Let me re-state your request so that you know I understand what you want -

You want to control some lamp circuits
        (how many sets?)  (how many watts per set?)
The control desired involves switching sets ON/OFF and
        varying the brightness of individual sets while ON
        (arbitrary levels, constant level; ambient + lamps, cyclic?)
Your lamps are 12V AC with each set having its own 240 ---> 12V transformer

You haven't mensioned the control system - rotary knobs, slider tracks,
light sensors, bcd encoders, dil switches, ?

Ok,
If you stick with AC you should detect the 'zero crossing' point (100 or 120
per second according to 50 or 60Hz supply).
Best efficiency (and safety) is to switch the low voltage.
Best lamp life (lowest in-rush shock) is to switch as near as possible to
the zero crossing.
Best brightness control is to switch on at the zero crossing of each HALF
cycle and switch off at some point into the Half cycle.

Problems are -
100Hz (or 120) hum.
Back emf into electronics/transformer secondary when switching off mid way
through HALF cycles.
Non linear brightness (peak just after mid point of HALF cycle)

If you convert to DC then forget about being locked to the AC.
Now your best choice is Pulse Width Modulation at a frequency of your
choice.
Frequency should be high enough to eliminate flicker, >256Hz is good.
If sound equipment or cables are nearby frequency must be >audio =>30KHz .
The pulse width should be divided into >100 divisions (256 is good).
When actively dimming and brightening NEVER reduce pulse width below about
15%
        (this keeps the lamps warm and greatly reduces in-rush current)
        Never go from zero (or 15%) to 100%, always rise slowly at the
bottom end
                [watch some traffic lights to see this in practice]

Now tell us exactly what you want from the above and we'll point you to the
documents you need to steal (ah, copy) the schematics and code from.

Bye.

> -----Original Message-----
> From: Mark Boyce [SMTP:mark@INTERHOUSE.REDBUS.CO.UK]
> Sent: Tuesday, 7 December 1999 22:49
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [OT]? Lighting Controllers
>
> Hi all,
>
> Hope this isn't too off topic for people :-)
>
> I need to be able to switch/dim some lighting from a pic
> controlled system.
>
> The actual target lighting will be pairs of 12v halogen spots
> such as you find in most low-voltage desk lamps or alike.
>
> The circuit I'm after should produce either 0<->12v or
> 0 <-> 240v on the output and take something I can connect
> to a pic on the other side.
>
> Software is my game with a little bit of 'plug the black boxes
> together' digital electronics so I'm out of my depth here :-)
>
> Does anyone have any pointers/urls/circuits/tips/etc that might
> help ?
>
> Cheers,
>
> Mark
>
> --
> "What contemptible scoundrel stole the cork from my lunch ?"
>      - W. C. Fields