The circuit was conventionally designed with eight LED audio level meter made out of two low power quad op-amps which utilizes LM324.The VU meter is calibrated in volume units that are numerically equal to the number of decibels above the reference level.
The VU meters were used to monitor audio power levels in recording and broadcast studios, so that the signal can be weakened or amplified to bring it within the optimal range for recording or broadcast. The meter effectively connects in parallel with the input of the receiver or amplifier.
The 1K resistors in the circuit are essential so that the LED’s turn on at different audio levels. There is no reason why you can’t change these resistors, although anything above 5K may cause some of the LED’s to never switch on. This circuit is easily expandable with more op-amps, and is not limited to use with the LM324. Pretty much any op-amp will work as long as you look up the pinouts and make sure everything is properly connected.
The 33K resistor on the schematic is to keep the signal input to the circuit at a low level. It is unlikely you will find a 33K resistor, so the closest you can get should do. The value of this resistor may need to be changed, so it is best you breadboard this circuit before actually constructing it on PCB. The circuit in it’s current form will accept line level inputs from sources such as the aux out on a Hi-Fi, all though could be easily modified to accept speaker inputs.
Visit Here for more.