I let the paint on the lexan dry for a week and re-milled the front panel. This time, it came out looking much better. I also tweaked the location of the mounting holes slightly to make it easier to line them up with the PCB (my PCB cad software prefers .05" spacing). Here is the piece lit by two LEDs from the back.
My first test lighting used regular off-the-shelf 5mm LEDs, the kind you see in DIY electronics kits. I was hoping these would work because they don't require too much current. The goal was to be able to power this project from USB which limits current to 500mA at 5V. Unfortunately, they weren't bright at all. This became a major time sink since I not only had to source new LEDs, but also redesign the product for external power. I ordered an assortment of high output LEDs of differing color and output angle and soldered them onto a test PCB. This also happened to be my first foray into surface mount soldering.
I picked two of the LEDs from the batch, one red, one amber to use in the project. They both pulled 100mA at 2V, so I'd need to provide 1.3A at 2V since I'm using 13 separate LEDs. Not a trivial amount of power! I first tried using a regular voltage regulator to bring my wall wart power down to 3.3V which the LEDs took with relatively small resistors, but there was so much excess voltage, the regulator became scalding hot within seconds. That wasn't going to work. Next up, why not try my hand at switching regulators. The aforementioned linear voltage regulator drops voltage by converting it to heat, hence, the enormous amount of heat. A switching regulator instead turns the power on when the voltage is below the desired voltage, then off when it goes above the required voltage. This extremely fast switching is much more efficient and produces very little heat. neat!
At this point, I had the circuit design finalized so I began making the PCBs. I started with the board that would hold the LEDs.
This board is pretty simple. It has LEDs (the little white things) and the accompanying resistors (little black things) that pull current through the circuit and light the LEDs. On the rear of the board, I attached a ribbon cable and connector as well as two extra wires that were difficult to etch on the front.
I re-milled the light tunnel that separates each lit section out of 3/4" MDF. MDF doesn't warp nearly as much as plywood and turned out to be a bit thicker so it lights more evenly. Here's the light tunnel sandwiched between the LED board and the front panel.
I then milled and put together the main board, which wasn't terribly exciting, just a standard PCB with a few components on it. The only thing that needed to be done after that was make a bracket to hold the LED board at a right angle with the main board and acquire the hardware to bold everything together. Here's the final product.
The main board is pretty simple. In the very back you see the USB connector, a power indication LED, the connector for the TEST/DIM switch, and the power connector (6-12VDC). Above that we have the main chip, a PIC18F4550. I chose this chip due to the easy to use package size and the native hardware USB interface. To the upper left we have a series of resistors which lead to the little black transistors next to where the LED board plugs in. These control the switching of the LEDs on and off. In the upper right, we have the switching regulator components. These components are way overkill for what I'm using them for, but it's what was recommended in the datasheet so I stuck with it. It's rated for 3A and I'm only using about 1A. The TIP120 chip controls the switching and only gets mildly warm. I've since added a small heatsink just in case.
I'll add a video of the Annunciator in action when I get the chance.
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