BY RICHARD SATTERLEE
must be greater than the forward
voltage of the diode (usually 1. 7-2.0V).
Also, there is little current flow (
practically none) when the diode is reverse
biased, or when the cathode is more
positive than the anode.
Since I’m using a common anode
display, each digit has a single connection to each of the anodes of the segment. The cathodes of each segment of
the digit are connected to the cathodes
of the other digit’s segments. So by
sourcing current in one chosen anode
and sinking current in one or more
cathodes (segments), a single digit can
be lit on the display. By sourcing
current in a different anode, another
digit is lit. By doing this rather quickly,
persistence of vision makes it seem that
all digits in the display are lit at once.
If either the anode voltage is low
and/or the cathode voltage is high,
then there is no current flow, and no
light from the segment. So if the
circuit drives one of the anodes high,
then optionally walks a zero though
each one of the cathodes, we can
have the display turn off and on each
individual segment. If we turn on only
the desired segments in rapid enough
succession, then we can have the
display show four unique digits at
minimum current draw.
Furthermore, if we limit the current
to what each PIC pin can tolerate, we
can get rid of the PNP
transistors all together.
We will still need a
current limiting resistor
though. The forward
current specification of
an LED is for continuous (DC) levels, but
if we have a duty
cycle, we can increase
the amount of current
through the device.
Since we are trying a
3% duty cycle, we can pass close to
the maximum current from a PIC
output through the LED. We just have
to be a little careful when debugging
the code and not leave a segment
stuck in the on position for too long.
So now we have the verbal
description of the problem. Oh wait,
there’s something else (isn’t there
always?). I want a small form factor for
the circuit without a lot of space taken
up around the display itself. But I still
want to use through hole design, not
surface mount. It might be good to
have an optional power supply on the
board, too as you never know what
type of power we will have around.
Now we have all of the requirements for the design. I’ve got a collection of four digit common anode, seven-segment displays that I bought from
RSR Electronics. In quantity 10, they are
■ FIGURE 2
FOUR DIGIT, 12 PIN
1 — Segment E 12 — DIGIT 4
2 — Segment D 11 — Segment A
3 — Decimal Pt. 10 — Segment F
4 — Segment C 9 — DIGIT 3
5 — Segment G 8 — DIGIT 2
6 — DIGIT 1 7 — Segment B
$1.25 each, which is not too bad. The
digits are about a half inch high and
have a right hand decimal point after
each digit. They didn’t come with any
specs, but a low voltage power supply
and a series resistor showed what the
pinouts of the display are (Figure 2).
Now, what if you have trouble
finding common anode displays (see
the Finding an LED Display sidebar)?
There are common cathode displays
out there with segments and digits
FINDING AN LED DISPLAY
LED displays don’t seem to be ber and came up with a few hits on the Fairchild MSQ6911C.
sourced by the “old name” OEMs search — all in Russian! After looking Jameco has a couple of parts that
much anymore. I don’t know why, but at the datasheets at www.rct.ru (the will work. The orange display (Jameco
I couldn’t find references to them in titles were not readable, but fortunate- P/N 335185PS) has a higher luminosi-many of the mail order catalogs that I ly schematics and ratings are), it sure ty than the green display that they ref-looked at. looked like the right part. So, although erence as 335193PS. The green display
I think that one of the original I didn’t have the name of the vendor, is a little dim in a brightly lit room with
partsoffered was the MSQ6911Cfrom I was on the right track. These are this design. I’m sure that it would
Fairchild. Fairchild has since sold their common anode displays. work well with more current through it
line to Everlight so you can find docu- A broader search on Google with and/or a higher duty cycle. It’s usable,
mentation for that part at either site. a reference to just 5641 brought up a just not very bright in this application.
In the article, I made reference to a lot more hits. So, you have a variety of options
display (or a bunch of them) that I pur- Futurlec sells a part with a on getting one of these displays. In
chased from Electronix Express. Their reference of 7FR5641BS. This looks the article, I discuss using either
stock number is 08FYT5641B. These like it would sure work. common anode or common cathode
devices have a part number of FYQ- Hobby Engineering has a part displays and you can also use either
5641 B on them. I Googled this num- H2276 that cross-references to red or green.
June 2007 39