How many times have you wondered and seen this... "What can I do with this old MHz display?".
They are basically 7-segment LED displays. 7-segment meaning there are 7 segments to form a digit ranging from 0 to 9, or even a few letters such as A, b, L, and h. If they are still in working order, meaning all 7 segments are still lighting up when powered, then we can use them for our PC modding purposes. We can make use of them as hardware system temps display, CPU or even RAM usage. Depending on how many 7-segment displays you salvage (or buy) or how many you want to use, and your knowledge on PC interfacing, you can do pretty much what you want, considering apparent limitations of course.
This is a simple (simple enough) project for hardware displays, compared to using LCDs or VFDs. For LCD or VFD users, we don't think you'll be using this mod, but for those, who are not one of those LCD/VFD users, this might be a good and cheap project, provided you have a free parallel port.
We will be creating a three-digit numerical display using three 7-segment displays controlled by the PC's parallel port.
- 2 BCD-to-Seven Segment Latch/Decoder/Driver (MC14511B) - look for "4511"
- 16 - 470ohm 1/4Watt Resistors
- DB-25 Male (Parallel Port Connector) and Ribbon Cable or you can use a spare Printer Cable and splice up the wires.
- Prototype PCB
- some wire
- basic soldering equipment
- soldering skills (can't buy this from the store though)
Cathode (-) / Anode (+)
You might already have a spare 7-segment-display from an old PC or maybe from any other thing, so we're gonna use that instead of buying one. But before we can use it we have to find if it's a common cathode 7-segment-display.
First of all, 7-segment displays come in two basic variants: Common Anode, and Common Cathode. In a common anode type, all 7 segments share one anode pin. meaning it has 7 cathodes and one anode. Imagine wiring 7 LEDs all connected together on their (+) legs. A common cathode has one cathode pin and 7 anodes.
Here's how you can determine which type your salvaged 7-segment is:
We need to have a 5V supply, probably just use the red and black cables in any of your molex.
First of all hook a 470ohm resistor to the red cable so that we won't burn any LED segment. Now put the black cable in any of the pins and start moving the red cable from pin to pin. If all segments didn't light up, move the black cable to the next pin and start moving the red cable from pin to pin again. Do this until you can light up all segments having the black cable in a single pin and putting the red cable in all other pins. Since one pin works as a ground line (cathode), we know it's a common cathode 7-segment-display. If you weren't able to light any segments up, it's probably a common anode type. Try tapping the red wire on a single pin and move the black wire on the rest of the pins. If that still doesn't work, then what you have there is a busted 7-segment. Sorry :)