AVR-Chat

I'm looking at developing a signaling application for 1.5" scale ride-on
railroads (a hobby of mine).  It involves a long signal bus (up to a couple of
thousand feet) based on RS-485.  I'll also need to run power to operate
ultra-bright LEDs as well as intermittently drive a couple of low voltage, low
power electric motors.  The cables will typically be buried along the track.

My design will involve and AVR-based module at locations along the track to
detect trains present on a section of track, signal other modules about track
occupancy, drive local signal (light) heads and possibly low voltage motors to
move throw track switches.

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).

My question at the moment is what is the best way to power the devices along the
network.  I don't expect a large current draw at each location (module power
plus several ultra-bright LEDs on at once) and at locations that have small
intermittent motors I could place a small rechargeable battery to take the surge
and trickle charge it from the power bus.  Some thoughts:

1) Something like 14-3 house wire carrying 12vdc (since 12vdc is common for
motors, lights, etc.) but I was worried about voltage drop over 1,000's of feet.
Obviously there would be voltage regulators at each module for local logic
power.

2) Same as #1 but use a higher DC voltage (48?) to reduce the current and hence
reduce the voltage drop.  How does one get from 48vdc to 3-5 vdc without huge
losses.  Cost is a factor here.  I realize that I could use a PWM regulator but
are there inexpensive chips for this?

3) Something like 48 VAC.  Easier to step down to a reasonable level but I fear
having a transformer, etc. at each module would increase costs too much.

I'm looking to keeping module costs as low as possible and still have a reliable
system.

Comments?  Other ideas?

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