[lug] Ethernet Enabled PLC/Linux Microcontroller

Nate Duehr nate at natetech.com
Sat Aug 1 02:08:21 MDT 2009

p.s. Disclaimer:  If you're talking about reading anything that  
outputs 120 VAC or uses it as an input, and you're building it  
yourself without knowing more than the basics about electronics, such  
devices have the ability to KILL someone, and you should find a  
professional EE or at the VERY least an EE student before proceeding  
-- that or embark on at least a few months of study and find said  
people you can ask questions of.

I always forget to mention that... when people show up on mailing  
lists asking about "simple" switches or circuits to measure "on and  

Measuring a low DC voltage, you can build things to your heart's  
content until you get what you want it to do, correct.  Messing around  
with 120 VAC from a wall socket, you can come real close to killing  
yourself, pretty easily.  Start to get into 220 VAC for motor and  
other controls, and you'd better be a professional or hire one.

Additionally, if other people are regularly going to come into contact  
with these devices, you need to take care to make them as  
intrinsically safe as you can.  The electronics become a bit more  
convoluted from a beginner's point of view, but are all well-known and  
well-documented techniques.  I've seen some really scary shit (and  
yes, I'm not censoring that word, because that is what they were)  
built by hobbyists become things that companies used for long periods  
of time until a shocked (hopefully not literally) EE walks by and  
reaches for the master power kill switch and tells no one to go  
ANYWHERE near it until it's been re-designed to be safe.

Just be careful and think... and you'll be fine.  But nowadays, I've  
run into SO many folks who don't... I figured I'd better follow up  
with the disclaimer.

Common sense, isn't so common anymore.  I've seen fully licensed  
Amateur Radio operators ask if they can transmit RF without an  
antenna.  Seriously.  Unbelievable.  "But I bought this radio!  Can't  
I use it somehow before I buy feedline and an antenna?!  Isn't it like  
a cell phone?"  When you explain that there's an antenna INSIDE the  
cell phone, they're usually quite surprised.  Then they want to know  
why a VHF radio doesn't include the same feature.  And these are folks  
tested to KNOW that a quarter-wavelength at 144 MHz is roughly 19"  
long, and the case of the radio is only 6" deep.  Seriously.  Really.   
It's that bad.

Okay so... all the above said, there's little that's truly "new" in  
Electronics.  A little study time at any well-stocked public library,  
or inter-library loan, and you can learn what's safe and what's not.   
Most folks today haven't had the "fun" of working on tube-based  
electronics where voltages were into the thousands of volts, even if  
low-current, and everyone passed along safety knowledge as part of the  
culture.  Lots of help "out there" though -- plenty of brilliant EE's  
out of work who'd enjoy teaching a bit...

(By the way, my learning of electronics is from almost two decades in  
Amateur Radio -- I do NOT consider myself and expert and EVERY design  
I create I "run by" some trusted friends with formal training and  
knowledge of electronics.  Over the years I've learned enough to "get  
myself by", but I can't offer much in the way of training... or at  
least I think I can't still... don't really want to fry someone to  
find out... heh heh.)


On Aug 1, 2009, at 1:52 AM, Nate Duehr wrote:

> On Jul 31, 2009, at 7:27 AM, Swavek Skret wrote:
>> I need to control a hardware unit that does not have a standard
>> interface (ethernet, RS-232, etc) but simply provides a binary
>> voltage to flag that it functions correctly or not. I am considering
>> using an ethernet enabled microcontroller with Linux OS ideally.
>> If anyone has related experience and would like to share or
>> recommend a particular hardware I would appreciate it.
>> Thanks,
>> Swavek
> Full blown embedded Linux is probably overkill.  Any microcontroller
> can be made to do simple voltage monitoring and spit out serial
> information to a connected PC, phone line via a modem, whatever...
> Microchip and Atmel are the most popular for "hobby" projects, and
> there's thousands of designs available for commonly used ones on the
> Net.
> Both have free Assembly and C programming language support, but you do
> have to learn a bit about how to program "at the hardware level", so
> to speak.  A month or two of study will yield a lifetime of cheap,
> virtually bullet-proof hardware control solutions.
> There are also development kits for both major chipsets that make
> programming them a breeze in languages like BASIC and similar.  Even
> some GNU/GCC projects for both, if you're willing to put up with the
> usual crappy open-source documentation and brain-damage.
> The forums at Microchip.com or avrfreaks.net are good starting points,
> if you already know basic electronics including Ohm's law and how to
> solder, both of which are pretty basic -- and there's plenty of "DIY"
> ways to learn both on the Net.
> There's no need to use an entire OS like Linux, when a few lines of C
> code compiled into machine language by a free compiler and dumped into
> a chip will do the job.  The logic involved is basically 1 for on, 0
> for off... something we all know, and the electronics involved in
> seeing if a device is "on or off" is not much more difficult than the
> circuit for a typical table lamp.
> Going all the way to Linux on a chip will be more "fun", but WAY more
> expensive both in the short and long term, unless you're only doing
> ONE of these.  I do suggest hunting the web for microcontroller
> circuits to do such a thing.
> Other "medium difficulty" options might be to use something like a
> PicAXE or Arudino where a hardware engineer has already developed the
> board, the layout, defined the chip(s) needed and provided a
> rudimentary real-time "OS" FAR better suited than Linux to handling
> the little (inexpensive) chipsets.
> http://www.rev-ed.co.uk/picaxe/picaxe.htm
> http://www.sparkfun.com/commerce/categories.php?c=103
> Etc etc... the possibilities are endless.  But Linux on the chip
> certainly sounds like SERIOUS overkill when a single chip can do the
> job...
> --
> Nate Duehr
> nate at natetech.com
> http://facebook.com/denverpilot
> http://twitter.com/denverpilot
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Nate Duehr
nate at natetech.com


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