I'm about to make a voltmeter from computer line-in. In some strange way I can not see direct current from computer. Why does it work only with alternating current? What information does captured wave contain? Could someone please explain basic principles of how does line-in work? How does sound capturing work in particular? Sorry if this isn't the right place to ask.
Electronic – Computer line-in voltmeter
computersvoltage measurement
Related Solutions
It depends a lot on which embedded system are you planning to use. A good alternative to desktop replacement (with all your requirements) is Intel DG45FC mini-atx. With a core 2 duo socket and an embedded graphic processor you can make any application run as in a desktop.
A cheaper and smaller alternative is D410PT (or D510MO for dual core processor) with intel atom processor. Its also an x86 processor which is pretty good if you're not planning to go on more embedded kind of operational systems, but still powerfull enought for the majority of the tasks.
Another alternative is beagle board. The built in processor is an core ARM. In this case you'll need to build your program based in Android, Win CE or some Linux flavors. I think that maybe this is the alternative that may suit you better.
Could be an issue with flat vs. upright mounting.
When the needle is too far towards the center at values close to the ends of the scale, the needle of an instrument intended for an upright mounting position might lack the gravity that puts it down just enough to yield a good reading.
When the needle is too far away from the center at values close to the ends of the scale, the needle of an instrument intended for a flat mounting position might be pulled down too far by the gravity that should not act in the direction of the needle's rotation.
Usually, little symbols indicate the specified mounting position, e.g. an upside-down T means you are to use the meter upright, and a table-like symbol similar to \$\sqcap\$ means the instrument must be used in a horizontal position.
Examples may be found in this link. (Sorry German only, but you will certainly understand vertikal and horizontal.)
Best Answer
Line-in is for audio signals. A DC signal represents silence, whether it is 0V 0.5V or 1V. Therefore the desgners of the Line-in circuits are not interested in DC levels at all.
As pjc50 commented, technically, DC is typically filtered out using a series capacitor.
Line-in is designed to be used to accept varying voltage levelswhich represent variations in air pressure over time. Typically such variations caused by spoken conversation or music near to a microphone (or indirectly from a recorder, or synthesised versions of this).
So long as the analogue signal is sampled at a high-enough frequency, the stored values can later be used to reproduce a good approximation of the analogue signal sufficient to potentially reproduce the variations in air pressure over time (using an amplified and loudspeaker/headphones/earphone).