Arduino Multi-Channel RTD Shield with RS232 and RS485

Our first shield design rolled out last week. You can find the Arduino Multi-Channel RTD Shield with RS232 and RS485 in our store.  We also strive to have the best documentation possible.  The docs for the RTD + RS232 + RS485 shield are available here.

We left it running for days to make sure everything was stable.  Everything seems to be running great.  We set it up to read the resistance values of precision resistors that were connected in a four-wire configuration.  It took about 40 milliseconds to cycle through all of the channels and get a new reading back to the Arduino.  On the shield it takes multiple readings and calculates the median value, which it saves and reports to the Arduino when queried.  This provides some basic filtering and eliminates any outliers that may have been encountered.

The shield is easy to use and has some nice features.  We tried to come up with examples that showed how all of the routines work.  If you have any questions, let us know!

Lessons Learned

  • Don’t try to access the EEPROM while in the I2C slave interrupt.  You can’t read the data from the EEPROM fast enough to properly service the I2C slave response to the master even with clock stretching enabled.

FCC Regulations and Open Source Hardware

We’ve been doing some research into how we can sell our designs without needing FCC testing.  There are a number of exemptions called out in CFR47 15.103.

One of the more interesting exemptions is the one related to “digital devices in which both the highest frequency generated and the highest frequency used are less than 1.705 MHz.”  We’ve read some discussion forums where some interpret this to include harmonics generated by the oscillator, which would make this exemption quite narrow in application.  However, there is a FAQ page at that seems to interpret this to mean “digital devices oscillating below 1.705 MHz.  See here (cached copy)  Take note of the answers to the following questions:

  • Can’t I be exempt from FCC regulations?
  • What kind of digital consumer products get exempted?

I’ve sent them a message asking if they believe the harmonics are excluded under this exemption.  I’ll update the post with their reply.

Another good resource on the interpretation of Part 15 is in the FCC’s OET Bulletin 62 (cached copy).

Update February 25, 2013

The response from Bureau Veritas is probably what you would expect: cautious with a slant for testing:

Digital devices using oscillator frequencies of 1 MHz (or even lower frequencies) are not necessarily exempt from FCC requirements. As you indicated, the potential for harmonics extending beyond 1.705 MHz exists, and these harmonics could conceivably emit at power levels that would require compliance testing.

Obviously, specific product design will ultimately determine the level of concern, but it is not a safe assumption that the oscillator frequency alone will define FCC exemption.

Reflow Oven

We got to wondering why no one sells a ready to use reflow toaster oven.  We’ve seen other sites where you can order a controller for your oven, but not a completely ready to use oven straight out of the box.  It seems like this is something that could be done without blatant and obvious legal repercussions.

For example, you can re-sell a product that you modify.  There’s a great discussion over on Avvo.  Maybe it’s hard to etch over the manufacturer’s name on some models.  Also, there doesn’t seem to be an issue with FCC compliance as this would fall under the exclusion for devices used exclusively in an appliance.

Obviously there would be no warranty available through the original manufacturer of the oven and any UL or other listing they obtained would no longer apply, but it would be a great way for makers to get up and started quickly with solder paste reflow.

Let us know what you think.

Hello world!

This is our first posting. We can’t wait to look back at it a few years from now and reflect…

We’re really excited about what we’re starting here.  We hope we can help enable people do creative things while sharing knowledge and doing what we love to do: make playgrounds for electrons.