sound-level indicator

This novel indicator is ideally suitable for use in a discotheque. It consists of eight equi-distant columns of eight LEDs arranged in a starlike pattern, so that corresponding LEDs in the eight columns form concentric circles, as shown in figure lb. The higher the sound level, the more circles light, giv- ing the impression of a star of con- stantly varying brightness. As can be seen in figure lb, the eight LEDs in any one of the eight circles are connected in series. Each of these series chains is driven by a transistor: T, . ..T8 in figure la. Dropping resistors are not required: the positive supply voltage provides just over 1.8 V per LED, which is a perfect value for red LEDs to show up nicely. Transistors Ti ...T8are driven by dif- ferential amplifiers Ai ...A8, which compare the audio-dependent direct voltage across C2, which is buffered by Al2, with the potential determined by D11and R11...R 18. If the result of the comparison is positive, the associ- ated driver transisto...