Category Archives: Where to get stuff

Notes about OSH Stencils

A few notes from listening to Brent of OSH Stencils: on The Amp Hour:

  • Planning to offer 1.6mm jigs to eliminate the error vis a visa common PCB thickness (hurray)
  • For those that didn’t know:
    • Offering high quality Kester solder paste (in checkout cart, as with the jigs)
    • Offering stainless steel as well as kapton (stainless steel are common 4mil thickness).
  • 5x5mil smallest feature size possible with kapton stencils. Minimum feature size for steel stencils is ONE MIL!
  • They see 70/30% split between 3mil and 5mil thickness of kapton. The 3mil is best for ordinary work, while the 5mil is appropriate for high power devices with large thermal pads, etc. where a lot of solder is needed. I can vouch for the fact that 5mil is a total disaster for fine pitch parts like a QFN package. Kapton in 4mil is unobtanium, which is why they don’t offer that thickness.
  • Name is OSH Stencils because Brent was originally going to collaborate with Laen of OSH Park, but a change in Laen’s circumstances caused them to remain separate entities and Laen was totally OK with the similarity of names.
  • Can reduce kapton stencil curl by “counter-rolling” the material and this will give temporary relief.
  • Started with hobby laser: That “exploded” the first week after starting business. Switched to Epilog 24 (roughly $30k)
  • Stainless steel stencils made with approx $300k LPKF fiber-based “flagship model” laser cutter
  • Have maintained 24 hour turn time from the beginning. They went with high end laser to be able to make *the best* stainless steel stencils. They are competitive by avoiding framed stencils and using a proprietary material loading system into the LPKF. Basic cost of framed stencil material is 5X the cost of raw. Could use “pneumatic frame” to simulate real frame, but that didn’t allow sheet sizes that were large enough to be cost effective.
  • Brent said their charter as they began making steel stencils was to offer the top quality at reasonable prices. He invited people to compare their metal stencils to Chinese stencils under a microscope as they consider potential “cost savings” by going off shore. (OSH Stencils is in the Salt Lake City area of Utah). The metal makeup is different. Most offshore vendors use “inexpensive stainless steel”. Looking at the apertures (the cutouts) you often see grooves to do with the kerf (diameter of laser beam). With low quality metals the edges of apertures have grooves making them look like washboards.  Also see minor warping and ripples in the Chinese metal.
  • Look for “exciting announcements” to do with the stencil process to be offered by OSH Stencils in coming months.
  • OSH Stencils makes stencils for customers that have nothing to do with electronics. Art projects, special “plates” for mechanical components, etc. They’re open to queries about whether your creative application can be handled by their equipment.

Battery Testing Resources


This is a constant current dummy load made from one of  Shane Trent’s  PCBs like those given away at a recent TriEmbed meeting. As mentioned on the  email list,  this PCB is a “fixed” version of a design from a “Sleepy Robot” blog of a guy named Wittenberg, which is itself a derivation of an original design by Dave Jones of EEVblog.  Wittenberg had made available  gerbers for his design (in early 2012)  that were unfortunately defective, and he didn’t allow for two way communication, forcing Shane to go to great lengths to correct the gerbers and get a run of PCBs fabricated. Shane’s blog article covers all this in depth and has a link to the corrected gerber files in zip format.

Fast forwarding to the present, here’s a recent tutorial by Dave going into depth about battery measurements.  Viewers will just have to put up with the axe-grinding, horse-beating treatment of a “battery life extender” Kickstarter that pushed Dave’s buttons. Apart from this, it’s an excellent treatment and a fantastic “essential subset” spreadsheet tutorial for folks that just want a hint about how to do cool things like the graph-making done in this video.

I assembled and tested a second of the PCBs recently.  It will sink up to one amp at up to around the 60 volt limit of the FET used (MTP3055VL) HOWEVER, unless you like to see magic smoke the 18 watt thermal limit of the FET/heatsink assembly has to be honored. So at a full ampere the voltage limit is around 18, and at that load be sure to avoid touching the transistor! At one ampere the shunt resistor will  be operating at it’s rated dissipation limit and will also be very hot. To summarize, this load has to be kept at an amp or less and at 18 watts of power dissipation or less. (Note: the shunt resistor is temporarily 5% tolerance due to an ordering blunder. That will be fixed.)

I’ve decided to make it available for borrowing by TriEmbed meeting attendees who can guarantee it’s return by them or their designee at the following month’s meeting. The transistor is not expensive and it won’t be any big disaster (just embarrassing)  if it’s accidentally destroyed, but blowing the traces off the PCB will be frowned upon (joke). So this (and perhaps some of the TriEmbed contact cards Paul made, hallway signs, etc) could be part of a shared resource that could expand over time.

The “UI” is currently two voltmeter test points, with the unit showing the load current as a one to one mapping from amperes to volts. A digital display with simple USB serial (current and “external voltage” aka battery voltage)  logging output and some temperature compensation/auto-calibration is planned, but it would be straight forward to tie the test points  to something like an Arduino analog pin or two.

Remotely controlling and/or making  the current limit programmable would be a bit harder, but a properly coordinated hack to provide an alternative control mechanism would be OK with me and make for a fun project for somebody.

Here are all the design-related links in one place:

Here are some BOM changes:

  • The load connection is just four bare plated through holes intended to get some wire loops. These Newark  12H8386  screw terminals solder in and work well.
  • As mentioned, a momentarily loose screw resulted in this Mouser part 660-MF1/4DCT52R10R0F  five watt resistor being substituted for the default 10 1/4 watt resistors. The bad news is this resistor has a 350ppm/C coefficient as well as being only 5% tolerance.  A better choice than either might be a pair of three watt, two ohm 1% Vishay resistors such as Mouser 71-RS02B2R000FE12. These have 50ppm/C coefficient so there would be about another 1/2% error at the point you could boil water on them.

Cary/Crossroads Radioshack just another crummy example now

In an apparent about face the Cary Crossroads Radioshack is now missing almost all the stuff that had kept it a huge exception to the rule up to this point. The repair shop is gone, the wall of Littlebits is gone, everything to do with soldering except a few temp controlled irons is gone, all the plastic boxes, perf boards, and other stuff to do with building things is gone, and the wad of Arduino-related stuff is gone except for a few lonely Arduino boards and a shield or two on the near end of the back. Much quizzing of the staff suggests their supply chain is now a hypothetical construct. Quoting Porky Pig, “That’s all, folks!”


Alternative short distance rangefinder: measures light transit time, not reflection angle

Here’s a device available on a pair of Sparkfun boards that Rod pointed us and Triangle Amateur Robotics to:

Update from Rod: A sensor + ARM M4 eval board for $20 from the chip maker:

At least one Radio Shack is upgrading itself with “Fix It Here!”


The Radioshack at Cary/Crossroads has set up a cellphone and tablet repair station in the corner of their store. It’s called “Fix It Here!”.  My eyes nearly popped out at the site of state of the art SMT rework tools right there, with a very sharp guy named Matt who knows how to use them. He’s there and ready to fix your broken glass, failed connectors, etc. The phone number is 919 851 0332.

And this particular Radioshack now has a wall full of Little Bits as well as a scrupulously maintained inventory of the other gadgets and gizmos embedded hobby enthusiasts need. I was encouraged that maybe they will adapt and pull out of their spiral.

(I have no connection to Radioshack, but I’ve been buying stuff mostly here since this store was opened. )

Why Indeed?

At a recent meeting Scott Hall told us about a $4 Cypress ARM system that his work group intends to use with a port of  an ultra-lightweight Linux called uClinux. Coincidentally there was a recent blurb on Slashdot about uClinux on a Motorola 68k and here is the article, the related Youtube page, and one of the subsequent (in my opinion, classic) /. comments about this subject. The Slashdot article is here.


The Amp Hour Redeems Itself Again

The Amp Hour has been terrible for the past couple of weeks. I’ve seriously wished for a refund of the few minutes of attention I paid them while listening and doing other things. ‘Nuff said.

However this week Chris and Dave have added one more data point to the curve suggesting that when they get somebody else into the conversation their podcast shines. This week’s episode is an interview of Eric Van Wyk, an engineer with amazing credentials. The specific Amp Hour web page for this episode contains many interesting details suggesting Van Wyk and company’s “Mooshimeter” will create immediate salivation if you’re part of the TriEmbed crowd.