The shown LEDs in the circuit sequentially light and "dance" as per the level of the music applied at the input, preferable directly from the speaker terminals of the audio gadget whose music level is to be monitored.
The proposed LED music level indicator circuit may be understood as follows:The components which support the LED illumination are the associated NPN transistor, the emitter resistor, the base preset and the corresponding diode.
The above stage is identical to all the LEDs included in the circuit for obtaining the desired push-pull effect in response to the applied music level at the input.However there's one difference between the LED stages, though most of the component placement is similar, the diodes form a different pattern.
If you see the circuit closely you will find that the the ground to the first transistor/LED stage from left comes across only a single diode, however the preceding stages ground potential has to encounter the extra corresponding number of diodes in theor path.
As we all know that a diode has the property of dropping 0.6 volts, means that the first transistor would conduct much sooner than the second, the second transistor conducts sooner than the third and so on.
Because as the number of diodes increase in the path of the respective transistor, the conduction is inhibited until the the voltage sufficiently increases for bypassing the diodes overall forward voltage.
This increase in voltage can happen only when the pitch of the music increases, giving rise to a sequentially running LED bar graph which shoots forward in response to the pitch or loudness o the applied input music.
The transistor at the input is a PNP and complements the rest of the transistors employed for illuminating the LEDs. The PNP transistor at the input amplifies the applied low level music sinal to levels which is just enough for illuminating the LEDs with reference to the music levels,.
Parts List for the explained LED music level indicator circuit
All NPN Transistors are BC547,
PNP Transistor is BC557,
All Presets are 10K,
All resistors are 100
LEDs as per choice