24" Wall Clock

From OpenCircuits
Revision as of 20:45, 24 December 2007 by 218.22.128.243 (Talk)

Jump to: navigation, search

in much so trouble world ashanti nipple slip used fire trucks evp sd100 form klund canon eos 20d review final fantasy 10 3d furniture cad free deficiency antique cars cartoons black tail rye bread nutritional information little girls lose virginity over the rainbow los angeles funeral flower static control wear house truro hobby people ccw getty images continuing medical education courses prince georges county tax jetnet.aa.com. as business doing typeperf windows 2000 doll houses accounts receivable factoring le chamois hotel whistler incest mother and daughter sony dvd201 review islamic museum turkey virtual hair studio nextel cellphones vols vs notre dame business marketing water fall owner finance home in georgia teen angel breathless television sales online madrid electronic game holdem texas tv hp 2100 repair los altos art home project schoolers gif images nylon rope anal beautiful girl java lava coffee safetycare video web hits brooke alva ok hotels baby polar bear lapd wilshire division storage lorain ohio monty hall probability problem ll bean yacht charters in bahamas myfico.com database siddh wholesale fragrance oil for candle marketing sales nortel contivity vpn ports launch at yahoo.com lewis laphams shaving pubic hair for girls kewanee newspapers dylan groene lewis human genetics pga professional program weight control book phcs lego instructions to build juciy fruit hilary duff give oral sex spyware adware remover download marjorie wilson artist free forums widened qrs complex ohio historical center survival kits werbach is environmentalism dead panasonic batteries sewing patterns ebay stores stripper ball america email list stockholder john maddens bus rosette spoonbills printing software abstract aromatherapy gifts nyc suicide hotline super famicom roms pipette tunu.com valentine s day clipart agreement rental template sitizenship r kelly video business brochures map of aberdeenshire scotland gold filled beads dodge logo love shack newport news kirby vacuum how do i live without you la gomera parador not asking for much wwe music family reunion ideas kaufmans airblue in pakistan minneapolis barbers microreactors intense pulsed light michaela bercu last minite club used dirt bike engines inuyasha in love pictures lapis talbot green shopping centre bit torrent wilcox az newspaper pictures of 2005 hairstyles maine real estate orpheus microsoft word undo save airporter.com quesnel lake porcelin sink experience extrude honing in production vortigerns machine mcdonald investments fantasy basketball gartner piper super cub softice free targets exploited com cbs show survivor aids life expectancy susan b anthony wikipedia rifle river michigan camping craft show listings black and white photos everest reinsurance holdings msn currency exchange {| align=right |- | Light Sticks |}

Contents

Project Scope

Description:
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.

LED7SEGLG 300.jpg

Here is an example of the customary 7-segment display. We aren't going to copy this directly, but close.

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 [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:

18:88 will be displayed worst case.

  • Each 8 is made up of 7 segments
  • 1 is made of 2 segments
  • : 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 8's x 7 segments * 2 sticks per segment = 42 sticks
1 1 x 2 segments * 2 sticks per segment = 4 sticks
1 : x 1 segments * 2 sticks per segment = 2 sticks

48 sticks needed overall
Minimum of 23 inidividual control channels needed


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).

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.

The control board will need some power as well. 5V for logic, 3.3V for GPS. But the overall consumption will be peanuts compare to the lighting. Probably under 50mA for the control board.


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.

We need to control 23 channels 18:88:

  • 1 has two segments, but the entire number is either on or off
  • : is always on, but we may way to flash it for seconds
  • 888 21 channels

I decided on 25 channels just in case I need some extra ['flare']. Perhaps an alarm of sorts. 'Beer Time' alarm at 4:30pm? We've got to keep our options open...

The need for 25 I/O pushed me towards the PIC 16F877A:

  • Easy to program (I've got a big code base)
  • Has enough I/O
  • It can be bootloaded!
  • It's got an RTC (when you add an external 32.768kHz xtal)
  • 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.


Light Control

Well if the PIC 16F877A has got the timing and control down, how exactly do we turn on/off the light stick?

Since the sticks only pull 60mA, we can use cheap BJT transistors. Let's go solid state! Many itty-bitty SOT-23 SMD BJTs can handle up to 1A with a max input of 40V. The maximum power output is 350mW though. Luckily, when forward biased, the MMBT2222 has around 0.5V drop.

So 60mA with 0.5V drop, we are looking at 30mW dissapation. That's atleast in theory. I'll believe it when I see it of course. For the time being, I'm going to assume this will work...

All I need is an NPN BJT for each channel:

Clock-Channel.jpg

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.


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:

Clock-Layout.jpg

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).


Things to Improve

Nothing at the moment. We have to wait for all the parts to come in. Should be about 4 weeks.

We may need a cover on the light sticks to give more of an opaque presentation. TBD.


Documents

SFE Wall Clock Main Board Schematic

Channel Control Schematic

Footprints:
SFE Footprint Library

FP Name: All sorts of FPs used.

Supplier Info:

Related Items:

Personal tools
Namespaces

Variants
Actions
Navigation
Toolbox