Poor Man's Oscilloscope:

Using A Computer With Sound Card As An Oscilloscope.

When experimenting with electronics, I think few tools are as powerful as an oscilloscope. Just a few minutes with an oscilloscope can save me hours of tinkering.

However, commercial oscilloscopes unfortunately can be quite expensive. To the electronics hobbyist, who perhaps has had no experience with an oscilloscope to understand its worth, and who may look to a junk heap for most parts and projects, the prospect of spending thousands for a tool of uncertain usefulness simply is beyond consideration. Here is a way to try a basic oscilloscope with almost no expense.

At its most basic level, an oscilloscope is a set of cables, an analog to digital converter, software, and a display system. A basic personal computer has most of these features already. The cables are relatively simple to build. The sound card or motherboard sound chip does a passable job of converting a voltage to digital data, and software can be had free or for a few bucks that can display the data on the screen.

Oscilloscope Cable:A three conductor shielded cable with a regular 3.5mm stereo phono plug will do the job. A stereo extension cable for a computer or headphones can be purchased if one does not already have a cable available. The stereo cable plugs into the "line in" jack on the sound card (the line-in jack is usually colored blue).

The sound card line input expects only a volt or two at most, so I used a voltage divider resistor network on each of the "right" and "left" conductors to ground for my experiments at 5 volts or so. I happened to have these resistor values handy, and they worked, but I think the values are not critical. I used 1/8 watt resistors, as the current is very small, but any watt value would work. I think the resistors also serve to protect the sound card if I goof and connect the cable to a higher voltage. The tip and middle conductor of the cable plug are right and left, and the base of the plug is ground. The 1/8 watt resistors are small enough to be included inside shrink wrap at the breakout of the cable from the single sheath to the three leads.

The Software: Software is used to capture the "audio" stream from the sound card line-in port, and display it in a graphical format. I used GoldWave v4.26, at the time a shareware program, but I think this version is still available as a trial from http://www.goldwave.com/release.php (see "Previous version" on this link). I configured a recording with the highest available sampling rate (96000 Hz) in stereo for two channel traces. The tracing shown here is a sample from GoldWave. The zoom controls allow picking a part of the trace to examine closely, and one can page through the tracings based on the size of the zoomed segment. Measurements of time are good to about 1 msec intervals using the zoom/measuring tool, but measurements to 0.1 to 0.01 msec are possible zooming in and using the scale on the tracing display.

Measuring absolute voltage is a little tricky, as the sound card will tend to quickly level the trace to eliminate a standing DC voltage. I measured transient voltage changes if needed by comparing to a standard size voltage pulse that I generated. The top trace in the GoldWave tracing at right is part of a LANC synchronized serial frame at 9600 baud. The bottom trace shows the video frame timing.

Try One, Buy One: So I used this for a quite a while and really had some fun with it. This actually works great! It does not have some of the more basic oscilloscope features like triggering, but makes up for it by having vast storage so that very long traces can be easily analyzed.

But once I could see the usefulness of an oscilloscope, I knew I had to have a more full featured unit. I purchased a used Fluke 199 Scopemeter (or more specifically a 199/A), which is a very tough, portable, and easy to use unit. The scope came with a nice attaché case, cables, and a computer interface with software for documenting tracings. Fluke certainly makes a nice scope!

A Dedicated Oscilloscope: The screen capture at right is from my Fluke 199/A and shows on the top trace the voltage drop across the shutter button of a Sony DSC-V3 camera when I took an exposure. One can see the oscilloscope trigger voltage marked half way down the voltage drop. The trace on the bottom shows the voltage across a photodiode in front of the internal flash during the resulting flash photo. One can seen the initial shorter pulse from the pre-flash of the TTL flash metering mode, and the slightly later and longer full flash pulse.

The Fluke screen capture at right is actually the full resolution of the scope's LCD screen, which after using the full resolution of a computer monitor, seems somewhat disappointing. However, much higher resolution data can be downloaded from the oscilloscope to a computer. Here is a sample page of downloaded oscilloscope data from the Fluke 199/A with quite a bit more resolution. The top trace is again the shutter button release, and the bottom trace is a combined square wave showing the video field timing and the smaller pulse of the flash hotshoe release.

Best Regards,
Rob Crockett
Copyright © 2010 All Rights Reserved.

Digital Stereo Photography Home