Dale Botkin wrote...

>My question relates to how to switch the 12VAC to the lights on and off.
>I'm thinking a sensitive-gate (logic) TRIAC...

TRIACs.  Ugh.  Voltage drop.  Lost power.  Unclean.

>Ebmarrassingly, I have never used a TRIAC before...

Be thankful, not embarrassed.

>Is there a better way to do this?

Yup.

No one ever said you can't use MOSFETs on AC; you just have to be a
little clever, is all.  Usually, that means you end up having to use two
of them in series-- one for each half-cycle-- because of the intrinsic
body diode that shunts the drain and source.

The circuit below uses no voltage-dropping, power-wasting TRIAC, no
likewise bridge rectifier, no relay^H^H^H^H^H "electromechanical logic",
and allows you to control your lamps either ON/OFF with a logic level
from a PIC (might want to boost that up to a little higher than 5V,
unless you use logic level FETs), or control lamp brightness with the
PIC's PWM.

           0--------------------------0

        FROM 12V                  TO LAMP
        XFORMER                    STRING

           0------+           +-------0
                  |           |
                  |           |
                  |           |
                  |           |
                  D           D
              Q1    G---+---G    Q2
             Nch  S     |     S  Nch
                  |     |     |
 GATE DRV  0------------+     |
                  |           |
                  |           |
 PIC GND   0------+-----------+

Voila...

Dave

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