Friday, February 26, 2016

I'm slacking.

I admit it, and I apologize to those who might be looking for tips and tricks.  I actually completed my build two months ago and have struggled to find the time to document things.  I was in a horrid rush to finish in time for Christmas that I had to stop even thinking about blogging the build, as I literally finished the last bits on Christmas eve.

But I did take photos and notes, so I'll try to explain things still.

When I left off, I was just about to start details on wiring.  But before I do that, there are some basics that really need to be covered.  This may be boring for some, but I'm hoping to get enough detail so that those unfamiliar with electronics can still build their own panel.

With that, let's talk about LEDs.  You need to know just a few basics to successfully build your panel.  First off, every LED has an anode and a cathode.  What does this mean to you if you're not into electronics but just want to build a panel?  Not much.  Just remember this - the anode on an LED is the long wire that comes off the back, the cathode is the short wire.  It will be critical not to get them mixed up, because current (electricity) flows only one way through an LED.  If you wire in an LED backwards, it will never light up.

So there's that basic.  Next, you need to know that every LED is a bit unique.  When you connect it to a power source (like a battery), it has a voltage drop across the LED that is often referred to as the forward voltage of the LED.  Hopefully your LEDs had that spec available for you. If not, you can build a simple circuit to test for it.  If anyone needs to know how to do this, let me know and I'll try to add those details.  You will need a voltmeter for that if you want to do it.  Most of the LEDs I used had a forward voltage of about 2 to 2.1 V.  Only the blue LEDs were significantly different, at 3.0V.

Why is this important to know?  Because if you connect an LED to a power source and don't limit the current flow through the LED, it will burn out.  Fast, in some cases.  The 10 mm red LEDs I purchased, for instance, if connected directly to a power source, would burn out instantly or get noticeably dimmer very fast.  So how do you stop this?  With a resistor. Attaching a resistor to each LED will limit the current flowing to the LED.  But what size resistor do you need to have?   Well, each LED also has a maximum current limit.  If you know this, you can simply use a current limiting resistor calculator such as at to calculate the value, remembering that the supply voltage will be 5V, the voltage the Arduino operates at.  The calculation is simple, but using the webpage is simpler.  That will give you a minimum value for the resistor.  But you can go higher.  I recommend you actually do go higher, at least until you notice the LED start to get dimmer.  Often you can double the resistance value with no noticeable decrease in the brightness of the LED.  If you don't know the maximum current for the LED, then generally you can simply start with a high resistor value (~680 ohm) and start reducing to lower values until the LED stops getting brighter.  If you need directions on how to do this, just let me know.  Odds are actually pretty good that the resistor values I chose (which I'll give later) will work well for you.

Next up, I'll get to talking about LED matrices.  This is where the detail on the wiring and programming will really start.