BC109 and ORP12
In this circuit, audio fed to the management channel is amplified and rectified by D1 and D2. This dc level activates LED D3 via Q2. the sunshine from D3 causes R9, a light-dependent resistor to decrease resistance. As R11 (audio gain) is about higher, a lot of audio is gift at the output of Q1. Audio fed into J2 is shunted to ground via R9 and fewer of this audio seems at J3. Therefore, audio at J1 controls the audio level fed to J3 from J2 and produces a fade impact.
