I built an easy single channel oscilloscope adapter that turns a typical PC into a functional oscilloscope. It is very simple to build and the software is also free. I searched around and found the perfect setup then I tweaked it.
No pot is needed to tweak the voltage. Four 1N4148 diodes handle the protection so you won’t kill the sound or computer. Do not go over 5 volts DC when testing a circuit! I wouldn’t try probing AC current as well!
When I was searching up on the subject I noticed everyone is using typical audio/speaker cable. I find this as a “no no” since you are around AC current noise the system will pick it up. I used an old TV coaxial cable since it is shielded and I have a crap load of it. Just cut off one end and solder a headphone jack on. Since I made a single channel I only used the tip and ground of a stereo headphone jack. You can easily turn this into a two channel but you will need two coaxial cables or build two single channel adapters and use the Mic and Line-in on the sound card. On the adapter side I used a coaxial extension so I didn’t have to strip both ends of the cable, also I can make a longer cable in the future or whatever. Also for the input I am using banana plugs so I can use my multimeter probes. If I make another one I’ll use a BNC connector so I can use a real oscilloscope probe. For the case I used an old GrandStream ATA case. The circuit is small enough that you can toss it into anything such as a TicTac box or even a Altoids box.
The software is called Soundcard Oscilloscope. It works very well and I can easily run it on an old AMD Athlon XP 2000+ with WindowsXP. The software isn’t freeware but it is free to use for private use.