The circuit LED volume indicator can be used in stereo amplifiers to indicate the balance level of volume. As human ear is not much sensitive to difference in sound level, LED volume indicator circuit indicates the difference in the sound level in any channel by means of LED.
Input to the circuit is taken from output terminals of each channel. The output signals are rectified by diodes D1 and D2, which is further filtered by capacitors C1 and C2. A DC voltage is developed across the voltage dividers compromising R1, R2 and R9, R10.
When there is same volume in both channels, voltage at points A and B are equal and the voltage difference between the points A and B are zero. So both transistors T1 and T2 remain cut –off. When the volume in the left channel is increased, the voltage at the point A also increases. When the voltage at point A increase by about 0.6 volt more than the voltage at the point B transistor T1 starts conducting and switches on the LED driving transistor T3 Transistor T2 remains cut-off.
Similarly when the voltage at point B increases by about 0.6 volt more than that at point A, transistor T2 conducts lightning RIGHT LED. Left channel LED remains off. In this way the respective channel LED lights up to indicate the rise in volume level.
Resistors (all ¼-watt, ± 5% Carbon)
R1, R2, R9, R10 = 470 Ω/0.W
R3, R8 = 10 KΩ
R4, R7 = 22 KΩ
R5, R6 = 680 Ω
R11 = 33 Ω
R12 = 68 Ω
C1, C2 = 220 µF/40V
D1, D2 = OA91
T1, T2 = BC148B
T3, T4 = BC158B
LED1, LED2 = 2 different color LEDs