AVR-Chat

(replies to multiple messages interspersed below)

> From: Javier Fiasche
> 
> Why dont you try 12 VAC (or less) and a switching regulator 
> at each location  (try using lm2575)

Thanks Javier, I just checked the data sheet on the LM2575 series.  Looks like
just the thing I need.  

(BTW: I assume you mean 12 VDC?  Otherwise I'd have to have a rectifier, etc. at
each location)

One question this brings up:  Do you think the ripple from the output would
greatly effect the A/D converters on the AVR?  The LM2575 data sheet shows an
additional ripple filter if required.  I am early in this design and I don't yet
know what my analog signal (rail occupancy detection) limits will be.  I hope
that they are in the range of 0 to VCC (as opposed to milivolts, etc.) but I
won't know until I do some field testing.  Just wondering if the ripple would
"grossly" effect the A/Ds making them unusable.  I don't think my needs will
require an external A/D reference.

Also, how sensitive is the AVR itself (ATMega16 for example) to ripple on VCC?

I realize I have a lot of characteristics to measure/calculate.  I'm just trying
to establish a basic direction to go, throwing out alternative designs which are
unworkable at a gross level.


> From: Zack Widup
> 
> Just a thought ... you can maybe use an implementation of 
> X-10 technology and power your devices at the same time. But 
> you'd have to use AC to power your devices.

Hi Zack,

I looked at X-10 but rejected it for several reasons.  One is that these ride-on
railroad tracks are many times located in public parks.  I doubt if they would
take kindly to running 120 VAC around and the conduit required would add to the
costs fast - and the grounds people have been known to sever cables when digging
in shrubs, etc. :-)

Another reason is that (to my knowledge) X-10 uses a "best effort" delivery
method (I think the message is sent three times) and there is no high level
acknowledgement.  This ACK could be added on top but my message traffic will
require a larger bandwidth than X-10 can support.


> From: James Hatley
> 
> A word of caution ... any voltage used where people might 
> contact across it must be properly installed and shielded. 
> Any voltage higher than about 30volts is considered 
> potentially lethal.

Hi James, I knew I didn't want to run 120 VAC but I thought 48 VAC was
considered "low voltage" (like in HVAC controls, etc.).  I think 12 or 24 VDC
will work fine with the switching regulators mentioned above.

> ....
> Somewhere here you must define a node, it's power 
> requirements, power cycle, does it just need 5 volts, or does 
> it require other voltages, and so on.

Understood, as I mentioned above, I'm trying to throw out some alternatives at a
gross level - like 120 VAC in the power area and, in the data bus area, methods
that don't have good noise immunity, have low bandwidth, etc. - before I start
field testing.

> .... (after determining power requirements) ....
> Then try 12volts, 18volts, 30volts, AC or DC or if you are 
> going to properly shield in conduit etc, 110vAC, solar power, 
> or whatever and so on. Study is required here before proceeding.

Something requiring conduit would probably have to be avoided (live steam clubs
don't usually have large budgets).  I do see a possibility of using solar power
to charge batteries at remote locations far from "an outlet".  Someday I can
foresee signal bus segments being "bridged" using RF links.

Thanks for the input.  I certainly agree that I need to design the module, field
test it, determine how many would be on a typical run, over what distance and
drawing what current, etc.

 
> From: Robert Adsett
> 
> ....
> Consider 24V, It's a common industrial DC bus voltage so 
> there is a fair amount of support for it 

I think 12 VDC is my current preference if the current draw requirements will
support it since then 12 volt items like light bulbs, motors, etc. can be
powered directly which would simplify things.  My next preference would be 24
VDC since, being a multiple of 12 VDC power can be supplied by common batteries
with solar cell chargers, trickle chargers, etc.

> (also 24VAC).

I'd like to stay away from AC if I can to minimize parts requirements.

> For local power storage you could also consider "supercaps".  
> Maxwell .... is making them in D cell 
> packages ....  Recharge is easy but cost and bulk 
> might be an issue.

Bulk is not an issue but cost would be.

> What about temperature?  Do you need to consider freezing?

Some clubs would have these units permanently installed so they would have to
stand freezing temps.  Others would collect them after a run day due to the
possibility of theft or vandalism.
 

> From: John Samperi
> ....
> At 05:12 AM 2/02/2005, you wrote:
> 
> >As I see it, I'll have to bury two cables along the track, one for 
> >power, and one for the RS-485 signal bus (either 2-wire or 4-wire with 
> >signal ground to control common-mode voltage differences).
>  
> Can you use the tracks as a ground wire ...just like the real thing?
> You will need to make sure there is continuity, but with the 
> size of the tracks you should get low voltage drop on the 
> common, so your power wires can just be used for positive/active.

Hi John, One of the problems with our scale rail is that it is an aluminum
extrusion in 10 or 12 foot lengths - good conductor, but typically the rails are
joined with small steel plates and steel (or SS) screws.  This leads to high
resistance joints between the rails after being in the weather for awhile.  When
using the rail as a ground these high resistance joints would play havoc with
ground potentials.  

Clubs that use relay based signaling typically have to "bond" all the joints
(attach the rails together with a wire jumper across the joints as in full
size).  As relates to detecting occupancy of a track, we also have the problem
that if the (wood) ties are wet, the conductivity of the track circuit varies
over time causing false signals.  I'm hoping to be able to get around both of
these problems by using the A/D in the AVRs to monitor the rail and detect
changes in conductivity as opposed to an absolute level of conductivity, which
is what the relay systems do (i.e.: the relays are set to pull in at a given
conductivity/current flow).

Thanks to all for your suggestions.  For those that have no idea what I'm
talking about when I say "live steam" tracks, here are some photos of my
locomotive:

http://www.whitetrout.net/Chuck/844 in the lower right you can see one of the
rail joints.  The lady behind me owns half of the locomotive - i.e.: she's my
wife :-)

http://www.whitetrout.net/Chuck/844/photos

I'm sure I'll have more specific AVR questions down the road (track?) as I
progress.  Even though I'm new to AVRs I enjoy the list and am learning a lot
just "lurking".  Hopefully some day I'll be knowledgeable enough to contribute
:-)

Cheers,

Chuck Hackett
"Good judgment comes from experience, experience comes from bad judgment"
7.5" gauge Union Pacific Northern (4-8-4) 844
http://www.whitetrout.net/Chuck