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Sound card oscilloscope

Im still working on a good circuit for sound card oscilloscope. I found the circuit that is attached and was wondering if its right. Shouldn't the resistors be switched around or am i missing something? Also i was thinking of using 2x1n4148 diodes in series with a resistor and get 1.5 V output. I want to stay within 1.5 v region so i don't destroy my sound card.


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The resistor ratio will depend on the voltage levels you want to measure. Use it as it is for high sensitivity and swap them around for less.
You could also replace the 82k resistors with 2+2 diodes (in antiparallell) for max sensitivity but with (bipolar) protection/limiting/clipping. (That'll be 8 diodes in total.)

Well my question was if the resistors in the schematic are right? Using the schematic i attached with a input of 5v the out was ~ 4.3v and if i switched (82k to 18k and the 18k to 82) i had output of ~ 0.7V(0.7v is ideal for my project. Could he have made a mistake or does the sound card play a role in how this voltage divider works?
new schem

This is the modified schematic. I added the anti-parallel diodes. so now its clipping 1.5v. Is it correct?


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If you use diodes, or zeners, then it's good to confirm the transfer ratio of input to output, as the diodes wil influence the ratiol before the nominal 0.7V 'clip' per diode.

You should also try to determine the voltage level at which your soundcard starts to clip the incoming signal (eg. look at the frequency spectrum of a sine wave and notice when the harmonics become significant - eg. with TrueRTA). Then you can tweak your probe to give the largest input voltage swing before external clipping. You should also try and introduce hf filtering.

Its quite easy to kill a soundcard (don't ask me how I know).

Ciao, Tim
Wether the resistors are right or not depends on the intended application, which you said zilch about..
Audio inputs usually has an impedance, which will modify the external divider ratio. Your voltage numbers disagree, but going by one of the sets input impedance is 52k.
Your modified diagram is incorrect. I said to replace the 82k resistors (as positioned in the original diagram, of course).
And as Tim says, the clipping will be soft, and it's not beneficial to clip it externally at a lower level than the card clips at itself.
I like what I have read so far and I wanted to add that you may see vastly different results just based on the frequency of the input signal you are using.

Do you already know the frequency response of th input in question? We know it is band pass because audio inputs are usually AC coupled so do you know at what frequency the input is in passband 10Hz, 50Hz,100Hz...? Perhaps if you have the ability you should characterize the frequency response then it may make better sense when you calculate your input limiting.

Remember that your input resistors add thermal noise.
The Soundcard Oscilloscope receives its data from the Soundcard with 44.1kHz and 16 Bit resolution. The data source can be selected in the Windows mixer (Microphone, Line-In or Wave). The frequency range depends on the sound card, but 20-20000Hz should be possible with all modern cards. The low frequency end is limited by the AC coupling of the line-in signal. Be aware, that most microphone inputs are only mono.
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