23 March 2021
Last week I sent all the files necessary to build the first draft at my Ilkley prototype to China. That means the plotting files to make the circuit boards, the list of all the components on them, the positions of all the components. The factory’s going to make the circuit boards and attach most of the components for me.
This is good, because many of the components are tiny.
The Ilkley prototype is on two boards: a ‘brain’ board that contains the microcontroller and almost all the electronics, and a separate ‘control’ board that is just some IO and inputs - knobs, buttons. I am focusing on the brain right now: its “revision A” board is the right size and shape to go in our housing; the current prototype of the control board is just designed to sit on my desk.
About six hours after I sent it all off, I got an email: I’d designed around the wrong sized part. (I’d picked a 3mmx3mm QFN part instead of a 4mm square part, because that’s what had been auto-selected by the component library). This meant they couldn’t place the part on the board: it wouldn’t fit.
“Should we ignore the part and go ahead with assembly?”
At this point blood rushes to my head. That part is one of the reasons I’m not building it myself: it’s not really possibly to attach with a soldering iron, and I don’t have a better tool available at home. So maybe I should quickly redesign around the right part? I hammered out a new design in Kicad.
But now I’d have to send new files, new placements over, and probably start the order again. This was going to add delays, might not even be accepted by the fabricator, and so on.
At this point, I took a step back, and had a cup of tea.
Over said tea, I made myself answer the question: what was this prototype for?
Was it only to test the functionality of that single chip that couldn’t be placed? The answer, of course, was no. There were lots of things it evaluated, and lots of things that could still be evaluated:
- many other sub-circuits - notably, a battery charger, a second amplifier, a mixer
- the integration with the ‘control board’ and the feasibility of my ribbon-cable prototype
- how several parts ended up being fitted, which the fabricator has not used for me before
- the fit/finish inside our final enclosure
- not to mention whether I’d made any other mistakes on the board.
So far on Ilkley, we have been lucky: every single first revision of our hardware’s worked. This doesn’t mean we’re brilliant at everything; it means nothing more than that we’re in credit with the gods of hardware. Something will go wrong at some point - that’s what
revision Bis for. All that had happened was I’d hit my first big snag.
Prototypes aren’t about answering every question, but they’re rarely also about answering one. I usually teach people to scope them by being able to answer the question what is being prototyped here? - the goal being to understand what’s in scope and what is not. Temporarily, in the panic of a 4am email from China, I forgot to answer that question myself. It was good to be reminded how many variables were at play in that Revision A, if only to acknowledge how many things I have going on with that board.
I wrote back to the factory; ignore the part and proceed. We’d still learn a lot from the prototype, and revision B would contain, at a minimum, a new footprint for that 4mm QFN chip. I saved the hasty changes I’d made to the circuit board after the email in a new branch called
revision_b- which I’d return to working on once the final boards arrived.