""on the air"" indicator

elektor july/august 1985 In radio and television studios it is customary to indicate to all concerned when the microphone or camera is "on the air". This is normally done with a red light at or near the relevant camera or microphone. The circuit described here is intended as an auxili- ary for a DIY mixer unit. To make the circuit automatic in action, stereo slide potentiometers are used at the audio inputs. When one section of these potentiometers is connected to the +15 V line, the potential at the wiper of this section is a measure of the potentiometer set- ting. This potential is amplified in opamp A, and applied to the invert- ing input of A2. The latter opamp tog- gles as soon as the level at its inverting input exceeds that at its +input, which has been set with preset P1. The slide potentiometers for this pur- pose are always logarithmic types, so that the voltage rise at the beginning of their travel is always pretty small. To ensure correct operation of the circuit even ...