Gas station without pumps

2012 July 28

Instrumentation amp protoboard rev2.1

Revision 2.1 of the instrumentation amp protoboard. This version allows the screw terminals for Gnd input to be disconnected from the board Gnd when a barrel connector is plugged in, or not, depending on jumper wires.

In Instrumentation amp protoboard, I showed an earlier draft design for the instrumentation amp protoboard. After tweaking the design a bit, I think I’m ready to try fabricating it.

I’m planning to try fabricating through iteadstudio this time, since they have the cheapest rates I’ve found yet: 10 boards for $9.90 + $3.90 shipping, for only $13.80.  Of course, the shipping is from China, so even air mail may take a while, and I don’t know what their turnaround time is for orders.  They also have small-batch orders that are even cheaper: $45 for 50 boards, $75 for 100 boards, making this very attractive for classroom use.  Of course, if the board gets just a tiny bit bigger, the price doubles or quadruples, since they have a 5cm×5cm bounding box for this price.

This will be the 4th different PCB prototype service I’ve tried. The others are 4pcb.com (good service, USA made, but a bit pricey), batchPCB (a little slower, but cheaper, probably from China), and OSH Park (USA made, cheap if boards are tiny or you need 3 copies , accepts Eagle brd file, don’t know how fast yet).

I think that someone using Eagle for design might do best with OSH Park, since learning how to use Eagle to produce the Gerber files needed by the other fab houses was a bit of a pain.

iteadstudio does provide Eagle design-rule check and CAM files, but they seem to be pretty much the same as the default Eagle ones—I’ve not gone through them carefully to see if there are any subtle changes.  They do have the most awkward way for delivering the Gerber files, requiring that they be e-mailed as a zip file, rather than accepting upload over the web like the other services.

Worksheet for designing layout. This PNG image is a little small, but Eagle can make a PDF output that is big enough for easy layout.

I improved the top documentation layer, so that the combination of the documentation and top silkscreen make a useful worksheet for students to design their layout on. If printed at 3.5× real size, the worksheet fits nicely on a page and leaves room in each resistor for adding a value.

I tried a couple of slightly different layouts for the EKG blinky circuit with optional Arduino output, and it seems to be fairly easy to lay out.  The hard parts for students will be coming up with the circuit design and soldering it up without shorting adjacent pads.

I’ll have to write up some documentation for the board, to explain the options for power (from an Arduino, from a DC power barrel, or from screw terminals; whether an RC filter is added to the power input or not; where the power and Gnd connections are available on the board, …).  I’ll probably also have to write some assembly/soldering instructions for the parts of the board that aren’t student customized.

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2012 July 16

DorkbotPDX, a cheaper hobbyist PCB place

Filed under: Printed Circuit Boards — gasstationwithoutpumps @ 21:37
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My son pointed me today to another PCB board consolidator (like BatchPCB) that provides small quantities of PCBs by the square inch for hobbyists: PCB Order | DorkbotPDX.

The DorkbotPDX pricing is even simpler than BatchPCB’s as there is no setup charge or shipping (in the US), just $5/square inch for 3 copies ($10/square inch for 4-layer boards).  They also sometimes do “medium-run” orders of 150 square inches or more for $1/square inch (about 40% cheaper).

They provide Eagle DRC files and accept either standard Gerber files or Eagle .brd files.  They claim the boards are made in the US also, and I don’t believe that BatchPCB’s are (though 4PCB‘s are).

I’ve not tried DorkbotPDX, but Chris Laplante, on whose blog my son found the reference, seems to like the service and pointed to several other bloggers who did also.  I’ll probably try them for my next board design, once I decide what that is (maybe something for the students to solder in the circuits course this winter).

2012 March 1

Sensor board for underwater ROV

Since I had bought the robotics club an I2C accelerometer and magnetometer, I decided to make a new PC board for them to mount the accelerometer, the magnetometer, and the pressure gauge on the same board.  I don’t have the SMD soldering skills to solder all the chips onto one board, and I already had breakout boards for the accelerometer and magnetometer from Sparkfun, so I decided just to put connectors for those breakout boards onto the back of the pressure sensor board.  (The back, because the pressure sensor on the front has to be stuck through a hole in the dry box and glued in place.

The new boards are tiny (1.05″ × 1.425″), so I decided to try BatchPCB (which has pricing by the square inch) rather than 4pcb.com (which has fixed pricing per board, up to a fairly large size).  The price from BatchPCB was $10 per order plus $2.50/square inch plus $0.90 for shipping, so ordering 3 copies of the board (though I only needed one), cost me $22.12, substantially less than a single board from 4pcb.com, which is $33 plus $17.30 shipping and handling per board (plus an extra $50 if your board has multiple boards on it).  The 4pcb price is lower if your board is bigger than about 15.76 square inches, so even my HexMotor boards (at 12.44 square inches) would be cheaper from BatchPCB.  If you get multiple boards from 4pcb.com on a single panel and cut them apart yourself, the breakeven point is about 35.76 square inches for a single design (so three HexMotor boards from a single 4pcb.com panel is cheaper than from BatchPCB). For multiple designs on a single panel, the 4pcb.com deal is better: for 3 different designs, a total of 27.04 square inches would make 4pcb.com the cheaper way to go.

If you want a copy of the board, you can order it from BatchPCB, or pick up the Eagle files from my web site and order copies from elsewhere.  I’ve put the HexMotor Eagle files on line also, but not put them on the BatchPCB site.  I should probably upload them there sometime.

Bottom line: BatchPCB is better for small numbers of tiny boards, but 4pcb.com is better for larger boards and multiple designs.

The BatchPCB orders came back quite quickly (12 days from order to delivery by mail), though I had been worried because their design-rule check, which they say takes minutes had taken about 8 hours.  The problem was that each check takes a few minutes, but they had hundreds in the queue over the weekend, and it took a full day to clear the queue.

I had less trouble soldering the pressure gauge this time (this was my second attempt at soldering surface mount devices).  You can see in the pictures above that the results are much cleaner than in my first attempt.

The robotics club has tested the pressure sensor on the new board (using their own code on the Arduino) and it seems to work ok,  have drilled the hole in the dry box for the port, and glued the sensor board in place using superglue.  It seems to be waterproof (at least down to 1 foot—we’ve not tested in deep water yet).

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