Chapter 4 - AC Circuits
PARTS AND MATERIALS
- IBM-compatible personal computer with sound card, running Windows 3.1 or better
- Winscope software, downloaded free from internet
- Electronic “keyboard” (musical)
- “Mono” (not stereo) headphone-type plug for keyboard
- “Mono” (not stereo) headphone-type plug for computer sound card microphone input
- 10 kΩ potentiometer
The Winscope program I’ve used was written by Dr. Constantin Zeldovich, for free personal and academic use. It plots waveforms on the computer screen in response to AC voltage signals interpreted by the sound card microphone input. A similar program, called Oscope, is made for the Linux operating system. If you don’t have access to either software, you may use the “sound recorder” utility that comes stock with most versions of Microsoft Windows to display crude waveshapes.
Lessons In Electric Circuits, Volume 2, chapter 7: “Mixed-Frequency AC Signals”
Lessons In Electric Circuits, Volume 2, chapter 12: “AC Metering Circuits”
- Computer use
- Basic oscilloscope function
The oscilloscope is an indispensable test instrument for the electronics student and professional. No serious electronics lab should be without one (or two!). Unfortunately, commercial oscilloscopes tend to be expensive, and it is almost impossible to design and build your own without another oscilloscope to troubleshoot it! However, the sound card of a personal computer is capable of “digitizing” low-voltage AC signals from a range of a few hundred Hertz to several thousand Hertz with respectable resolution, and free software is available for displaying these signals in oscilloscope form on the computer screen. Since most people either have a personal computer or can obtain one for less cost than an oscilloscope, this becomes a viable alternative for the experimenter on a budget.
One word of caution: you can cause significant hardware damage to your computer if signals of excessive voltage are connected to the sound card’s microphone input! The AC voltages produced by a musical keyboard are too low to cause damage to your computer through the sound card, but other voltage sources might be hazardous to your computer’s health. Use this “oscilloscope” at your own risk!
Using the keyboard and plug arrangement described in the previous experiment, connect the keyboard output to the outer terminals of a 10 kΩ potentiometer. Solder two wires to the connection points on the sound card microphone input plug, so that you have a set of “test leads” for the “oscilloscope.” Connect these test leads to the potentiometer: between the middle terminal (the wiper) and either of the outer terminals.
Start the Winscope program and click on the “arrow” icon in the upper-left corner (it looks like the “play” arrow seen on tape player and CD player control buttons). If you press a key on the musical keyboard, you should see some kind of waveform displayed on the screen. Choose the “panflute” or some other flute-like voice on the musical keyboard for the best sine-wave shape. If the computer displays a waveform that looks kind of like a square wave, you need to adjust the potentiometer for a lower-amplitude signal. Almost any waveshape will be “clipped” to look like a square wave if it exceeds the amplitude limit of the sound card.
Test different instrument “voices” on the musical keyboard and note the different waveshapes. Note how complex some of the waveshapes are, compared to the panflute voice. Experiment with the different controls in the Winscope window, noting how they change the appearance of the waveform.
As a test instrument, this “oscilloscope” is quite poor. It has almost no capability to make precision measurements of voltage, although its frequency precision is surprisingly good. It is very limited in the range of voltage and frequency it can display, relegating it to the analysis of low- and mid-range audio tones. I have had very little success getting the “oscilloscope” to display good square waves, presumably because of its limited frequency response. Also, the coupling capacitor found in sound card microphone input circuits prevents it from measuring DC voltage: it is as though the “AC coupling” feature of a normal oscilloscope were stuck “on.”
Despite these shortcomings, it is useful as a demonstration tool, and for initial explorations into waveform analysis for the beginning student of electronics. For those who are interested, there are several professional-quality oscilloscope adapter devices manufactured for personal computers whose performance is far beyond that of a sound card, and they are typically sold at less cost than a complete stand-alone oscilloscope (around $400, the year 2002). Radio Shack sells one made by Velleman, catalog # 910-3914. Having a computer serve as the display medium brings many advantages, not the least of which is the ability to easily store waveform pictures as digital files.
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