Tuesday, June 7, 2011

Linksys LED Matrix

     My first electronics project of the summer is now finished. Overall, it didn't take me too long to complete, and I'm very pleased with the results. Watch this video overview and check after the break for all the juicy details.


     Picking up from where I left off, Linksys LED Matrix (Intro) , I finished assembly of the LED Matrix along the front of the board.

All 39 LEDs in place

Wires connecting the right and left sides together
All anodes connected together with short red wires
750 ohm resistors for "always on" center LEDs

     The AC adapter I'm using measured 12.63 V DC and measuring the output of the AN6884 gave me 16.3 mA per LED. So using simple Ohm's law, I calculated that I needed 775 ohm resistors. I happened to have two 750 ohm resistors and calculated that would give me about 15.1 mA, which is perfect for LEDs.


     I mimicked the application circuit almost exactly. The one thing that I did change was a 10 uF capacitor on pin 7 to a 4.7 uF one (as shown in the image above). This capacitor adjusts how quickly the LEDs react to the sound. Less capacitance produces a quicker response. As stated, I'm using a Vcc of about 12.6 V so I choose to use a value of 68 ohms for R. I knew I would never run this circuit above 60C (140 F!).

Finished circuit on protoboard

     While assembling this circuit on protoboard, I kept all the components along one side. I had originally thought about putting another AN6884 chip in and making this run on both the left and right channels. Instead, I managed to squeeze a 8 ohm speaker into the case. I ran the audio (Vin) from the left channel, because, according to this diagram, both mono and stereo audio jacks (TRS connectors) share the left channel through the tip. I used a screw terminal to hold the three wires (ground, left, and right channels). I directly connected these to a jack plug that works as a feedthrough for the audio, so that I can attach external speakers. I also have a double spring clip for my input voltage and ground.

Power and audio inputs (with nail polish color coding)

Switches and potentiometer mounted beside LEDs

     I mounted two switches to the front; one for turning the internal speaker on and off, and the other to remove power from the circuit (which turns the LEDs off, but leaves the audio feedthrough untouched). Along with the switches is a 10 Kohm potentiometer that is used as a voltage divider to adjust the incoming signal. It is recessed into the case and needs to be turned with a screwdriver, but I don't change the input volume from my computer so it isn't a big deal (I have a volume adjust on my external speakers that I use with my computer's audio jack always turned to 100%). I have some copper foil soldered between the switches to keep them aligned to the holes in the case.




     After completely assembling it, I didn't like how the green LEDs and blue case went together. I decided to spray paint the entire case flat black. While I was doing that, I cut out a very thin plastic sheet to affix to the front to diverge the light from the LEDs a bit. Along with that, I placed a small piece of foil around the yellow LED because the green LEDs washed the color out. I like how it looks much more, but it was too plain. I added a OSHW decal from Mr. Decals. I just wish it was cut out more evenly and didn't have the white border in some places.

Finished project with lights on

Final placement on my desk
     I hope you enjoyed my project write-up. Let me know in the comments if you liked it or if you have any suggestions or if there's anything else I can elaborate on. Stay tuned to my RSS feed for any future projects!