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[on:]

Hi All,

I am fairly new to the Arduino/hardware scene. I decided that the best way to jump right in was to start a project.

I found a great one called a word clock, and theres already an existing project that can be found at http://www.dougswordclock.com/

I would like to use some RGB LEDs and make quite a large clock. My sketches at the moment use 26 groups of LEDs, totalling 130 LEDs all up.

I have the ability to wire it all up and construct the clock, however, I have no idea how I would control them!

At the moment, I am looking at wiring all the LEDs in a single circuit and using a relay on the grounds to control which groups of LEDs turn on. The relays would then be controlled by a couple of 8 bit registers.

(TLDR;)My question is: would this be an appropriate way to go about the project? Would there be a better way of setting up the LEDs and controlling them and is there any points that would not work/might cause me a lot of hassle.

Thanks in advance for your help!

[Reply #1 on:]

Relays are great when you need to switch mains / line voltage, like for a 1/2 horsepower motor. For low voltage DC things like LEDs, most prefer to use transistors or other ICs instead of noisy, bulky relays.

(Of course, if you're going for a steam-punk / retro kind of effect, then relays are perfect -- use the biggest, clackiest ones you can find.)

Anyway, if you use a vanilla shift register like the 74HC595, you can only power up to 70mA of LEDs at a time per chip unless you wire in external transistors to switch the actual load. And, you have to use a lot of current-limiting resistors. Instead, consider an IC like the STP16C596: it's a shift register designed to drive a bunch of LEDs directly without additional transistors or resistors.

[Reply #2 on:]

Take a look at ClockTHREE (launched on kickstarter a while ago), a word clock with individually controlled RGB LEDs.

[Reply #3 on:]

Here is a monochrome version based on ClockTHREE Jr
http://arduino.cc/forum/index.php/topic,118338.0.html

[Reply #4 on:]

Wow, those clocks look awesome!

I think transistors are the way to go. I am going for an extremely modern/neat looking design... something I could put in the data centre I'm working at.

Next question: is this transistor good enough?
http://www.ebay.com.au/itm/Fairchild-2N5089-NPN-GP-Amp-25V-100mA-TO-92-BC560C-/120717085331?pt=AU_Pro_Audio&hash=item1c1b4c8a93#ht_731wt_922

I had a look and I am really struggling to understand the different mA capacity I will need etc.  I was thinking of using these LEDs:
https://www.sparkfun.com/products/105

[Reply #5 on:]

I had a look and I am really struggling to understand the different mA capacity I will need etc.  I was thinking of using these LEDs:
https://www.sparkfun.com/products/105

Great as Sparkfun is, for 130 RGB LED's your better off (price wise) looking on ebay. You can get 200 of them here http://www.ebay.com/itm/200-pcs-5mm-4pin-Tri-Color-RGB-Common-Anode-Red-Green-Blue-LED-/120953667490?pt=US_Car_Lighting&hash=item1c29667fa2 for just under $40 where Sparkfun would cost just over $200 for 130 of them. It's worth getting extra on ebay as you can sometimes end up with mixed common anode/cathode or the occasional dead led.

[Reply #6 on:]

Awesome! I knew they were overpriced but hadn't begun researching where to buy them from yet. I knew I'd find them cheaper and you've saved me the hard work! Thanks!