 7. It was just difficult to draw it like that.

Just two 33 K and a 47 K and a 0.1 uF capacitor.

The waveform is a square wave except above 5 KHz when it becomes triangular.

Seems like you could make a useful instrument out of it if you can borrow a frequency counter to calibrate it.
 
  • #9
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5Khz is a pretty high pitch right? What makes it change?

So for clarification, there will be a 33k resistor before the +4.5v gets to pin 7
 
  • #10
vk6kro
Science Advisor
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The new 33K resistor just connects to pin 7 as well as the +4.5 volts.

5 KHz is a pretty high pitch. Like a mosquito, I guess.
 
  • #11
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ugh. I finished it all, tested...didn't work. The amplitude pot worked fine, but only noise came out

I looked for my error and found i used a 47 ohm resistor instead of 47k. careless mistake, but radio shack is closed and it would be great to finish tonight. Is there a way of reconfiguring things to replace the 47k ohm resistor with one i have?

I'm currently searching through old tvs and vcrs for the golden 47k resister
 
  • #12
vk6kro
Science Advisor
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The 47K should not stop it working. It is only there to stop the coverage being too great when the pot resistance is reduced towards zero.

If you set the pot resistance to maximum, the frequency should be about 1000 Hz, according to my simulator.

If you have the inverting and non inverting inputs of the IC wrongly connected, this would stop it working.

Which opamp are you using, and how are you checking the output?

Check that all the wires go where they are supposed to go.
 
  • #13
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Im using the radio shack 741 op amp. I havent checked the output. All i did was give it power, and p[lug it into a headphone jack. The headphones just buzzed, but the 10k vol pot changed the vol of the buzz.

Im about to go to bed, so i cant recheck now.

sine im using the same perfboard, ive been using these pictures as a guideline for my own. I matched most of it up to this. One thing that confused me was replacing the 2 10ks with one, because some stuff would be skiped over using one resister vs 2. (see picture for what i mean)

http://www.instructables.com/files/orig/FBJ/5FKP/GFRWMOU7/FBJ5FKPGFRWMOU7.jpg
http://www.instructables.com/files/orig/FLK/SKKY/GFRWMOU6/FLKSKKYGFRWMOU6.jpg
 

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