# 7 segment led displays?

Discussion in 'Electronic Basics' started by kash, Jul 14, 2003.

1. ### kashGuest

Hello to all,
Question about 7 segment displays:
Do I need to light up the segments individually, or is there
some sort of encoding. What else do I need if I want three
7 segment displays to display a voltage ?
Tia

2. ### Jonathan KirwanGuest

Well, you have to decide which segments you want lit up for each
digit to be displayed. It's pretty standard, but most people do
either use the term "encoded" or "decoded" for the process of
converting a binary value into the right segments to use. For
example, a binary value for '8' would want to be decoded so as
to light up all 7 segments, A-G.

But in the case of using three of these digits, it's common
practice to first light up all the right segments for the first
digit, while the other two digits aren't powered and are blank,
then do the same for the second digit, then the same for the
third, and then back again to the first in a continual loop.
Each digit gets 1/3rd "duty cycle," which if the current is high
enough and the loop speed fast enough is just fine and the
blanking of two out of three digits all the time won't be
noticed.

In doing it that way, you can wire-or all the A-G lines and just
enable each digit through it's common anode or common cathode
line. Saves I/O pins, if that's desired. There are other ways
of doing it, but that one blends some simplicity with reducing
the number of distinct I/O lines needed.

Of course, you'll still need to work out how to measure the
voltage range you are looking for, with the precision you want,
and figure out what value to display.

Jon

3. ### Sir Charles W. Shults IIIGuest

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

4. ### Peter BennettGuest

A basic 7 segment display is just 7 LEDs with their cathodes (or
anodes, depending on type) connected together. It is up to you to
decide which segments to light.

Some 7 segment displays do include a decoder and possibly a data
latch, so you can just supply binary or BCD data, and they will
display the right pattern.

If you want several displays to display a voltage (as for a
voltmeter), you need to design a circuit including an
analog-to-digital converter, binary-to-BCD decoder, and BCD-to-seven
segment decoder. (It is probably cheaper and easier to buy a

5. ### Mo MorganGuest

Hi,

I have a problem regarding supplying DC to a variety of different pieces
of equipment. Fairly basic, I know, but it's not the kind of thing you
want to bungle into and get wrong.

I'm a musician, and I have lots of equipment that I take on the road in
rack cases. Some of this equipment has internal transformers and feeds
straight off the mains; others require external PSUs. This causes two
problems - the first being the obvious one of having to plug in and
accommodate loads of PSUs and cable, the other is that these units
frequently overheat and burn out.

So, my idea is to build a power distribution unit in a 1U 19" box for
each rack case. On the back of the box will be a single power connection
taking AC from the mains, and a series of fans to provide adequate
ventilation. On the front will be various connections: eight standard AC
mains outputs and eight adjustable DC outputs.

Obviously, wiring up the AC outputs is straight-forward, but when it
comes to the DC outputs I'm afraid my knowledge of electronics falls
short. The requirement for each of these outputs is as follows: a switch
to isolate it from the mains, a switch to control the voltage output
and, ideally, a switch to control the level of resistance. Some of the
equipment I want to power runs at 9V/600mA, some is at 12V/300mA etc -
so I'd like each of the DC outputs to be configurable depending on what
gets plugged into them. Basically, it's eight PSUs in a box.

Put precisely, I need to know the following: how to convert AC to DC,
how to step voltage down to a user-defined level (never more than 12V)
and how to adjust resistance to a user-defined level (between 200mA and
600mA).

I'm in the UK, so I'm starting with 230-240V AC. Any help with this
project would be much appreciated.

MM

6. ### Bill BowdenGuest

There is drawing on my site for a simple regulated power supply.
The usual parts are:

1. Mains transformer to reduce 240VAC to 12VAC.
2. Bridge rectifier (4 diodes) to change AC to DC.
3. Filter capacitor to maintain the DC fairly constant.
Note: the 22,000uF shown is a little large and can
be around 4700uF for light loads of 600mA.
4. Adjustable voltage regulator to maintain a constant
output voltage for varying loads.
5. You just need 2 resistors to set the output voltage to
what you want. There is a table showing values to use.

You will not have to adjust the resistance for different
loads. The regulator will do that for you. The output
voltage will always be whatever is set by the resistors
and the load current will be controlled by the regulator.
But the regulator (LM317T) may get a little warm under load,
so you might have to mount it on a metal surface to keep
it cool.

http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#317.gif

-Bill

7. ### Terry KingGuest

MM, Bill has given you the right idea.

(Almost) all these devices use a positive supply, and the
negative supply connection is connected to their 'common
earth ground'. This means you can power a number of them
with their grounds (American for Earth) connected to
a common ground. This means you can build one 'raw' supply
of, say, +18 volts and then have several simple regulators
for several outputs.

This all assumes you have 'common grounding' and all the
audio ground loop issues are under control, or minimized with
balanced lines etc.

The LM317T is a versatile regulator. If you have trouble finding
them, drop me a note and your mailing address. Right now I'd die
with a couple hundred 317's...