Wednesday, December 10, 2014

The PCB Part 4: Drilling, Populating, and Soldering

I finally got around to finishing the PCB last night.  I didn't have any drill bits near small enough so I used some Amazon points to order this micro drill set from Amazon which included a swivel head pin vice (the "handle").  It's a pretty nice set but it only took me two holes to realize I didn't want to do the rest by hand.

It would've been best to use a small drill press, which I don't have, so I figured I'd just use a rotary tool.  I have the full Dremel kit (minus the drill press base, of course) including the pen attachment but that still seemed too bulky.  I bought one of those cheapo 80 piece rotary tool kits from Harbor Freight quite some time back.  It's nice and small, which would make it easier to keep the bit from wandering, so I thought I'd give it a try.

I planned to wrap tape around the 1mm drill bit from the Amazon kit to fit it into a collet but, low-and-behold, there is a 1mm collet as well as a 1mm drill bit in the HF kit!  This made short work of the remaining holes though it did have a tendency to walk slightly.

After drilling was complete, I used the real Dremel with the side cutting bit and the small, plastic router base to cut the PCB to shape.  This worked fairly well, so long as I went slow and kept a firm grip on the Dremel.  I used a hand file afterwards to clean up the edges.

Then, on to the soldering.  I discovered two areas following the etching process where the traces had breaks.  I addressed those while soldering but using a small jumper wire.  I also decided to "coat" the copper traces with solder.  My reasoning was to reinforce them, in case any others were marginal, as well as to provide a little corrosion resistance.  It looks ugly as hell but it works.

For the wired leads, I went with standard Yamaha colors (at least for circa 1980): Black = Ground, Blue = Indicator, Yellow = Brake.

The LEDs were purchased online from  Their pricing was reasonable and they were shipped extremely quickly.  You can find cheaper LEDs from Asia on eBay but I was quite pleased with my order from these guys.  As an added bonus, the box they shipped in, although over-sized for the 50 LEDs I order, ended up being a perfect size to carry around the various parts of the light (including the housing pattern/form) while I work on them.

Each section of the light (indicator and brake) consists of three parallel strings of four LEDs and one resistor.  I calculated the required resistor for each to be 260 Ohm but, with 270 Ohm being the closest standard size, I went with that.  (Okay, okay... I didn't exactly calculate the size at first.  I used an online calculator but I did go back and confirm it mathematically, just to make sure I remembered how to do so.)  I also found I only had five of the six required 270 Ohm resistors so I had to put two 560 Ohm resistors in parallel to get it back down to the required resistance.  Good ol' Ohm's Law!

And best of all, I got the circuit correct on the first attempt!! Alright, not exactly the first attempt.  This actually was version 1.1 where I switched from standard 5mm "dome" LEDs to the four conductor flat mount type.  In doing so, I did find that I had placed one of the LEDs backwards in the circuit which was remedied in this version.  Either way, we have light!!

Indicator/Tail LampBrake LightBoth Circuits

Lessons learned/things I'd do different next time:
  • Using overhead transparencies instead of photo paper was definitely the way to go.
  • I'd make the traces thicker as well as the component pads.  This would help in the etching phase but also in the drilling and soldering step.
  • A drill press, though not required, would've made drilling so many holes a little easier.

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