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Step 4The display board
THINK FIRST
Before you do anything, have a think about the size you would like the project to be. In this version, I used high intensity LEDs. they have a beamwidth of about 10 degrees, which means that I had to mount the stencil about 30mm from the LEDs to get an even illumination. If you purchase 'Flat Top' LEDs, then you can make the clock much thinner.
MOUNTING THE LEDs
I decided to make the display board using a piece of fiberglass material, with the LEDs mounted into a series of holes that I pre-drilled. This is a different mounting system than the original, but I feel that it is probably more accessible, as it doesn't rely on another expensive PCB. The LEDs are soldered on the back of the board to a piece of 26 way ribbon cable that is terminated with a 26 way IDC socket.You can use a piece of thin plywood, or acrylic if you like - Just use whatever material you can that will hold the LEDs stable.
MAKING IT (Hey thats a great name for a magazine :-)
I started with a square of fibergless sheet, upon which I measured a 12cmx12cm square. I divided that into a grid that was 13 dots x 9 dots (which turned out to have a spacing of 1cm x 1.5cm), and drilled starter holes using a small drill. DRILL HOLES
Then I measured the LEDs, and they were indeed 5mm diameter, so I drilled LOTS of 5mm holes.....Before I started drilling, I was aware that there was going to be a very big mess. There was board material everywhere. I did the drilling outside, and made sure I wore a dust mask.So many little holes....
MOUNTING LEDs
Once that was done, I slid each led into a hole, being careful to make sure that they were all oriented the same way. A drizzle of hot melt glue was useful in holding the leds in place. When assembling, I mounted every second row on the board, and gently folded the leads down as time came to mount the final sets of rows.
SOLDERING IT TOGETHER
Then I trimmed the leads, and soldered in the current limiting resistors, and soldered the LEDs together, following the layout diagram exactly. When you are following the diagram, remember that you are working on the BACK of the board, so you need to mentally reverse the layout. Also - don't forget to use 36R resitors (see below) when you are connected to a set of 3 LEDs and 360R (see below) resistors when you are connecting to a set of 2 - It alters the current flow to make the LEDs a uniform brightness.DO I *REALLY* USE 360R AND 36R RESISTORS?
As an update, I have been using Blue Flat Top LEDs, with a forward voltage drop of about 3.3v @ 20mA. The supply that I was using was about 10.5VDC. With the original design, with 2 LEDs, the voltage drop across the LEDs was 6.6v, meaning that 3.9v was being dropped across the resistor (11-6.6). With a 360R resistor, this limited the current to 10.8mA, and with 3 LEDs, the voltage drop across the LEDs was 9.9v, meaning that 0.6v was being dropped across the resistor (11-6.6). With a 36R resistor, this limited the current to 16mA.What I have subsequently discovered is that with supply voltages of 12V, the current through the leds ends up being way too high (15 & 58mA), which is a problem.
In my most recent versions, I have setled on 680R for the 2 Leds and 360R for the 3 Leds, providing 7.9 and 7.8 mA respectively. Much lower current, and still ample illumination.
ADD A RIBBON CABLE
When everything is finished, prepare and solder on the ribbon cable with the 26 way connector.The connector plugs into the controller board. (But I guess you figured that out!)
The photos below should show the process that was used.
To be completely honest, I think next time I will simply use the display board from the first WordClock that I built. Yes, it would mean making another circuit board, but it was so much simpler to make, and the result looks much nicer.
WordClock-Arduino-LED-WIRING.pdf(595x842) 34 KB| « Previous Step | Download PDFView All Steps | Next Step » |
If you take the task on methodically, you will find that it isn't very complex. Many people have made slocks so far!
If it helps, I have a PCB that does all of the 'scary' wiring for you. You just solder the parts in - It is from version 1 of the PIC Microprocessor clock.
V=IR
(12-6.6)=.02 R
5.4 = .02 R
270 = R
So a 270 Ohm resistor would be used to drive the series of 2 LEDs at 20mA with a 12V power supply. But you say you use a 680 Ohm resistor?
The Anodes of all the LEDS are connected to a common supply line (typically between 9 and 12VDC) The ribbon cable does ground each LED string. If the string is grounded - it turns on - if it is floated, then it turns off.
The reason why the ULN2003s are there is for two reasons, firstly, it provides the ability to drive a couple of hundred mA pre LED string, allowing really big arrays to be constructed, and secondly, when the ULN2003 is not turned on - the output floats. If the LED array was connected to the 4094s then the output would either be at 0v, or 5v. If it is at 5v, then there is still 12v-5v (7v) across the LED array.
The reason why I didnt use the common ground was that the ULN2003 output transistors had to be in the ground end of the circuit - The ULN2003 has NPN transistors in its output, and the common of the ULN2003 is the ground.