Thread: Designing Digital Clock, need some suggestions

Hello,

I've wanted to learn about microcontrollers so I have decided to start a digital clock project just for fun. I have the general circuit laid out in my mind but I need some suggestions for specific parts relating to some analog issues.

I'm thinking about using just one 7-segment decoder and multiplexing the displays to make wiring a little easier. But I'm not exactly sure how I should drive the displays with the 7-segment decoder. In my digital circuits class, whenever we needed to drive LEDs we used the IC as a sink because it can sink more current than source. However, I want a fairly large display that can be seen across the room and I think these generally use more current.

Will I have to use a transistor on every segment of each display to be able to source more current to the displays? Or are there large displays that can be driven directly from the 7-segment IC. If anyone can suggest some part numbers for the IC or the display that would be helpful.

The common node of each display is going to be connected to a MOSFET to be used as a switch for turning the displays on/off for multiplexing. I am thinking of using the 2N7000 mosfet just because that is the one we used in our lab class and from what I can tell from the specs sheet, with 5V on the gate it can supply around 800mA which should be enough. Is this a good choice for a transistor?

Finally, can anyone recommend an AVR chip that will be good for this project? The only requirements I can think of is to be able to run off an external crystal clock, and to have at least 13 I/O pins.

Thanks for any suggestions!

Okay well, I decided to go with these LED displays

Lot of 10 pcs 7 Segment Red LED Display 3'' - eBay (item 250453984981 end time Jul-28-09 01:05:07 PDT)

They're common anode so I want to hook then up directly to an active-low BCD to 7-segment converter acting as a sink. In class, we used the 7447, but when I look this up on mouser they're either out of stock or "obsolete". Is there an equivalent chip I can use? I've looked around for some but mainly I just find active-high drivers.