Editing 24" Wall Clock

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[[Image:Sign light Three 400x400.jpg|Light Sticks]]
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===Project Scope===
 
===Project Scope===
<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.
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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.
  
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.
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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.  
  
 
[[Image:LED7SEGLG 300.jpg|150px]]
 
[[Image:LED7SEGLG 300.jpg|150px]]
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===Power Regulation===
 
===Power Regulation===
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).  
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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 cheap 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.
 
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.
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===Clock Control===
 
===Clock Control===
 
Who wants to set the real time when we've got GPS to set it for us?! :) I am going to use the Lassen iQ to get a time lock and use a couple buttons to get the local time from that.
 
Who wants to set the real time when we've got GPS to set it for us?! :) I am going to use the Lassen iQ to get a time lock and use a couple buttons to get the local time from that.
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*UART to read the incoming GPS NMEA from Lassen iQ
 
*UART to read the incoming GPS NMEA from Lassen iQ
  
 
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.
 
 
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===Light Control===
 
===Light Control===
 
Well if the PIC 16F877A has got the timing and control down, how exactly do we turn on/off the light stick?
 
Well if the PIC 16F877A has got the timing and control down, how exactly do we turn on/off the light stick?
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While this should work, it's going to leave a bunch of connections with one pin 'hot'. A better design would probably utilize a PNP connected to RAW. Ehh - no big deal.
 
While this should work, it's going to leave a bunch of connections with one pin 'hot'. A better design would probably utilize a PNP connected to RAW. Ehh - no big deal.
  
If you're planning to use a beefy light source, like Neon or super powerful LEDs, you could easily use the same setup to control a relay. [[Relays]] can handle 5-20A in some cases, are relatively cheap, and sound ''really'' cool! The only problem is they are big. And for this project, we wanted a relatively small control board. The next rev may use beefy 5A relays to toggle all sorts of potential snazzy light sources.
 
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===Board Layout===
 
 
The PIC is connected to 25 channels, the GPS, a couple buttons, and a status LED. Here's the current layout in all its random glory:
 
The PIC is connected to 25 channels, the GPS, a couple buttons, and a status LED. Here's the current layout in all its random glory:
  
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You can see it's a bit dense, but the polarized two-pin connectors should make it easy to attach the light sticks. Everything was SMD to ease the assembly as much as possible. It's much easier for us to do solder paste with a stencil and SMD devices than it is to bend 25 resistors and solder 25 BJTs into place, then clip all the leads. PTH is actually pretty time consuming. I digress...
 
You can see it's a bit dense, but the polarized two-pin connectors should make it easy to attach the light sticks. Everything was SMD to ease the assembly as much as possible. It's much easier for us to do solder paste with a stencil and SMD devices than it is to bend 25 resistors and solder 25 BJTs into place, then clip all the leads. PTH is actually pretty time consuming. I digress...
  
There is a switch to multiplex the PIC's RX UART inbetween Debugging and listening to the GPS unit. Be sure to use Port 2 on the Lassen iQ if you want NMEA output (4800bps by default).
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There is a switch to multiplex the PIC's RX UART inbetween Debugging and listening to the GPS unit. Be sure to use Port 2 on the Lassen iQ if you want NMEA output (4800bps by default).  
 
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<b>Related Items:</b><br>
 
<b>Related Items:</b><br>
 
* [[POV display]]
 
* [[PointLess LED Array]]
 
 
=== External References ===
 
 
[http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=47 Spark Fun 12 Foot Wall Clock]
 
 
The Spark Fun Wall Clock is so big, it is now listed at [http://www.hackersbench.com/giants.html "Registry of Giant Digital Clocks"].
 
Congratulations!
 
 
 
[[Category:Projects]]
 

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