# LED Lit 7-Segmented Clock

Discussion in 'The Projects Forum' started by JustinS, Oct 25, 2014.

1. ### JustinS Thread Starter New Member

Oct 25, 2014
2
0
Hey guys I'm a total beginner when it comes to circuitry and the only background I have in it is in highschool physics (further proving my point.)

I want to create a 7-segment clock like the one here:

Except each segment will have several LEDs and will be behind an opaque plastic or glass to evenly distribute the light.

Here is a rudimentary representation of one set of number segments:

Furthermore the LEDs will lead into an arduino board in order to function as a clock. Now my major issue with this project is the circuitry because this will add up the wires fast, and I have almost 0 knowledge of how to do so. Any help would be much appreciated and with time I'll post my progress and any other potential problems!

Also here's a little preview of my envisioned finished project:

2. ### adam555 Active Member

Aug 17, 2013
858
40
We did one of those in electronics class; except we used a PC 286 instead of an arduino. If you arrange the LEDs on each segment so they match in the PCB, or tie the leads together if you are going to place them elsewhere, then the only wires you will need are the 7 main ones in that circuit.

Then for the different digits I suggest you connect all the same segments on all the digits together, so you will still only use 7 in total for all; plus 4 more for multiplexing -11 in total-.

You can get those 11 down to half if you use an IC; but I'm not sure you want to do it that way.

Last edited: Oct 25, 2014
3. ### JustinS Thread Starter New Member

Oct 25, 2014
2
0
Maybe I understated how little my knowledge was but I really don't understand your answer. You saying "7 main ones in that circuit" are you referring to how my picture is laid out because that also ends up with 7 main ones. Then attach all of those to one board, resulting in 4 boards (1 for each digit)? After that you lose me upon mentioning multiplexing and the number 11 lol

4. ### adam555 Active Member

Aug 17, 2013
858
40
Maybe this helps:

In your circuit you only laid out the "7" segment wires; but you will need one more to multiplex each digit (4 in your case for your 4 digits).

Notice how all the same segments from all the digits are connected together. And since you are not going to use 7-segment displays, but individual LEDs to form each segment, you will need to tie together every anode of every LED on each segment, and all the cathodes of all the LEDs on each digit (or the other way around if you're going for common anode, which I don't recommend).

Bear in mind that an Arduino can only output a maximum of 40mA per I/O pin; so be careful with using too many LEDs per segment.

Last edited: Oct 25, 2014
5. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,971
616
Hello Adam. I have a question about the 8th pin which is used to select the digit. On your pic it is the blue wire. I have not done anything with 7 segment led units. Do they normally have this 8th pin?

6. ### k7elp60 Distinguished Member

Nov 4, 2008
509
90
Most 7 segment displays are constructed the same way. Each segment has a designation A through G. And each display has an 8th lead known as the common. Either common anode or common cathode. The individual segments need a resistor connected to it to limit the current. The value of this resistor depends upon the driver circuit. (circuit that is operating the display) I am attaching a file that shows the location of each segment and a suggested circuit for each segment. Each segment should have the same number of LED's. With four displays all the common leads need to be tied together.

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7. ### MrChips Moderator

Oct 2, 2009
14,510
4,278
Firstly, I would decide what power supply you intend to use to power the LEDs. The number of LEDs per segment will be limited by the supply voltage.

You can put the current limiting resistor in series with the LEDs at any location in the chain. It doesn't matter if you choose common anode or common cathode.

I would probably choose common cathode.
You will need seven PNP transistors such as 2N3906 to drive the anodes
and four NPN transistors such as 2N3904 to select the cathodes (digits).

Hence to drive three LEDs per segment will probably require a minimum of 9V supply.

Stuart Matthews likes this.