Close

PCB Layout, Engineering challenges

A project log for Time Circuits Timepiece

A practical-size, Back to the Future themed digital clock mimicking the "Present Time" portion of the DeLorean's time circuit displays.

stephen-holdawayStephen Holdaway 02/22/2017 at 00:171 Comment

Earlier this week...

It became clear pretty quickly that 340 x 40mm isn't a whole lot of space for PCB layout when there are 234 through-holes spread evenly across your board.

Since KiCAD requires some manual prodding to create multiple boards from a single schematic, I had a play with putting all the components on one double-sided board before breaking the project into multiple boards. A bit of fiddling confirmed that both the control and display boards are too complex to route singled sided (without some trickery and a lot of time), so the display board will be a single PCB that I'll have manufactered by a fab house instead of doing it myself.

The display drivers just fit inside the footprint of a single segment, which is great because there's not really any other place to put them:

Another challenging part to place was the rotary encoder - the primary control for this device. In order to not have the knob stick out the back too far, this needs to be mounted on the display PCB, however it also needs to line up with one of the access ports in the back of the enclosure. With these constraints, there was really only one place to put it, and it's a tight squeeze:

The 16 segment displays I'm using have a 1mm air gap under them when mounted, which should be just enough to clear the pins from the rotary encoder once they're trimmed. Not ideal mounting it this way since the overlapping display segment would need to be desoldered if the rotary encoder needed to be replaced at some point.

As part of figuring out this positioning, I also went and 3D printed a knob for the rotary encoder:

Flicking between enclosure design, positioning controls, PCB layout, and figuring out how to use limited available space really makes me appreciate how much combined engineering effort goes into even the most mundane consumer electronic device. That said, I've drawn myself into a corner somewhat by designing as I go, rather than planning the entire product before starting anything. Makes for interesting challenges in any case.


Present day - PCB layout done

To keep the control board small, the piezo buzzer and light sensor have stayed on the display board along with the rotary encoder. Everything else is on a 40 x 55mm board that connects via a 14 pin header:

The display board hasn't changed much from earlier:

Schematics have been tweaked, but mostly just moving things around:

Testing display footprint

I've also done a quick test to confirm that the display segment footprint based on the datasheet has enough tolerence for the segments to fit together. Works a treat:

Next step is to get these boards manufactured. A little bit scary as this is only the third PCB I've designed... hopefully no major mistakes.

Discussions

Jan wrote 02/25/2017 at 12:02 point

"That said, I've drawn myself into a corner somewhat by designing as I go, rather than planning the entire product before starting anything. Makes for interesting challenges in any case."

Exactly what I did with my latest clock (which is much less complex compared to yours). For my next project I am really gonna try to keep myself focused and on the "finish the design before building it" path :)

  Are you sure? yes | no