The circuit is described for one channel of the stereo amplifier. The other channel will of course be identical to it. A parallel point from loudspeaker of one channel is connected to the input of the three transistors! Fig. I). With 4.7k preset, the bias of T1, T2 and T3 is adjusted such that: (a) at low level of music only Tl conducts, (b) at medium level of music Tl and T2 conduct, Cc) at high level of music Tl, T2 and T3 conduct.As Tl, T2 or T3 conducts, it extends a ground potential to its respective LED through a buffer amplifier. Now it is obvious that at low volume only LED1 will light, at medium volume LED 1 and LED2 both will light, and at high volume all the three LEDs will light. LEDO (green) will be lit permanently to show that the channel is 'on'. Depending upon the liking, different colour LEDs can be used. 1C 7407 contains hex buffer amplifiers and hence one 1C is sufficient to accommodate both the channels.
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8 years ago