Macrofab Synthesizer Round 2
I mentioned in my first Macrofab Synthesizer post that the hosts on the Macrofab Engineering Podcast had a contest to design an LED array for feedback on a synthesizer adjustment potentiometer. Well, I won the competition so they're going to go ahead and manufacture my design. This of course meant that I have to actually finish the design which required many, many small adjustments and changes.
First off, I had to actually decide how I wanted to drive the LED's. In my first run at this I considered either a dedicated 16 channel buffer chip or individual transistors for each LED. The material price of each solution is pretty similar so my concerns were mostly based on if I had the room to fit the chip on the already cluttered driver board. As I started to seriously look at the layout of the driver board I realized that I could get pretty much everything before the counter output on one side of the board leaving most of the other layer to route the buffer signals. Once I knew I could fit the chip it became the obvious choice. The TSSOP-48 package with .5mm pin spacing would make this a very challenging board to hand solder, but because Macrofab's business is "End-to-End Electronics Manufacturing & Operations" I didn't have to worry about that.
With my schematic pretty much settled I went on to finalize the layout and routing. This would have been very challenging using the fairly conservative design rules that I normally use when I send boards off to cheaper board houses, but luckily Macrofab's minimum manufacturing rules allow for very compact and designs. Particularly, the ability to design down to 5 mil trace spacing and 6 mil annular rings is what allowed me to get the entire design to be within the 1.2" x 1.2" requirement from the original challenge. I put all of the external inputs and outputs in a line with through holes so once the design is manufactured it should be relatively easy to integrate into Stephen's synthesizer. The last step was to leave myself a test point for both the clock signal and the multivibrator pulse so if (realistically when) this doesn't work exactly as designed first try I have a good entry point into figuring out what's going wrong.
Because of how far along my LED board was when I submitted my idea to the podcast I had fewer alterations to make to that design. The only major change was that the buffer chip I chose sinks rather than sources so I had to reverse the polarity of all of the LED's. I also had lots of fun fighting the board cutout tool in Circuit Maker trying to make the hole and alignment slot for the potentiometer. Lastly, I added a few mounting holes to the LED board to make things easier on the body and solder joints of the potentiometer.
Board schematics and layouts (click to enlarge)
It took a fairly normal amount of fiddling to get Circuit Maker and the Macrofab website to agree on a few small details, but after a bit of tweaking I was able to get both designs uploaded and ready for manufacture. Overall, the price for the LEDs specifically drove the cost a bit higher than I was hoping for. The price that Macrofab quoted is in line with what I was finding at large online suppliers like Arrow, Digikey, and Mouser. I just feel that those websites tend to charge a ridiculous amount for LEDs even when bought in reasonably large quantity. For example, currently if I buy Arrow's entire stock of 3244 Vishay TLLG4400 LEDs they still want to change $0.11 per LED. If I was actually going to do a manufacturing run of these I would probably source the LEDs myself and then send them to Macrofab to store and use in the product. You can see the price breakdowns for the two different boards below. All tolled, for a single piece to go from gerber files to a fully assembled unit the driver board would be $28.45 and the LED board would be $27.61 for a total of about $56. Considering that is for a manufacturing run of 1 unit I think that's pretty reasonable. For a more normal run of 1000 units the price goes down to about $16 per assembled unit.
For now, my part is done and I have handed things over to Macrofab to get everything made as a part of Stephen's larger synthesizer project. I've been pretty lucky with projects working as designed, but I can't believe I'll get lucky enough for this to work 100% first try so I'm sure I'll have something interesting to say make once this design is created.
