"ZUUL is the most transparent noise gate on the market! Studio quality specifications with simple straight forward operation. The ZUUL’s threshold knob also controls the dynamics of the gate circuit. Perfect for guitar, bass or any signal source as it has studio quality specifications. Use in front of an amplifier or in a series effects loop. Dual colour LED for visual feedback gating action: Green = Open, Orange = Closed, Off = Bypass. The KEY input signal can be directly from your guitar (ex. split from ROACH or SPLIFF) or other signal source which lets you go from clean to high gain without changing the Threshold knob. This powerful feature lets you control the gate with a different signal than what flows through the input/output jacks of the ZUUL. Side switch or internal jumper to change the threshold range if you are injecting a large signal into the input and not using the key input"
You can find PedalPCB's schematic, info and pcb on their website here.