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AC on breadboard

  1. Mar 4, 2013 #1
    I was wanting to get some hands on experience with AC current circuits. I have been tinkering with DC circuits on a breadboard and was wondering what the best way to get AC on a breadboard would be. I can't really spend 200+ on a function generator. I might spend 30 or so on a kit function generator if that is what I need. I also of course can't buy an oscilloscope.

    So my question I suppose is, is a kit function generator worth it for an amateur hobbyist?

    And, why can't I just mimic AC using oscillating circuits of some sort?

  2. jcsd
  3. Mar 4, 2013 #2
    I just posted something like this the other day - look to a PC Soundcard solution like: http://www.zeitnitz.de/Christian/scope_en - I have not used this product and there are android scopes / signal gens as well - but for the $.... this has to be what you are looking for.
  4. Mar 4, 2013 #3
    Getting a function generator would be useless without an oscilloscope. I'm in the same position as you. I still don't have these two equipments because I think that I don't need them that much at this level, although I'm a fourth year engineering student, but I get away with this problem by using PSpice simulation software. Testing different concepts, theories and some applications with this incredible software, is really entertaining.
  5. Mar 4, 2013 #4
    Out of curiosity, a year ago or so I downloaded the zeitnitz soundcard scope that Windadct suggested. It is pretty impressive for a freebie (although I have no practical use for it). Basically an audio frequency scope.

    For a source you can use your IPhone. It contains an app for driving an audio sinewave out to headset port.

    So, If you have an IPhone and a PC, you have everything you need for free.
  6. Mar 4, 2013 #5


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    yeah simulations are ok sorta ... problem is most of them assume ideal conditions
    there's nothing like building an actual circuit and measuring the results with real test gear
    where real world components have wide and varied tolerances that add a whole different effect to the operation of the circuit being experimented with

  7. Mar 5, 2013 #6


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    I found this link It could be a way in for you. It's a fairly straightforward thing to do and there are a number of freebies that will give you a scope picture and allow you to source low frequency (audio) signals with a PC.
  8. Mar 9, 2013 #7

    I have the cable to try this, but I don't have a line-in jack on my laptop.

    Would this work with a mic jack? I guess not since its just mono.

    Otherwise I might have to buy an external usb sound card.
  9. Mar 9, 2013 #8


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    The mic input jack should work just as well. You may need to adjust the gain with the control panel. The only problem with using the PC is that you will have a low(ish) input impedance compared with the 1MOhm or 10MOhm of a 'scope and also a lower sensitivity. Not a problem for what you need, I think.
  10. Mar 9, 2013 #9
    I think I got it going, but am not sure:

    There ended up being a way to switch the mic input to a stereo mix using windows and also the software that came with my sound card drivers. I verified it was stereo after putting this together.

    So, first I stripped the stereo extension cable and found 2 wires in a copper sheath (I was expecting 3 wires). The sheath was ground. It was easy to verify the other wire connections to the 3.5 plug using my multimeter.

    Instead of soldering immediately, I decided to just build the voltage dividers on my breadboard, so I could play around with resistor values. I wanted to make sure I wasn't going over the limit for my netbook (ASUS 1000he). The other site referenced above by Windadct suggested 0.7 so I went with that. I was using a 9V (which was actually closer to 7V) and so ended up with resistors of 10k and 1k.

    I attached the wires from the stereo cable to the voltage dividers on my breadboard using alligator clips. Then for the "probe leads" I just used some jumper wires.

    I then built an astable 555 timer leading into an LED. My RC components affecting the flash frequency were 33uF and 10k ohms. I put one lead of the probe next to the anode of the LED and the other on pin 1 of the 555.

    I plugged the stereo cable into my computer and set up goldwave to start recording. As soon as I powered my circuit I got quite a surprise! My computer made the same sound as if I had connected an 8ohm loudspeaker to my circuit where the LED is. That makes sense, but still it was somewhat shocking. This however was with different RC values, when I changed to the ones mentioned above it became inaudible, but I could still hear small clicking that matched my LED lighting.

    I printed the screen to show you.


    I'm not really sure this is as it should be, especially the huge spikes. My digital multimeter seems to max around 0.5, is it as simple as the vertical axis in goldwave being voltage?

    I would be very grateful if someone could build the same circuit and test with a real oscilloscope. I will draw my circuit in circuitlab and post later.
  11. Mar 9, 2013 #10
    Here is a zoomed in view for a 1 second interval.

  12. Mar 9, 2013 #11
    Ok, here is where that wave is supposedly coming from. And only while I was making this did I realize connecting channel B to pin 1 wasn't exactly what I wanted to do. Regardless, can anyone show me a real oscilloscope's output to compare?

  13. Mar 9, 2013 #12
  14. Mar 24, 2013 #13
    I wanted to share some more if anyone is interested. I moved the probe voltage dividers off the breadboard and onto a terminal block to make it more convenient to use. I'm still hesitant to solder it together because I'm not really sure on the values I'm using, I always check with the multimeter first before slamming it into my computer.

    Heres some pics, the circuit is just some potentiometers and a PUT making an LED blink. Goldwave recordings have time stamps so I can see exactly how much is between blinks. The loose grey wire in the second pic is the unconnected positive for the battery.


    The yellow is ground and green channel A on the probe. I'm going to put a stranded wire across the terminal strip and do another voltage divider on the opposite side if I ever need channel B.

    I'm still not really convinced this is doing what an oscilloscope would. All I can say for sure is that I'm throwing voltage pulses through my soundcard with no noticeable damage to my computer.
  15. Mar 26, 2013 #14


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    Nice job mishma! This is some great hacking. All those plots look AC coupled to me which is why they likely look a little different then what you are expecting. Basically, your "scope" is measuring the derivative of the input signal. I think this is likely because you are using the microphone input. I know the microphone input on my laptop is AC coupled. I suspect yours is as well.

    Apparently some laptops have a DC coupling mode on the mic input, like the OLPC kit. So you might check that.
  16. Mar 26, 2013 #15


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    Some questions for you:

    You have 2 resistors in series and are calling it a voltage divider. You are then connecting your probe to what I would call the input to the divider? Where is your signal input? As it now stands you could replace the 2 series resisters with 1 equivalent. How familiar are you with voltage dividers?

    What is IC2? If I were to attempt to recreate what you have done I need that info.
  17. Mar 26, 2013 #16
    Hi Integral, the resistors aren't in series, although I admit it does appear that way. If you look closely at the second to the last pic, you can see on the left side of my terminal strip there is a copper wire, a white wire, and a red wire in that order leading downwards from the top.

    The copper wire goes to ground on the input jack.

    The white wire is unconnected right now, but could be used for a second "channel" later. It goes to the middle section of my jack.

    The red is connected to the point where the two resistors meet. There is a small cylindrical metal tube stretching across it and you can screw 2 wires in from either side to connect them. So the 2 resistors are connected to the red wire. That's how it makes a voltage divider. Red leads to the tip of the jack and is having its behavior recorded in green in the pics. I will get a better picture of the terminal strip when I get home.

    The point of doing a voltage divider at all, as I understand it, is because mic inputs on laptops were only designed to handle 0.7 V or so. So I have to step it down before putting it in. That breadboard circuit is being powered by a 9V battery. I couldn't tell you how a real oscilloscope probe is designed to handle a similar circumstance, I assume the voltage limitation is something ridiculously high.

    IC2 is a 555 timer. I mentioned that in the post text but omitted it in the pic, sorry.

    eq1, looks like I will have to read up on that, thank you! I need to just break down and fix my old tower which has a decent sound-card with line-in, if that's the case.
  18. Mar 26, 2013 #17
    Here's what I was trying to say, Integral: on terminal strips connections aren't only made by putting wires into the same hole, but there is a connection through the strip. So the resistors are in one hole together but there is a third (red wire) that completes the node. On the second image below, red lines are drawn across the strip to indicate connections.

  19. Mar 27, 2013 #18


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    Depending on what's wrong with the tower a cheaper option might be a USB ADC kit, a.k.a. a DSO (digital sampling oscilloscope). For $30 you can get a 2 channel 8Ksps USB kit on Ebay. http://compare.ebay.com/like/110833685985?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar [Broken]

    There are tons of other options if you're willing to spend more.
    Last edited by a moderator: May 6, 2017
  20. Mar 27, 2013 #19


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    I have used that connector, so am familiar with it,. Thanks for pics. but I would rather have a schematic drawing of the connection.

    What waveform do you expect from that configuration of the 555?

    Looks like it will not be straight forward, I would recommend that you wire up a simple multivariator circuit with a frequency of ~1kHz, this would be well within your PCscope capabilities and you will learn if your connection is working the way you think it is.
  21. Mar 27, 2013 #20
    Integral, there is a webpage referenced by sophiecentaur earlier that I am using. You can find a schematic there as well as more information. The only difference is I am using 10k and 1k for my resistor values instead of 18k and 82k as shown there. Here is the link again:


    I will certainly try the circuit you suggest, thank you.

    eq1, you're right, I should probably just step up to something like that at this point. I haven't found any further information about my mic jack for my laptop model. This has been fun to mess with though.
  22. Apr 10, 2013 #21
    I am playing with this again.

    I built a sine-wave generator on breadboard using a schematic from Forrest Mims Op-Amp Mini-Notebook (not sure posting this schematic is ok, its on pg 47 of that text). The expected frequency as stated in the text was 1356 Hz. My frequency as measured from Goldwave was 1470 Hz. Here are some pics.

    http://s24.postimg.org/7t9q49ew5/sinewave.jpg [Broken]
    I eyeballed the start and end of the wavelength, then subtracted the two values on Goldwaves scale and took the inverse to find frequency.
    http://s18.postimg.org/m3pjolszd/circ.jpg [Broken]
    The green lead is going to the output on the opamp from my probe, yellow is ground. Batteries not connected here, obviously.

    Now, earlier there was the suspicion that my mic port was taking the derivative of the function. So I guess this could be a cos wave? There is a triangle wave generator here in the same book I will try; that should be conclusive.
    Last edited by a moderator: May 6, 2017
  23. Apr 10, 2013 #22
    Here is the same but using the much more convenient program recommended in post #2. This program displays frequency.

    http://s24.postimg.org/qqueemox1/otherprog2.png [Broken]
    http://s18.postimg.org/vfz11n5dl/otherprog.png [Broken]
    (showing program's display of frequency, somewhat in agreement with expected 1356)
    Last edited by a moderator: May 6, 2017
  24. Apr 10, 2013 #23


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    Since you got sine to sine (with an unknown phase shift) I think you can conclude it can work.

    Then if you get square instead of triangle I would think that would be conclusive.
    Maybe try a couple of different slopes on the triangle just to be sure.

    Since you don't know the gain vs freq of the port it will be hard to predict levels before doing the experiment but faster should make the amplitude of the square higher and slower should make it lower.

    I am very interested to see what you get.
  25. Apr 10, 2013 #24
    Alright! Now were talking: triangle / square wave generator built according to the schematic found here, run with +/-9V


    Here's a pic showing the probe locations on the breadboard, red is square, blue is triangle.
    http://s21.postimg.org/ufirfiwlj/DSCF1654.jpg [Broken]

    Here's the probe connected to the circuit at the triangle output:
    http://s17.postimg.org/y0ul6e4n3/DSCF1652.jpg [Broken]

    And the triangle wave!
    http://s14.postimg.org/nghwfjbmp/tri.png [Broken]

    Triangle frequency:
    http://s16.postimg.org/50tpjnyol/tri2.png [Broken]

    Then I just left everything alone and simply moved the probe to the square output:
    http://s23.postimg.org/6cn8cody3/squ1.png [Broken]

    Square frequency was the same.
    Last edited by a moderator: May 6, 2017
  26. Apr 10, 2013 #25
    Added a second channel to my terminal, successfully probed both square and triangle outputs of previous circuit simultaneously (:D):

    http://s21.postimg.org/4ggvil2fr/dualchan.png [Broken]

    The cost of the 741 opamps was something like less than 5 dollars for 5. I only really needed 3 to get sine, tri, and square. I've heard there are better alternatives to the 741. Other stuff like capacitors etc I had already.

    I will upload pics of the probe with second channel added if interested. I feel like this is a good place to rest for now.
    Last edited by a moderator: May 6, 2017
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