On Sun, 03 Jan 2010 13:17:52 -0500, Karl Townsend wrote:

How much I/O do you need? Motor up/down takes 2, limit switches take
2, and 2 for thermostat or serial temperature sensor. Then if you have
an operator interface you can get a serial LCD and 4 inputs for select
up/down, 1 for Enter, 1 for Back (or Esc). Parallax (among others)
also sells a ULN2803A chip that can drive 8 external relays up to 50VDC
1/2A, complete with built in diodes for inductive loads.

I also installed a 1/rev contact to detect/count motor movement on each
motor. There are four of these motors total for a total of 20 IO here .
I also sense outside temp in addition to two inside temps and want to
leave room for a wind velocity sensor (future) - four more. Then there
will be two soil moisuture sensors and a water pump - three more. I'll
need a couple or four for an LCD read panel. I'd use any extras for a
small operator interface to do things like open or close everything.
From experience, I know to leave room for more IO, always seem to need
it.

I'm not aware of this ULN2903A chip, I'll look into it




I bought a PIC32 starter kit from Digi-Key (Microchip has them for same
price), has a lot of I/O, 512K program memory, 32K ram, and a
optimizing C compiler with a 64K limit, all for $49.


Basic stamp is easy to program and well documented. I'm not at all
unconfortable with it. I feel just the opposite about working with C or
assembler. Plus, I'd be on my own for driving an Opto 22 board.

Karl

If you have the $$ to be extravagant with processors, use one per motor
and have them all talk on an RS-485 bus (assuming you can easily get set
up with RS-485 hardware).

Or don't -- it's how I'd design the system if I were going to sell it,
but half-duplex networking can be a bitch to get working the first time.

If I were anywhere close I'd offer to drop by and help out, but IIRC
you're in Wisconsin or Michigan or some other place influenced by large
amounts of fresh water and generally flat land.

--
www.wescottdesign.com