1/7/2024 0 Comments Signalscope 3.5 mm![]() There are many design for such DC shifting circuits, but many of them need somewhat complicated external powering (dual polarity DC supply usually needed for this kind of opamp circuits). So what we need is a circuit that would scale the around +-100 mV DC input at around 0V to voltage in around 2-2.2V DC range on soundcard microphone input, preferably with some form of DC bias adjustment. So if I just short circuit capacitor C6, when there is nothing connected to input the voltage CM119 mic in is over the top of the operating range, and when 0V is connected to input it is below lower range. The DC potential on the CM119 mic in should be around 2-2.2V DC range for everything to work well. ![]() First the sound card mic input has bias current feed (designed for electret mic powering) that is approximately 4.3V fed through around 4.7 kohm resistor. Some modifications and additions needs to be done besides short circuiting C6 to make this USB sound card to be useful DC capable measuring instrument. That’s OK for many uses (use attenuator if you want higher range). I measured that the mic input is by default designed to take around 100 mV AC signal for full output. ![]() Even if this does not look very good it works well. The SMD components on the circuit board are quite small, so the wire looks quite thick on the picture and it is hard to make a nice looking solder joint. I just soldered a wire that short circuits C6. On my device that capacitor is market with symbol C6 and one end of the capacitor measures low resistance to the mic input 3.5 mm connector tip.Ī closer look at my modification. The capacitor can be pretty easily found on circuit board when you know what you are looking for. So after modification the device should works on your computer with a bandwidth of DC to 15kHz, which is enough for many purposes of DIY electronics. The CM119 IC has, unlike most other sound card controller ICs, no digital highpass inside, which means that it can measure also DC, if the input capacitor is removed. No extra drivers needed in Windows (should work well on Linux according to comments but I have not tried that myself). It seems to be good device, cheap ($3.70) and plug-and-play – easy to use. It is based CM119 USB soundcard IC according to product comments. The product I selected for my project is USB Virtual 7.1 Channel External Sound Card Adapter. USB based sound cards are cheap, and some can even be modified a DC measurements capable measuring instrument. Using an external USB sound card instead of one built onto computer has several benefits: you don’t have to disturb normal PC sound operations with your measurements and mistakes do not break your mains sound card. For example “zeitnitz” is a very fine program! There are many free oscilloscope software that you can use. Sound card based oscilloscope is not new to me, I have used it sometimes. I bought USB Virtual 7.1 Channel External Sound Card Adapter to build a cheap PC based digital storage oscilloscope. Circuits by Tomi, Computers, DIY, Electronics Design, Hardware hacks.USB soundcard to digital storage oscilloscope
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