Wednesday, 30 April 2014

LED Desk Light – n x n matrix

I’ve decided that it’s too dark at my desk using just the overhead light for illumination. Hell, I’m getting old and my eyes aren’t much good in the dark anymore.
What I’m building is a very simple matrix of LED lights soldered to a perf-board. The idea is very simple, a matrix of LED connected in a parallel circuit that kind of snakes around to make a 5 x 4 (20) LED configuration.
I’ve designed a simple 2 x 2 LED circuit that I will simply extend in both the X and Y axis to make my 5 x 4 circuit. The circuit was designed using Fritzing.
Fritzing - 2x2 LED Matrix_pcb
I’m only using this as a design guide, since I’m going to be making it up on perf-board rather than PCB, and I’m orienting the resisters vertically, rather than horizontally to conserve space on my perf-board. The main reason for that is that I want to set my board up in as little space as I can (with a reasonable amount of space, that is) and using through hole components.
The circuit uses 20 x 5mm Ultra Bright White LED, 20 x 150 Ω resistors (120 Ω is the right size … but I don’t have any of them) and 2 x 90o pin headers to attach to power (a 5.7V AC Adaptor). I used a LED Design Wizard ( to help me to design the matrix based on the properties of the LED that I am using (Forward Voltage = 3.3V, Forward Current = 24 mA and a source voltage of 5.7V AC).
Later on, I’m planning on making the same thing using surface mount technology (SMT) with 0603 components.
Fritzing - 2x2 LED Matrix - 0603 SMD_pcb
That’s going to be a bit of an adventure in surface mount by hand for me. I’ve never done any SMT stuff before and this is a simple enough circuit. But that’s for later (I still have to order the parts, and then wait for up to a month for them to arrive).
So far, I’ve soldered up 4 LED and 4 resistors. My LED are spaced about 5mm above  the perf-board and I’ve made myself a spacer that will allow me to position the LED consistently above the board. The spacer is nothing more than a paddle pop stick that I’ve cut down to size. The spacer fits between the LED legs and, when I’ve bent the LED legs, the fit is snug. The spacer allows me to orient the LED rows while I’m soldering and then (with a little wiggling) I slide the spacer out and use it on the next row. I can also use the spacer to set the resistors up, too.
Now that I’ve done the first 4 LED (2 x 2), I can see that I should have started the first row off one row in from the edge so that I can better accommodate the column jumpers. I’ve also noticed that the silk print on the cheap perf-board is not very well laid out. The grid is 5 x 5 then 5 x 6 then 5 x 5 so it doesn’t end up even. Oh, well … I’ll survive. It just looks a bit wrong.
4x5 LED Matrix - LED with Guide
Above is a picture of the LED matrix showing one row of LED aligned using the spacer. This is how the LED legs will be bent evenly so that the LED are all set to the same height above the perf-board.
4x5 LED Matrix - Horizontally Aligned Resistors
And here is a view of the matrix showing the 4 LED that have been soldered to the board. These 4 LED were soldered without the spacer (before I had the idea to use the paddle pop stick as a spacer, at least). You can also see the silk screen grid on the perf-board a little better. Let’s play “Count the Holes!”. The horizontally aligned resistors save me a little space.
4x5 LED Matrix - Solder View
Finally, here is the under side of the perf-board showing the beginnings of the solder grid. The height of the LED above the board has the advantage of setting the length of the leg under the board to the anode of the next LED in the column. This makes it a little easier when I’m soldering and I don’t have to cut any of the anode leg off. The cathode leg of the LED is a different matter, it gets soldered to the resistor and then chopped off. I’ve bent the negative side of the resistor so that the leads from the resistor forms the negative line. The positive and negative  columns then get connected to the next positive or negative column, respectively, with some shielded jumper wire … but we’ll get to that later.
When soldering the LED into the perf-board, I use a heat sink clip on the upper side of the LED to avoid overheating the component. I start by heat sinking the unsoldered anode and then soldering the anode from the previous LED to it. The solder has some trouble forming a good fillet, so I leave the iron against the copper pad and make sure it gets a decent amount of solder flowing in. Some of my soldering is a little messy, but, hey … it’s a prototype, so I can cope with it.
When the anode is soldered, I then solder in the cathode (with the heat sink attached on the other side) and clip the lead where it will intersect with the lead from the resistor. Then it’s a matter of soldering the resistor in place, cutting the crossing lead from the resistor (the one that attaches to the LED cathode) and bend the other lead down into it’s column.
So far, so good.

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