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# the OUTPUT voltage of an mp3 phone jack? Answered

i want to build a vu meter using the lm3914 now it it voltage coming to it is equal to 1.7v all the led with work so i need to devise that on 10 to know the lowest voltage to, turn on the first led. that's the clear part now i don't have an oscilloscope in my home so i cant know the out put of my Samsung galaxy ace4 / i want to know the out put so i can build an amplifier wish amplified the signal to  around 2v

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In place of an oscilloscope you might be able to use a computer's sound card input (preferably "line-in" rather than "mic", but "mic" might work too) and some free audio editing software, like Audacity

http://www.audacityteam.org/

So the audio software can tell you the instantaneous magnitude of an audio signal it is reading, but it won't tell you what that magnitude is, in volts. The units it uses will be dB, or just some arbitrary units.

There may be a standard for "line level" voltage that defines 1.0 volt peak-to-peak as 0 dB, 0.3 volts peak-to-peak as -10 dB, and so forth,

https://en.wikipedia.org/wiki/Line_level

Another approach to figure out the conversion from real voltage to audio software numbers, might be to build an oscillator whose peak-to-peak output voltage is known, e.g. a square wave with peak to peak amplitude of 1.0 volts. Then measure that known signal using the sound card input and sound editing software.

By the way, there are probably a few instructables already written, on the subject of using a computer's sound card as an oscilloscope, uh, here:

https://www.instructables.com/howto/soundcard+as+os...

According to kevmatic at overclock.net you don't need an oscilloscope.

"You don't really need a scope to measure the output of a port, anyway. Use Audacity or something to make a sine wave file then use your everyday DMM in AC voltage to measure it - that'll get you close, as long as you don't drive the output into clipping! That'll get you the RMS value."

http://www.overclock.net/t/1359473/amps-voltage-3-...