A seven segment display has individual terminals for each of the segments.
One is a common, one is typically a decimal point, and the rest access the
segments, one for one.
Different types of seven segment display will sometimes have more that one
common, and they are available as both common cathode or common anode. You
should have a slash sheet for the display you decide to use, if you build your
display from scratch.
To display something like a voltage, you must first convert it from analog
to digital format, then you must have a circuit that drives each of the segments
in each of the displays. There are two common methods of doing this.
The first is to write the bits for turning each segment on or off into a
latch of some sort. This means that for each digit you display, you must have a
separate latch device. Also, it means that your circuit must convert each digit
into seven segment format.
The second is to multiplex the display, meaning that you wire all the A
segments together, all the B segments together, etc. The common terminals
remain separate. Then, you will turn on the proper segments for the first digit
and apply power (or ground, whichever your display requires) to the first
digit's common terminal.
After a brief interval, typically milliseconds, you will then set the
segments for the second digit and apply power to its common terminal... be
repeating this for each digit, you can sequentially light them and make it
appear that the whole display is lit all the time. The display is updated
hundreds or thousands of times per second, creating the illusion of having all
the different digits lit at once.
The reason why multiplexing is so common is that it greatly reduces the
number of wires needed to light up the display. Consider a three digit display,
hard wired to latches- you would have to have 24 wires for any arbitrary
display. But with three digits multiplexed, you only need ten wires. Decimal
points are often set with a jumper wire in this case.
If you have your data in BCD format (unlikely, considering your question),
then you need four bits of data for each digit. A decoder would convert the 4
digit code into the seven segment output patterns.
This is not a project for somebody to try without a little expertise. You
will find that ready made chips with analog to digital converters and internal
multiplexing are your best bet. The ICL71xx family is a good place to start.
Here is a link to the PDF slash sheet for these devices.
http://www.electronic-kits-and-projects.com/kit-files/datasheets/ilc7106_7.pdf
Cheers!
Chip Shults
My robotics, space and CGI web page -
http://home.cfl.rr.com/aichip