Arrays of 7-segment displays are used in all radios, the OBI and of course in the mode control panel. In my cockpit, they are connected to EPIC 32-Digit Display Modules. Each of these supports 4 x 8, i.e. 32 digits. Up to 4 modules can be daisy-chained. That brings the maximum number of digits to 128, enough for even the most demanding cockpit project.
EPIC requires common anode displays. They come with either red, green or lately even white LED segments. When I started my project, only red and green were available. Both colors look inappropriate in the cockpit, as if the displays had been ripped out of a cheap 80’s style wristwatch. After a lot of tinkering and (finally) some hard thinking, I found a solution to this problem: If the green displays are installed behind a purple colored translucent foil, the result are yellow-orange digits that do look like they belong in an airliner cockpit.
There were companies in the late ninties that made pre-assembled red 2×4+1 digit units for use with EPIC. They even came with a 20 pin connector, ready to be hooked up to the display module. Since I did not like the color and also wanted more flexibility in digit spacing, I decided to build my own displays.
What follows is a collection of hints on how to do this.
Let’s start with some basics:
The picture on the left shows the front of the display. Close observation reveals what gave the seven segment display it’s name: It consists of seven LED segments. When lit, they can display any digit from zero to ten (and some more stuff). Is seems there is a norm for the denomination of these segments, so no matter what make your display is, the “A”-segment will always be on top, “B” on the right, and so on.
As a matter of fact, the correct name for this tiny piece of hardware should be Eight Segment Display, because there is another segment for the decimal point. But I’m not going to split hairs…
The back side of the same seven segment display. I found this drawing particularily useful during soldering, because it shows the side of the display that you look at as you are working on it.
10 pins are grouped in two rows of five. The pin for common anode exists twice, probabely for convenience during soldering. There is one pin for each segment, and one for the decimal point. I recommend you use your multimeter to verify the pin arrangement for your own displays.
This diagram shows how the connections are made between the displays and a flat ribbon cable (and ultimately to the EPIC display module).
Essentially, all pins of the same type – say, all “A” pins – are connected to each other and to the corresponding lead on the flat cable. Flat ribbon cables always have the 1-lead marked, usually in red. Solder the cable from the “A” pin to lead 1, the cable from “B” to lead 2, and so on.
Leads 17 and 18 are unused. Voltage limiting resistors are not a must, but recommended.
Pin diagram of a male connector on the EPIC display module. The 20 pin flat ribbon cable is plugged in here. An opening in the connectors case (marked with the red arrow) and a corresponding protrusion on the female plug make an accidential reversal of the plug impossible.
On the casing of both male and femal connectors, the “1” pin is always marked by a small imprinted triangle.
After this long theoretical introduction, here comes the practical part. The photo on the left shows the raw materials:
Utmost care should be taken not to shorten any pins during soldering. Since there are two wires connected to each pin, I found it easiest to first solderthese two wires together, fix them with shrinking tube and ony then solder them to a pin on the display.
It can not be avoided: The more cables are soldered to the display, the messier the job gets. As a fellow builder wrote somewhere: Wiring these displays is possible, but it leaves you cross-eyed before you finish… I admit this is not far from the truth. Wiring the 7 digits shown in the photo on the right took over 4 hours!
Hardwire whatever you can: for example the leading 1 in the frequency display of a COM radio, or the decimal point. That saves EPIC digits and makes programing easier. When you hardwire a segment, the anode goes to “+” on lead 19, cathode pins are daisy chained and connected to GND on lead 20. Don’t forget resistors! They control the brightness of the hardwired segments. Brightness should of course be identical on all segments. Some experiments might be required to find the right resistor.
Conrad Electronic, my source for displays. Look for Artikel-Nr.: 184675 – 62
Blue Side Up. Paying homage to the pioneers: on this site I first found information on how to wire these darn displays! One of my graphics on this page is a remake of what they published back in 2002
http://en.wikipedia.org/wiki/Seven-segment_display. Wikipedia on 7-segment displays