And then you just connect your circuit with PC soundcard jack input?
Do you mean the protection circuit that I posted, or the circuit you are working with?
What type of circuits are you building or testing?
While you
might be able to connect directly to the soundcard's input jack, line level on consumer kit is usually -10dBV. That means that the full scale voltage is only 0.447V peak, above and below zero. Any more than that and it would probably just 'clip' at the maximum level. Too much and, of course, the soundcard would suffer.
So, unless you are only working on low-level signals like that, you need to use some form of scaling circuit on the input. Protection is good also, so that you don't accidentally blow up your soundcard (and even PC!)
In the protection circuit I posted above, the there is an amplifier that can be switched between x1 and x10, so that can be used to amplify smaller signals. You could easily add a simple voltage divider just before the amplifier's input to divide larger signals down. I would probably use /10 and /100 ranges so that you could measure up to about +/- 45V. You only need a few resistors to do that.
This method will never yield high accuracy of voltage levels though. Good enough to see what is happening, but not for making any critical measurements. It should be good enough for frequency measurements though.
Does any of that help?
- - - Updated - - -
Here is a very simple way to do it. There is no input protection, just a simple voltage divider to allow signals around 5V peak to be measured:
http://www.ledametrix.com/oscope/index.html
Be careful what you connect the probes to, though.
---
Another, simpler input circuit that the first one I posted is here:
http://www.sciencetronics.com/geocities/electronics/projects/soundcard_osci.html
It does not really protect the sound card though. What it does is give a high input impedance. See below.
---
There are a number of issues that might arise using a soundcard.
One is the input impedance of the soundcard's line in circuit. We do not know what it is! It's probably low enough that it might load the circuit under test and cause it to change when tested. That's not useful, so a useful addition to the input is a high impedance buffer. That is what the simpler input circuit above accomplishes.
Another thing to keep in mind is that the soundcard's line input ground is actually connected to the continuity earth in the building wiring. If your circuit's power supply also has the negative or ground side earthed, then make sure to connect ground to ground. Otherwise, if you might accidentally short out the power supply. I've even done this myself
- things get hot and I even melted a 'scope probe once! If you work a lot with audio stuff though, be aware that this can cause a 'ground loop', the effects of which you are likely familiar with already.
(This second point is a minor one, as long as you remember to connect ground of the soundcard input to ground of the circuit under test.)
---
Don't let the seeming complications put you off what you are thinking, by the way. It's not that complicated and there is always help available here. I tend to look at what might go wrong, then make sure it doesn't, before I jump into doing something and sometimes it makes things sound worse than they are.