This is Stereo Balance Indicator Circuit.
You have a stereo system. But the level of both channels right and left are not the same. Yes, you can adjust with the Balance button.
But it’s quite difficult to adjust fine and accurate.
Like Water level gauge in construction. We need to find a balanced level of 2 points in order to install the shelf on the wall.
So in this, we use Stereo Balance Indicator to check it.
How to use
Connect both input right and left to a power amplifier.
Adjust a sound signal until both LEDs go out. Shows that both audio signals are well balanced.
In contrast, if the sound signals are not equal level. One of the LEDs will light up.
How it works
Sure, I try to keep a simple circuit. This circuit too.
Include 4 transistors, two LEDs, 4 resistor and 2 capacitors only.
Shall I explain its process?
Apply 9V power supply(battery) to the circuit.
Enter the audio stereo signal into both the input left and right channel.
C1 and C2 pass audio signal and block DC to the base of Q4 and Q2.
If the stereo signal is equal. Make Q1 and Q3 conduct the current is the same too.
So not the difference of voltage. They cause both LED1 and LED2 goes out.
But if the stereo signals are not the same.
The Left channel is louder than right. It causes Q4, Q3 works. R2 passes the current to LED2. It glows up. But LED 1 goes out. Because Q1 and Q2 do not work.
On others hand, the right channel is higher than the left. Make Q1 and Q2 work well. LED1 so glow up instead.
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Parts you will need
Q1-Q4: BC147 NPN transistor
C1, C2: 0.01uF 50V Ceramic
0.25W Resistors, 5% Tolerance
R1, R2: 2K
R3, R4: 470 ohms
LED1: 5mm RED LED
LED2, 5mm Blue LED
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