How to Build a Simple LED Audio Level Indicator

Music: A Part of Our Life

For ages music has continued to be a part of human life and has always intrigued us. Today through some huge advancement in electronics, we have been able to refine and shape up the clarity of music to outstanding levels. The addition of external visual effects like rhythmic lights to the various music systems has become quite common nowadays. These multicolored rhythmic music light effects are really fascinating to see. They are mostly used in clubs, discotheques, party halls, etc. to fill the atmosphere with great fun and enthusiasm. The little circuit of a LED audio level indicator presented here though may not be as dazzling as its “elder brothers,” but once connected to your audio system, its smart visual rhythmic display will surely be able to enhance and lift your listening moods.

A music signal is generally rich in harmonics and may contain frequencies as low as 20 Hz up to 20 KHz. The present circuit is basically a multi-stage voltage sensor that converts the music content’s varying voltage (frequency) into subsequent visual indications through LEDs. Before moving on to construction clues, let’s try to understand how the circuit functions.

How does the circuit Function?

led audio level indicator, Circuit Diagram, Image

LED Audio Level Indicator Mounted Over Speaker Box, Image

Referring to the given circuit schematic, the whole circuit operation may be understood through the following points:

  • The music or the audio frequency is applied to the base of T1 through the capacitor C1. Here C1 performs the function of blocking the DC and allowing only the audio signal to pass through.
  • The signal is amplified by T1 up to a suitable level so that its variations may be properly recognized by the following stages.
  • The amplified music directly available at the collector of T1 is fed to an array of 7 voltage sensor transistors.
  • Transistors T2 to T7 are all configured to sense varying music voltages at different levels.
  • The presets connected at the base of each of the transistors are adjusted so that the transistor is able to sense the voltages in a step wise manner, i.e. music signals with lower most amplitude levels will be able to reach and bias only transistor T2 and will only go on to reach the subsequent transistors as its amplitude rises.
  • Thus the LEDs connected at the collector of each transistor are able to light up in accordance to the rising music patterns or the amplitude.
  • For an optimum volume level a perfect audio level indication may be received from the circuit.
  • If the treble part in the music is high only few numbers of LEDs will oscillate to and fro, say may be the first four LEDs, but may go on to shoot and light up the whole array of LEDs as soon as the bass level of the input music signal is increased.

Parts List

R1—R7 = 150 Ohms,

R8—R11 = 47 Ohms,

P1—P7 = 2K5 PRESET,

ALL DIODES = 1N4148,


T1 = BC 557,

T2—T8 = BC 547,


C1 = 100uF/25V,




Construction Hints And Testing

With the help of the given circuit diagram the whole circuit may be easily assembled over a general PCB. Remember to keep the height of the LEDs to a higher level than the other components so that they may easily protrude out of the box in which the circuit may be enclosed.

LED Audio Level Indicator, Monitor Graph, Diagram, Image

The testing procedure of this led audio level indicator is also quite simple. Connect a 12 volt supply to the circuit and apply a music signal to its input. The signal may be directly derived from the terminals of your speaker system. Keep the volume to a minimum level, Keep the position of P1 somewhere in the midway and adjust P3 to P8 to shut off the respective LEDs. Now set P2 such that the relevant LED just starts flickering. Increase the volume slightly, set P3 to let the second LED flash, go on increasing the volume gradually and repeat the above procedure until you finish setting all the presets. Now at an optimum volume all the LEDs should move in a to and fro manner as per the pitch of the music to provide a beautiful indication of the music strength.

This wraps up the setting procedure of the circuit; you may arrange the LEDs in a rising curve sort of design and put a graph at its background to give it a more technical and a professional look.