Okay, okay, now that I’ve researched the actual voltage levels on computer inputs, I can revisit the idea of building a microphone input-based oscilloscope with more confidence.
A resistive divide-by-1000 voltage divider is a good starting point to get 5 V logic down to a voltage level similar to that from a “dynamic” moving coil microphone.
- Bottom leg: 100 ohm
- Top leg: To divide by 1000… 100 * 1000 = 100 K ohm
Yeah, this is the easiest selection of resistors, but technically the total resistance is 100.1 K ohm, so the division factor is 1001, not
- And is this sufficient resistor values for the current drain of the sensor? I’m pretty sure it is.
Are dynamic moving coil microphones those common ones you see on stage? Yes, these are the typical kind of microphones used on stage amplification, where the listeners are in the same room as the performer and it is important to use this type of microphone to supress the “howling” feedback loops that result when the microphone picks up the amplified output. Try plugging one of these into your audio input and see what kind of digital signal levels you get from your particular microphone input.
Otherwise, if you really don’t have one of these on hand, just try divide by 1000 to be on the safe side, and if you still have plenty of headroom on your digital input, you can step up to divide by 100.
What exactly is the voltage level output of moving coil microphones? How does that compare to electret microphones?
20200426/https://en.wikipedia.org/wiki/Microphone
20200426/https://en.wikipedia.org/wiki/Electret_microphone
Unfortunately Wikipedia doesn’t give numbers to these different types of microphones, so I have to search elsewhere.
20200426/DuckDuckGo output voltage dynamic microphone
20200426/https://www.sweetwater.com/insync/understanding-signal-levels-audio-gear/
Ah yes, so this article labels microphone voltage levels into just two different general classes of microphones, when they output less than line-in levels.
- 2.5 mV low-output mics (i.e. dynamic microphones)
- 23 mV high-output mics (i.e. electret microphones)
Then, of course, consumer line-in starts at 316 mV (0.3 V) and voltages go up from there to the more familiar voltage levels for digital logic I/O connectors.
Indeed the numbers agree with my previous recorded numbers… just that I wasn’t exactly sure about which microphones those numbers were supposed to refer to.
Now, revisiting the instructable that details how to build a cheap oscilloscope using a PC microphone input… they recommend picking the resistors to divide by 1.3 at minimum, and divide by up to 11 maximum. Clearly, they are recommending resistive divider values only suitable for line-in input, not microphone input.
- 22K high leg, 82K low leg.
- Up to 820 K high leg.
- Plus an additional 50K or 100K linear potentiometer on the high leg.
And they say the potentiometer is only necessary to adjust if the input is above 5 V.
20200426/https://www.instructables.com/id/Use-Your-Laptop-as-Oscilloscope/
And they recommend plugging the 3.5mm stereo jack into LINE-IN/MIC. Plugging into line-in for sure is safe with 0.8 V or less, 0.3 V being a safer starting point if you really don’t know about the specific limits of your device.
Inductive coupling is also another approach that can work well. This may be an easier approach for stepping down really high voltages, and it has the advantage of using separate electrical current loops.
Okay, let’s fact check… here, they are recommending that the microphone input accepts up to 100 mV. Line-in can do up to 1 V. I would agree without any further knowledge of the hardware.
DuckDuckGo can laptop microphone input handle line in voltages
20200426/https://forums.tomsguide.com/threads/pc-microphone-in-voltage.239703/
You can’t measure DC signals with microphone input, can you? Yes indeed, sound cards are AC coupled and won’t measure frequencies below 20 Hz. Either that, or they use a high-pass filter on the input to get the same effect.
This is an old but good info article on the electrical construction of the line-in input on old PC sound cards. Indeed, most sound cards have a capacitor to act as a high-pass filter on the input of line-in, and therefore especially also microphone input.
20200426/https://www.qsl.net/om3cph/sb/dcwithsb.htm?_ga=2.162187132.1923014167.1587942445-807676586.1587942445
But hey, here’s another approach for measuring DC. If the frequency is too low, you can modulate the signal with a higher frequency carrier wave. Just a simple transistor amplifier to multiply the output of an oscillator by your signal under measurement will do the trick. Need a cheap oscillator source? Of course, use PC audio output. A USB audio device that provides both an audio output and a microphone input is really handy and really cheap for this specific purpose.
However, if you do modulate a low frequency waveform, please note that you will not be able to determine if the input is positive or negative by analyzing the output. If you want to know the polarity, you can setup two polarized modulator circuits and determine which one results in a signal. So, you have to determine the polarity via a manual configuration switch flip which of the two signals is forwarded, two microphone inputs, or just try flipping around the test terminals manually.