At its most basic, an oscilloscope is an instrument that graphs an electronic signal over time. They’re extremely useful when evaluating electronics—particularly when you’re working with analog signals. Most modern oscilloscopes offer a lot of additional features that help you really dig into those signals, but simple oscilloscope functions can be handled by a small amount of hardware. Most of the cost in that case comes from the display itself, which is why you may want to follow this tutorial to turn a USB sound card into an oscilloscope that can be used with your computer.
A USB sound card that has both a microphone input and a headphone / speaker output contains two very important components: a DAC (Digital-to-Analog Converter) and an ADC (Analog-to-Digital Converter). The ADC is normally used for the microphone and can be used to translate an analog signal into digital information that your computer understands. The DAC does the opposite, and takes digital data, like the sound information in an MP3 file, and sends it out through the headphone output as an analog signal. In this case, that can be used for the signal generation functions of an oscilloscope.
The problem is that both of those can only handle very small voltages and currents. If you try to measure the signal on a 12V stepper motor driver, for example, you’ll almost certainly fry the sound card. At the very least, you won’t get any useful information. This project describes how to create an “analog front end” for cheap USB sound cards that scales those signals down to levels that are suitable. It can work with both DC and AC signals, and has attenuation ratios ranging from 1:1 to 100:1.
The PCB design for that analog front end is provided. Most of the components are through-hole, but there are a few SMD chips that are used. The board contains two switches, two potentiometer knobs, and the signal inputs. Two 3.5mm jacks connect the analog front end to the USB sound card. The board can be housed within an off-the-shelf enclosure, or a 3D-printed one. With Christian Zeitnitz’s Soundcard Oscilloscope software, you can use the hardware to graph out signals just like you would with a real oscilloscope.