Build a soundcard scope

soundcard scope article

I recently became aware of some Windows software to turn your PC into an oscilloscope. Having had a fair bit of experience with oscilloscopes over the years I thought I’d have a look at it. This in turn led to me looking at other PC soundcard scope software. If you’re interested you can read my article on PC oscilloscope software here.

After looking at the software I decided I’d go about building the required hardware to turn a PC into a functioning oscilloscope. I then realised there were ready made alternatives that you could buy which in turn lead to an article on USB oscilloscopes that you can read here.

Make your own oscilloscope

Anyway back to the original idea of building an oscilloscope. As the probe connects to the soundcard it’s a recipe for disaster connecting straight to it. You’re just asking to blow the input of you soundcard. Even worse if it’s the soundcard in your laptop. You’re going to need something to act as a buffer between the probe and the soundcard input.

I found a circuit on the internet from Timothy Witham which was adapted from a circuit from “The art of electronics”. I’ve adapted the circuit to use parts that I had lying about so it’s not actually cost me anything.

I’ve used a LM351 op amp, the diodes are all 1N4148’s everything else is pretty straight forward. You need a power supply of + and – 12 volts. Connecting the end of R4 to 0 volts switches the probe between X1 and X10.

The buffer should ensure that if you’re going to blow anything it will be the buffer and you can easily replace anything in this circuit rather than ending up with a blown soundcard.

At the time I built this buffer I was experimenting with building PCB’s again and I’d just got into using the excellent DesignSpark software so I thought I’d give the software a go at a real application, always a better way to get a feel for it's real usefulness. I can report that I found the software to be extremely useful. I would quite happily have paid for it but it’s free. It’s amazing when you look at what it can do compared to the first PCB layout software we used to have at the university which cost about three thousand pound as I remember.

The buffer circuit was drawn into DesignSpark. You can see it in the picture below.

scope buffer circuit diagram

From the circuit, the program generates a net list. You can then manually move and position components or let the program have a go. I made the layout single sided because that’s all I can make at the moment, which isn’t a problem considering the simple circuit and came up with the PCB layout shown below.

soundcard scope pcb layout

You can also render a 3D graphical representation of what the assembled board will look like. You can tilt and move it around and turn it over on screen, much like if you were physically holding the board in your hand and look at anything you want. Again this is really impressive and gives you a real feel for what the completed board will be like.

soundcard scope 3D

I just used a couple of clips instead of an oscilloscope probe while I was experimenting, you can see the completed board in the picture below.

soundcard scope complete board

The connections to and from the board that you can see are the thick black cable to the soundcard. The black and red with clips to the circuit on test and the black, red and green, to 0, +12 and -12 volts.

If you can’t make PCB’s this is an easy enough circuit to knock up on a small piece on veroboard. If you’re not too sure about using veroboard I have an article here.

Once the hardware is complete you just need the software. You can read my full article on PC oscilloscope software here or just use what i did in the end.

The above picture shows a screen shot of the soundcard scope buffer circuit running Scope V1.46 connected to an oscillator that I built on veroboard. To be honest I'm amazed at how well it shows the wave form and how smooth it is.

Oscilloscope for audio testing

Obviously this soundcard scope won’t be able to compete with a fully functional oscilloscope. If you are on the lookout to get an oscilloscope read my article on the best hobbyist oscilloscope here. Because of the soundcard you won’t get a very large bandwidth but for basic use on audio circuits it’s a lot better than having nothing. It will also enable you to learn what an oscilloscope does without having to spend much money at all. If you want to learn how an oscilloscope works you can read here.