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I am working on building a wordclock using one of the several design ideas that you can find all over the Internet. Something like a mix of this one: http://www.instructables.com/id/Sleek-word-clock/ and this one: http://www.instructables.com/id/Personalised-Word-Clock/ A lot of the guides suggested building your own Arduino to save money, but I found these Arduino Nano clones for $7/apiece: http://www.amazon.com/gp/product/B00SGMEH7G.

I've done a few simple projects before, but nothing with the same kind of power demand. How can I figure out the power requirements? My plan is to use approximately 64 3mm LEDs via shift register, or possibly the equivalent segments of an LED strip.

The Nano clone does not have a barrel connector for external power adaptor. Should I directly solder on the power cables from an external adaptor? Use a screw terminal or similar? Is the USB adaptor a good enough source of power for 64 LEDs? Or do I have to do something else to supply external power to that many LEDs?

Thanks for any advice! I am experienced in programming but not in electrical engineering so many of the concepts are something I don't have a good handle on yet.

  • If you use shiftregisters, and don't do any multiplexing, you probably don't want to run the leds at their maximum brightness. Running them at 5mA will probably be more that bright enough. That would, in worst case, mean 64*5=320mA. Which is not that much, though I'd suggest using a 5v adapter, as the tiny voltage regulator in the nano board probably won't handle that much current without getting extremely hot. You can easily find 5v usb chargers that handle upto 2A. – Gerben Oct 8 '15 at 18:20
  • I'm trying to find some videos that explain it better. How should I tie in the 5v USB charger to the Nano? – Quinten Oct 8 '15 at 18:44
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    Just use the mini-usb plug, or connect ground and 5v on the nano to the corresponding wires on the charger. – Gerben Oct 9 '15 at 13:02
  • Are you ever going to power all 64 LEDs simultaneously? I would imagine a word clock would have just a fraction of the LEDs switched on at any given time, which would greatly reduce the current demands. – Adrian McCarthy Mar 16 '18 at 17:58
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If you are running the LEDs from shift registers then the power rating of the Nano is irrelevant.

If you are wanting to use the Nano as the power source (not recommended) then you would need to know both your expected current and what the regulator on the Nano would be able to supply. My immediate guess is "not enough".

64 LEDs at full brightness would be expected to use a maximum of say 20mA each, so 1.28A just for the LEDs. That's a huge amount, and way more than any linear regulator can be expected to supply without melting everything around it.

So you can reduce your current requirements by not having the LEDs as bright. Half the current does not equate to half the apparent brightness. In fact at half the current you probably won't notice much difference at all. At a tenth of the current you will see them a bit dimmer, but not as much as you would expect.

So you can come up with a compromise between the brightness you desire and the current consumed by each LED.

Say you end up deciding on (as Gerben has suggested) a current of 5mA per LED. When all LEDs are on that would be (5 × 64) 320mA. That's reasonable for a linear regulator, but still will generate more heat than is really advisable.

So you should, for this project (assuming you aren't going to be using multiplexing, which is a whole other ballgame), investigate using a switching (aka "buck") regulator. You can pick these up on eBay for just a couple of dollars and can quite happily supply amps as opposed to milliamps and will allow you to run far more LEDs without going into thermonuclear meltdown.

Another important consideration to think about is how much current an individual shift register can provide - both through its output pins and in total for the whole chip. There are special constant current LED driving shift registers that remove the need for individual resistors on each LED and give much better control over the LEDs than traditional logic output shift registers.

  • Thank you. I was planning to use 3 of these: SN74HC595N to control the LEDs. Are you talking about something like the MAX7221/7219? I found a guide that mentioned that part but I didn't realize one of the benefits was not needing resistors. That sounds really useful, I will use it! Also: do you have any good references I can use to learn about how to safely connect the power? EDIT: looking at this guide, playground.arduino.cc/Main/MAX72XXHardware, does this reduce power usage to safely supplied by the USB port? I would use a small AC adapter with USB micro port. – Quinten Oct 8 '15 at 20:43
  • I often use the TLC59116 which is an I2C device - you can have up to 14 on one bus and each one supports 16 LEDs (so 224 LEDs max). Connecting the power is easy - the best thing to do is just use the buck regulator for everything - connect it to the 5V/GND pins of the Nano and also the Vcc or 5V and GND pins of any external devices. – Majenko Oct 8 '15 at 20:46
  • Oh, the TLC59116 is also a PWM device, so you can set the brightness of each individual LED - just like the PWM outputs on the Arduino. I even have a library to control them: github.com/MajenkoLibraries/TLC59116 – Majenko Oct 8 '15 at 20:49

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