Meter backpack.

Well dammit.  I spent a few hours testing traces and resoldering points on the not-working board only to finally realize I'd soldered the transistors on backwards.  Fixed that.  It's still not working, but it was more not working before.

Perils of DIY Etching #62:  no silk screen means no markings to align your components correctly.  I might actually print up a silkscreen layer next time and iron it on after I've tinned the traces.

I'm seriously considering just bailing out on this board and etching a new one with vias to make almost all component soldering happen on the back.  This one has been soldered and resoldered so many times I don't know if I trust it to last a month.

Perils of DIY Etching #23:  no through-hole plating means that header pins and other components that can expect some stress are only soldered to a tiny, poorly-adhered wafer of copper.   You are in a maze of twisted copper traces, all alike.  You are likely to rip one up.


Not sure how to best address the strain relief issue, but if I redo the board I'm gonna make sure I solder it nice and clean on both sides.

Here's what the board-with-vias might look like:

Like this, except I really need header pads on the top to help hold the pins in place.

Meter board with control backpack *updated*

Finished soldering the control backpack last night, but I haven't had a chance to fire it up and test yet.  I am certain there will be problems, but an initial continuity check with a meter shows all traces to be doing what they're supposed to do.

Here's the final front and back layouts for the control board:

Top and bottom layers, as I print them for toner transfer.

It's got two TPIC6B595 shift registers with built-in darlington to sink the 11 rows of LEDs, one 74HC595 shift register that's driving 7 high-side PNP transistors to push the 3.3v up each column, and resistors for the LEDs and the transistor bases.   On one side is 11 pin headers to plug into the columns of the meters board, and on the bottom are 7 header pins for the rows.

Even through there's only 3 control wires and VCC/GND, I used a 5x2 header block to interface with the offboard microprocessor because 5x2 is the easiest ribbon cable connector to find around here, and I figure it's easiest for all of the submodules to use the same connector.  Maybe this is dumb, I'll find out eventually :)

Finally, here's a shot of this board plugged into the meters board.   I haven't cut the boards down to their final size or cleaned them up yet, that would be wasted time if the thing is broke.

I had to stand the 7-pin headerblock off the board by a few mm to get the iron in underneath.  Only afterward did I realize I could solder the headerpins 'upside down' and then just push the plastic retainer down against the board.   Live and learn.

The biggest lesson I've learned in this stage is that etching your own board is easy, but soldering to your home-etched boards can be a major pain in the ass.  Specifically, since there's no through-hole plating, you can't just solder your through-hole components on either side and be done;  you have to solder on the side where the pad connects to the trace.  If this is the same side as the component, you're stuck trying to work the soldering iron in underneath the component.   This can be really difficult, and fixing mistakes becomes even more difficult.

I will need to keep this in mind as I lay out future boards, as I really did not enjoy doing this at all and it made for a sloppy solder job.  If I can't get it right in layout perhaps I will add pads right before the component pads and solder to that, then use a wire to jump to the other side.

*update*
shit's broke,   Feels like a solder bridge somewhere, though it's of course entirely possible the whole layout is wrong.   Losing is Fun!

Progress update

Had some extra time this week to tinker.  Here's what I've got:

1) Finally screened and etched the SSOP24 breakout widget.  This is needed so I can breadboard the chip and find its magic.

SSOP-24 breakout.  Toner transfer method is surprisingly efffective.

2) I worked up a shift-register backpack for the LCD screen.   I figured since 4-bit uperation of the LCD screen requires only 7 pins, why not control them with a shift register and reduce the micro pins count by four?   why not indeed.  We will soon-ish see if this is wise or dumb, but I tossed together a simple interface board to help test.

You can see the evidence of a sloppy laser printer:  the ground pours are splotchy because the printer  is too.

3) I'd always planned on an LED Meter bridge backpack to hold the actual control ICs.  I had assumed something like the TLC5940 sinking the rows and a 595 through high-side transistors on the columns.  Trouble for me is, I don't see a great way to control the TLC5940 without using more pins than I have available...   so I'm falling back to plain shift registers.   I'd already made the LED meter board with plan A in mind, so I'll be driving columns no matter what.  unless I remake the board.
Here's the backpack, it'll hold one 74hc595 to fire seven transistors for the cols, and then two TPIC6B595s to sink the rows.

4) I drilled and tinned all of these, and began populating the meter backpack. The tinning ain't pretty, but it seems to work well.

Duh moment

I just realized I've been routing audio directly into the digital pots.  And that is dumb.  All this mess about biasing the signal to center around 2.5v and whatnot, so dumb. I have no idea why I went down this path, and I feel pretty foolish.

HEY ANDREW:  USE THE POT TO CONTROL THE OPAMPS.  OR NOT.  MAYBE I WAS RIGHT TO BEGIN WITH.

SO CONFLICTED RIGHT NOW

Meter board, double-sided PCB etching

With the success of the SSOP breakout experiment, I was eager to try it on a larger scale.   If everything goes well, I may end up making all of the boards in-house!  This would be awesome, because I make a lot of mistakes and wasn't looking forward to the one-month turnaround times just to learn my board was crap.

The next most-simple board I have planned is the meter bridge.  It's a simple matrix of 77 LEDs, to be controlled by a TLC5940 PWM LED driver chip and a 74HC595 shift register.  In order to fit everything within the front panel of a 1U rack unit, I've opted to move the chips onto a daughter board that will backpack onto the front-facing meter board.  

The meter board is a simple affair, but absolutely requires a two-sided PCB due to the nature of a matrix.  Step one:  design the board.

Front and back of a board for 7 10-LED bargraphs and a single 5mm LED at the top of each column.

Some close-ish traces, but after doing the SSOP I felt confident.

Several people suggested printing both sides and aligning them  face-to-face, taping the paper together on one side, and ironing them both at once.  This sounded a lot better than the one-side-at-a-time technique I had imagined, and in practice it worked perfectly.

Hey, cool.

 I'll solder it up tomorrow night, but dang I am excited about this.   I chose the right drill size this time (1/32") and everything just came together.  Assuming everything is good, this is a major win for my productivity - all of those LEDs and their associated 150+ wires were a total mess on two breadboards.  Consolidating all of that down to just 18 pins is wonderful.  If the soldering goes well, I'll prototype my control chips and then make the daughter board for this, further reducing the pin count to 5.  Yes!

si4735: AM/FM LW/SW radio on a chip, DIY circuit etching

I noticed this chip on sparkfun the other day, and immediately knew I had to have it in the project.   Since this mixer is meant to sit next to the media PC and handle all audio, it's just too cool to have a radio in the box as well.

The chip looks awesome, but there's a small problem - the largest package available is SSOP in a narrower-than-normal size.  This means that any commonly available SSOP-24 breakout board is worthless.   However, I've gotta break it out to bread-boardable pins because I have no hope of getting it set up correctly without testing first.

That left me with only one real option - I've gotta make the breakout myself.   Instead of using one of the many PCB fab companies, I thought I'd try my hand at etching a circuit myself.

First, a design.  Seems simple enough for a first attempt:

Narrow SSOP-24 breakout.  Click for full size

So there are several methods for DIY etching.   After some research, I decided to try the toner transfer technique, where you print your design on glossy paper and then iron the plastic toner right onto the copper.   The toner resists the etchant, and boom hot dog.

My first attempts were failures of course - it took me a while to find the right combination of paper and heat and time, but in the end I got a good transfer. Etched it, and all traces were good!

Soldering to a board without plated pin holes is trickier than I'm used to.   Also, I drilled the holes a little big, so I had to bridge the gap between the pad and the pin.   It took some re-working and the result is ugly as hell, but all pins are clean and working.

SSOP breakout board.  This is an earlier design than the one above, and the solders are /ugly/

I'm extremely encouraged to know I can cheaply and quickly make PCBs with tiny SMDs.

Maple Mini

I ordered a Maple Mini last week and it showed up on Saturday.  It's a fantastic piece of work, it's extremely fast, lots of hardware interrupts, lots of memory, and I'm having a blast playing with it.  I was running up against many limits with the ATMega328, and I was adding lots of components to work around them.  With the Maple, I can get a little lazier!