Gas station without pumps

2011 August 2

Boards arrived

The hexmotor rev 1.3 boards before cutting them apart.

The printed circuit boards for hexmotor rev1.3 came today from 4pcb.com. I ordered the boards on July 21, so the total turnaround time from order to delivery was only 12 days. No wonder the company is so popular for student projects—low prices and fast service will do that!

The boards looked pretty good, so the first thing I did was to check some crucial dimensions:

  • I put headers into the holes for the connection to the Arduino board and checked that it would plug in.  No problem!
  • I checked that I could spread the pins of the TLE-5206-2S chips to fit the holes where they were supposed to plug in.  No problem!  In fact, it was surprisingly easy to get the right spacing, by using the board itself as a tool for bending the pins.
  • I checked that I could put in both the MTR2 TLE-5206-2 chip and capacitor C5 without conflict (as I’d noticed that the silk screen outlines overlapped only after I’d sent in the order).  This also turned out not to be a problem
  • I checked that the mounting holes on the board lined up with those on the Arduino.  Only 2 out of 3 do.  What is going on???I got the hole placement from the Adafruit library for Eagle, and it has 3 holes, but they are not in the same places as the Arduino board.  It turns out that two of the holes correspond to holes on the Arduino board, and two to holes on the Arduino Mega board, which has holes in different locations.  Neither pair corresponds to the holes on the  Adafruit motor shield, which has only 2 diagonally opposite holes, the two holes that are actually in common between the Arduino and the Arduino Mega footprints.  For the next revision of the boards, I have to decide whether to use the 3-hole design of this board, or to go with the pair of diagonally opposite holes.  Or put in 4 holes, so that either Arduino or Arduino Mega can be securely attached with 3 screws. I think that there is room for the fourth hole, if I move one of the electrolytic capacitors.
  • I also did some continuity checks on the power lines.  The power and ground lines connect to the right pins on the TLE-5206-2 footprints, and they aren’t shorted.  The 6.25v and 5v lines also seem fine.
  • I had to make some Eagle library components for the screw terminal blocks I was using, and they seem to be fine, except for two little problems: I put a silkscreen mark for the bump on the side of the 5mm block on the wrong side, and the holes specified by the manufacturer seem a bit too big for the pins.  I’ll specify a smaller drill hole (closer to the spec for the pin size) in the next revision.  Neither of these little problems will interfere with functionality.
  • I checked out the Sparkfun header pattern, with the slightly staggered holes to hold the headers while soldering.  The holes must be a bit bigger than Sparkfun gets from their manufacturer, or my headers smaller, because the headers were not held by the staggered holes.  They fit fine, but the holes are loose enough that they aren’t held.

Tonight, I’ll fix up my Eagle library to have a 4-hole Arduino template and to have a better 5mm screw terminal component.

Here's a 12" board shear that costs $379 from Circuit Specialists.

Tomorrow I’ll go up to campus and see if I can cut the boards apart with the board shears.

I took a look at  printed circuit board shears on the web (you need to include “printed circuit” if you search, or you get book-binding equipment), and they range in price from $125 to $500, depending on size and quality.  I don’t plan to buy one for myself, but it seems like the sort of low-price, special-purpose tool that a place like Makers Factory ought to get.

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