Thursday, December 11, 2014

Soundcard Scope Interface

Soundcard Interface
A computer soundcard can be used as an oscilloscope for testing frequencies below 20kHz. This circuit is a simple protection and control interface, with BNC connectors for oscilloscope probes.

This device is useful for generating a sine tone into audio equipment, and testing the output. The amplitude control knob of the input circuit allows testing of line level audio (1V), and up to speaker level audio (100V).

This circuit is designed to be used on a work site where a full size scope is impractical. It contains only passive components (therefore has a very low input impedance), and as such should be used only for rough troubleshooting.

Circuit Diagram
The protection circuit ensures voltage at the soundcard input is clipped at 1.2Vp-p.
Veroboard layout
Circuit will mount nicely on the back of a 16mm potentiometer.
Probe Input Circuit
Mounted on a small piece of veroboard.

Completed Unit
BNC connectors are used to connect test probes.
6.5mm socket for use with audio test leads.
Banana socket to connect ground lead.

Visual Analyser (Sillanumsoft) is the software I have chosen to use in this project.
It is available for free download here:

Visual Analyser uses your soundcard to display oscilloscope traces, as well as frequency spectrum. However, the soundcard must first be set up correctly.

Right click on your volume control icon in the task bar.
Select "Recording Devices".
Choose the soundcard input
Select "Properties".
Select the "Custom" tab.
Un-check AGC (automated gain control).
Select "Levels" tab.
Increase microphone to 100%.
Visual Analyser is now ready to use.


Always connect the ground clip of the device to the chassis of the equipment to be tested. Using the ground clip as a probe will short live circuits to ground through your test device and PC (This goes for any oscilloscope that is not isolated from ground).