I really wanted to increase the audio->video->audio feedback loop effects so I built an audio amplifier that responds to the brightness of the area pointed at by the light gun. The rainbow video effect circuit takes input from the amplifier which produces another feedback loop.
This was the last step of the project and I hadn't done anything with the zapper's trigger button yet. I set it up to saturate the brightness input causing the amplifier to go full volume regardless of the light source.
The edge detector output is only on/off, it doesn't change in voltage levels based on the brightness of the monitor. It does give off more pulses per frame when the monitor is brighter because scanlines adjacent to the one it's pointed directly at give off enough light to produce a pulse. The first stage of this circuit generates an analog voltage based on pulses per frame by using each pulse to increasing the charge of a capacitor that is gradually discharging.
Project is complete. Here's me playing it at VIVO Media Arts.