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# Do audio devices put out alternating current? Does the current ever switch itself? Answered

Hi

I'm making a prop Geiger counter for Halloween, and i was going to have bi-polar LEDs on it depicting if it is safe or not to proceed.

I have worked out all the main issues with making a prop Geiger counter, Producing the sound, amplifying it, and distributing it. I was going to have one channel of my MP3 player play Geiger counter clicks and the other  channel square waves to drive a transistor to drive the Bi-polar LEDs

I was going to have Red LEDs light up to the positive part of the wave, and the Green to the negative. This is where i ran into a small problem, i don't really remember if Electronically produced sound waves have both Parts of a wave (look at the picture), or of it just produces a positive pulse without reversing polarity.

I will be using a computer speaker amplifier for this project

Thank you!

(basically, i need to know if Computer speaker amplifiers produce AC current)

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## Discussions

Voltage is always relative to some reference point -- which can be chosen pretty arbitrarily. A sine wave that goes from 0 to 20 volts can equally well be described as going from -10 to +10 if you pick a different reference. You can call it AC with a DC component (which may be zero) or pulsating DC; they're really the same thing.

The signal is probably all positive, or all negative, when measured with reference to the signal ground. (Just because that's the easiest way to build a D-to-A converter.)

If you want it to officially be "AC", you can filter out the DC component; that establishes a reference that floats somewhere around the signal's average voltage, so when measured relative to that reference your signal will go both positive and negative.

Does that help?

So if the amplifier puts out +20v when connected to the output of the amplifier and it drives a speaker (ignore the LEDs for now). If i Connect one lead to the output and another to a +10v source, i should be able to get the speaker to move both back and forth (produce real AC voltage)

Am i understanding you correctly?

See NachoMama's comment. You can probably assume that the output is capacitor-coupled, in which case current *will* flow both directions between signal and signal-ground.

Apologies for complicating matters

. Technically, it's not AC since it doesn't change direction periodically.
.
. To come at it from a slightly different angle than the others, the LEDs see relative voltage. Eg, if one side is at +5V and the other at +7V, then the LED sees 2V with the +5V side being negative (relatively).
.  Just connect your LEDs, in parallel, in place of the speakers (wire up one of them "backwards" so that the + of one LED is connected to the - of the other. A current-limiting/impedance-matching resistor would probably be a good idea, but I don't think your small amp will burn out your LEDs unless you crank it all the way up; keep the volume low until you know for sure.

I'm not completely convinced that will work, Nach; the DC component of the output may prevent current from ever actually reversing. Remember, to get a negative voltage relative to signal ground requires using both positive and negative rails to feed the output amplifier stage (or the DAC before it)... and if they're taking low-level output directly off a chip, they may not have made the effort. Or it requires a DC blocking capacitor at the sound card's output, which strikes me as more likely but I'm still somewhat skeptical.

Interesting question, actually; has implications for other (ab)uses of soundcards as general analog outputs...

Proper way to check this is with a 'scope, or by looking at the soundcard's circuitry. Unfortunately, while I have a logic analyzer (long story) I do not have a simple analog scope.... and I'm not sure whether the answer will be the same for all soundcards. (Gods know the circuitry on my motherboard and in my Transit are likely to be significantly different, not least because the Transit is USB-powered.)

. There is no need for a ground or any other zero reference - it's all relative. The differential voltage at the amp output will swing positive and negative. If the differential always stayed positive (or negative) the speaker would be inefficient (only pushing (or pulling) relative to its neutral position).
.
> Or it requires a DC blocking capacitor at the sound card's output,
.  Been years since I glanced at a soundcard schematic, but that's exactly how they used to do it.

.... Ah. And in fact that does make sense from several points of view. I sit corrected.

What comes out of a PC is "AC coupled", so you can apply whatever reference to it you like.

you COULD make a relaxation oscillator which can click with a variable frequency.

Steve