This circuit has 3 main sections, an inverting op-amp amplifier, a rectifier and LED display. The input can be from almost any low level source, but if not using an electret microphone, then omit R1 as it's not necessary. The gain of the amplifier based around IC1 is controlled by VR1. The output is rectified by D1,2 and filtered using C4,R6.The final stage is a standard design for the LM3914. Alter R8 to control LED brightness but be careful not to over-drive the IC's. It's recommended that you don't
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