Hi all,

Many thanks for replies so far. Given the questions floating around
in response to the original one I thought it best to have another stab
at re-asking it with a bit more detail ;-)

Quick Summary;
    Trying to create a 'black box' with 240v AC and Serial/I2C as inputs
    and 0-12v (variable) DC 150w(ish) independantly controlled channels
    as outputs for use with low voltage lighting.

Target Lighting;
    12v DC low voltage dichroic halogen lamps as purchased from many a
       highstreet retailer.
      (http://www.arklighting.com/4000series.cfm is the sort
      or thing I'm talking about.  This isn't who I'm getting the kit
      from they just have a handy web page. They'll wonder why they're
      getting so many hits suddenly ;-)
    Each unit could be up to 50w.
    Arranged in a number of banks of 2 or 3 units.
    Inital spec. calls for 4 banks, although I can see the need for more.

Voltages.
    I'm in the UK so I'm starting with 240v AC.
    I have no preference as to wether the circuit delivers 0-12v DC or
      0-240v AC which I can then drop through a dimmable transformer.

Control.
    This will be controlled by an as yet undefined Unix box. My plan
      is to get a PIC to do all the hard work and then control the PIC
      by I2C/Serial/etc from the unix box.  I feel this should help the
      scalability of the whole thing.

    Lighting banks should be able to be switched on/off as well as
      dimmed to specific levels.

    All user interaction will come via the unix box so, AFAIK, should be
      ignored here.


Misc.
    Electrical noise is possible issue as this is being used along
      side audio/visual presentation kit.

    If this system can be designed to directly control 12v loads it will
      provide a better example for others to follow without worrying
      about differences between 110/240v ?

      http://www.radioshack.com/ do a part # 990-0039 which is a
      DC Controlled AC Dimmer Kit which looks the part but not
      not available in UK and is 110v ;-( Does anyone have the
      circuit for this ? The manual apparently has an example
      for use with halogen lamps.


Questions.
    (Questions that I have seem pass in EMail that I cannot answer
      or comment on)

    Which is more efficient, safer, etc; Controlling 240v AC then using
      a transformer to step down to 12v DC or stepping down the voltage
      and controlling 12v DC.

    Should the lamps ever be switched totally off or should they have
      a low 'pre-heat'(?) voltage applied when in use but 'off'

    Halogen lamps are not ment to be dimmable, but it appears to work ;-)


Ascii Art Attempt ....
                              Black Box
                              +---------------+
    Mains In [240v AC] -----> | Kit to Switch,|>----[Lamp bank 1]
                              | dim,etc  and  |
          +-[Serial/I2C]----> | output as     |>----[Lamp bank 2]
          |                   | 12v DC        |
          |                   |               |>----[Lamp bank 3]
       Computer               +---------------+
          |
          |                   Black Box
          |                   +---------------+
          +-----------------> | Kit to Switch,|>----[Lamp bank 1b]
                              | dim,etc  and  |
     Mains In [240v AC]-----> | output as     |>----[Lamp bank 2b]
                              | 12v DC        |
                              |               |>----[Lamp bank 3b]
                              +---------------+



Further comments / suggestions / etc :-)


Mark.
--
"What contemptible scoundrel stole the cork from my lunch ?"
     - W. C. Fields