I built a three channel, 4 control color organ for our band to use during
shows. It works well, but I have a question: I want another bank of lights
to operate *opposite* the first bank.

The schematic is similar to this:

http://tinyurl.com/cnyr6d

with the exception of three level controls to control the level of signal
to each channel. The gate triggering is similar.

I would think some kind of op amp could be used to invert the signal and
trigger the second bank of lights, so the second bank's lights are on when
the first bank's are off, but I don't have the expertise to create the
circuit. Adding another bank of lights is no big deal, but getting it to
trigger opposite is the tough part.

Note that I modified the circuit to make full use of the SCRs supplied.
The circuit was rated at 150W per channel; I offloaded the SCRs from the
circuit board and isolated the hot side of the AC input to the cathode of
the SCRs, the anode connects from the tab of the SCR to the hot side of
the outlets to the lights, and the return side goes directly back to the
mains. The only part connected to the circuit board is the gate for the
trigger. I have run ~220 watts on it for over an hour, and the SCRs don't
even get warm. They are rated RMS on-state=4 A, with an average forward
current of 2.55 for 306W. I figure in this configuration it should be good
for 225W, or three 75W lamps.

Any ideas how to invert the trigger signal are appreciated. Note I am
trying to keep this as cheap as possible. So far, for parts, a decent
case, lights, and an amp to drive the input we're up to ~$145, but
anything commercial this good would be $275 just for the color organ
itself.

Note the box I modified to house everything had a power supply with +/-5V
and +/-12V, so DC voltages are readily available.