beginner here -> 7-segment led clock out of single leds

hey all,

i want to build a large 7-segment clock. since most 7-segment parts are fairly small, i was thinking about building my own 7-segment unit with blue leds.
each segment would have 2 to 3 single leds (depends on how bright they are and how even i can distribute the light). it would look like someting like this:

o o o
o o
o o
o o
o o o
o o
o o
o o
o o o

the segment should be about 15cm high. i would need four of those segments (hrs/mins) plus two leds for the blinking seconds between hrs/mins.
each led array would be in a separate compartment to 'simulate' the different segments. it would be really cool if they could dim depending of ambient light.

i'm more of a programmer than a hardware guy an need someone to point in the right direction :wink:

as far as hardware goes, i have the leds (+resistors), an arduino mega, arduino nano, a DS1302 CMOS RTC Realtime-Clock, 10x 74HC595 shift registers and 1x MAX7219 led driver.

i was thinking about wiring each 3 led array in series and then creating one logic 1 x 7 array (the one column is actually 3 leds in series) to be controlled by a 595. does this make sense?

i'm not so sure about the power setup. could the arduino power all the leds?

any help is appreciated.


I would start with the Arduino Nano and use the MAX7219 or the 74HC595.

If you use the 74HC595 with transistors to drive the leds, you can build everything yourself just how you would like it. You could use PWM to control the brightness if you use a single transistor or mosfet to control all the leds.

For the DS1302, you can use this : Arduino Playground - DS1302
You could combine that with this : Arduino Playground - Time
But I think that is not needed at the moment. Only the DS1302 driver will do.

The power supply on the Arduino Board can't deliver the +5V for the leds.
So you need a seperate +5V power supply.

I would like to know what leds you have. If they have a small beam, the are not very good for a clock. A diffuse led would be better. But you can make them diffuse by yourself with sandpaper.
How much current do the leds need ?
Did you buy cheap 20mA leds on Ebay ? Well, I did, and after 3 month they were half as bright. It turned out that the 20mA leds lasted longer with 5mA.

I would suggest that you have a dedicated driver, like a npn transistor for each segment / set of leds. This will help in terms of power supply selection or led arrangement.

yeah, i got the leds off ebay (these) ->
they're 20ma / 3,2-3,6v

i'm still working on the 'diffusing' part. maybe i will use cut acryllic segments that gets lit from the sides. thats what they use on commercial led light panels.

You can use aTPIC6B595 to sink current through whole strings of LEDs , driven from up to 50v, and they all latch. ( run at full brightness all the time )

You can send the data via shiftOut or SPI for higher speed, and cascade as many numbers as you like.

a while back i wired up a 8x10 matrix with two 595s to drive the columns and one 595 to sink 8x12 led matrix with arduino mega and three shift registers (595) - YouTube (video).
this is all driven by a arduino mega. since i roughly know how this works, i was hoping to 'recycle' this in some way.