soundcard oscilloscope

[eddieprice19]

I have built a control circuit for a small motor and would like to examine the pwm waveform. I downloaded software for a scope based on a PC soundcard. It works fine off the internal mic and when I whistle I get a nice sine wave, but I can't get anything through the external mic connection. I have never used an external mic on my laptop so don't know how well or if it works. I have a stereo plug and a cable, but can't get any waveforms onto the screen. Is there something I need to do, or is there some other way of accessing the sound card?

 

[Sceadwian]

You wouldn't use the microphone input, you would use the line-in input. You have to be careful, the line-in signal is only around .2 to 1V tolerant so you can't just feed it a signal from anywhere you want. A PC sound card is going to give you HORRIBLE wave from rendition, what specifically are you trying to see in your PWM waveform?

 

[MikeMl]

Also, be advised that most laptops only have a provision for a mono mic. Even though the A/D converter has two channels, the laptop makers are too cheap to wire up both tip and ring in the stereo mic input. They only hook up tip, not ring.

Sound card oscilloscopes do not have response to dc. There is a blocking capacitor in series with the input, so the frequency rolls off below ~30Hz.

 

[eddieprice19]

I have just checked my laptop user manual and I have two inputs, microphone and line-input. Would it be possible to use an oscilloscope probe with say a 10x reduction. I did see a reference to a Virtins Sound Card Probe, but they appear to be in the States. Is there a UK version? What sort of plug fits the line-input? Is it a stereo plug like the microphone input?

The reason for my interest is that I am having a problem with using the pwm waveform to drive an h-bridge. My only piece of test equipment is a cheap digital dvm. The waveform is about 100Hz with a max amplitude of 5v. The simulated waveform from mplab looks fine. With no motor attached the average dc voltage, as measured by the dvm increases from about 0.7v to 4.5v, as I increase the speed control, but when I attach the motor to the bridge the average voltage only rises from 0.7 to about 0.8. I would just like to see what is happening. The motor is a small 12v device for a model train.

I am using a Zetex ZHB6718 h-bridge. This is a bipolar based bridge, presumably with Darlingtons, since the quoted small signal gains are typically 450. The pwm signal is applied to one side of the bridge while the other side is at 0v. The motor runs, but only slowly.

 

[mdanh2002]

Oscilloscope 10x probe normally has a very large internal resistance (around 9 M ohm) which, when combined with the oscilloscope input impedance around 1MOhm, will attenuate the input signal by 10%. Sound card does not have the same input impedance as an oscilloscope, so an oscilloscope probe can't be used for a sound card oscilloscope.

See this **broken link removed**

 

[Sceadwian]

DVM's can not be used to read PWM signals, the results you get will be unpredictable and will have little bearing on circuit functionality.
Can you post a schematic of the circuit you've built?

 

[eddieprice19]

I will try to post a schematic. It will be in pdf format. Don't know how well it will reproduce. What do you normally use for schematics? I am using TinyCad.

Just noticed in another thread a schematic saved as a gif file.

Thinking about what you say about the use of a dvm. The pwm signal from the microprocessor is unidirectional, and so as the duty cycle increases the average value will increase and should be noted with a dvm.