Editing 24" Wall Clock

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<b>Description:</b><br> Sometime ago a friend challenged us to build a 6 foot digital clock - just like the alarm clock next to your bed. Well, because of scaling issues, 6 foot was a bit too expensive. Instead, we are going with a 2 foot clock. I'm not entire sure about the end size, but you get the idea. And just to be geeky, let's use GPS to snag the time with 100ns accuracy.

The control system is fairly straight foward. Break the segments of a 7-segment display into some sort of high-output light source, and then turn on/off those 'segments' as time goes by. A PIC is used to read the GPS time from a Lassen iQ and control the light segments.

[[Image:LED7SEGLG 300.jpg|150px]]

 

We need to use something to illuminate each segment of the display. How about a light bar. What's a light bar? Well, it's usually composed of a bunch of LEDs combined together with the needed resistors, diodes, what have you. We could build our own, but I'm lazy and [[http://www.besthongkong.com/ Best Hong Kong]] has something called a ''Sign Light Module'' that looks like it could do the job really well, for only $2.39 a piece! That's cheaper than I can put a stick together, so I put 50pcs on order. We'll see how they look when they come in.

 

Here's the plan:

 

<b>18:88</b> will be displayed worst case.  

*Each <b>8</b> is made up of 7 segments

*<b>:</b> is made of 1/2 segments each

 

To make the clock slightly larger, we are going to use two light 'sticks' for each segment. Each stick looks like it's about 6" long, which will make a segment aprox. 12", which whill make the number height ~24" overall. Each dot in the ':' will be made out of one stick (6" tall). This may look a bit odd, only time will tell.

 

3 <b>8</b>'s x 7 segments * 2 sticks per segment = 42 sticks<br>

1 <b>1</b> x 2 segments * 2 sticks per segment = 4 sticks<br>

1 <b>:</b> x 1 segments * 2 sticks per segment = 2 sticks<br>

 

Quick math : Each stick is 3 LEDs. Each Piranha LED is 20mA according to the website (I don't trust the Hong Kong Website very much, but we'll assume). So each stick is 12V @ 60mA. 50 sticks * 60mA = 3A @ 12V. This is a bit juicy for a wall-wart. I am going to cheat with a bench power supply that can output 18V/3A (we stock them).  

 

3A is really the worst case scenario.  We only have 48 sticks (not 50) and 10:08 seems to be the real time with the most number of segments lit (44 sticks turned on). We obviously should never see a time of 18:88. At 10:08 we'll need 44 * 60mA = 2640mA = 2.6A. Not too horrendous.

 

 

ToDo : It's possible to reduce the number of channels by multiplexing. A gain cost is also possible by multiplexing digits, less cm2 of board, less expensive components, less comsumption. 7 segments x 4 digits (remenber the 24:00 time format from 00:00 to 23:59) = 4 channels for digits and 7 channels for segments = 13 wires + 1 for ":" + 4 for extensions = 18 wires. Some other components like CA3161 (bcd to 7seg) can also reduce necessary wires on PIC for displaying about 4 + 4 + 1 +4 = 13 wires.