Let's Make Robots!

Beat Flash Box

Flashes light to beats in music

Flashes LEDs to beats in music.

The concept is simple. Two FETs act as switches, and turn the LEDs ON when sufficient current flows through its base terminal.

The current from your sound card is maximum when music has low frequency tones (bass) or beats so LEDs are bright during beats, giving a nice visual effect.



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OMG, I just made something similar, to my project dancing robot :)

Only, I used transistors, and added a cap to the collector, which results in a more "analoge feel", with the light fading out after a beat.

A good idea is to split the current of your sound card in 3 parts and when the current for example is in the first part

the flash beat box will light the first leds (right and left).

What you have to do is to measure somehow the maximum current of your sound card split the measurement in 3 parts 

with the help of a microcontroller and then light the proper leds every time.

a simple diagramm of how this will work but with words.


set sound cards maximum current as maxcurrent;

measure sound cards current at this moment;

set measurement = get sound cards current;

set a = 1/3 *  maxcurrent;

set b = 2/3 * maxcurrent;

set c = 3/3 * maxcurrent;

if(measurment > 0 && measurement <= a){

    light led1right;

    light led1left;


if(measurement > a && measurement <= b){

    light led2right;

    light led2left;


if(measurement > b && measurement <= c){

    light led3right;

    light led3left;


go to start

i did try to make an audio splitter with an opamp Lm324. Maybe i chose wrong components for the job because there was a lot of hum in my speakers

The leds worked fine though.

i want to try this.could you give schematic?

Here it is. You can basically use any FET or transistor with a high hfe (>100) and a base resistor R3,R4 in the range 1k to 10k (depends on your soundcard)

You might also want to increase the current limiting resistor R1,R2 in series with the LEDs unless they can handle the current.

I tried  to do something similar, just used one LED, one FET and 4V instead of 9V for experimenting. Everything works, but the voltage LED receives is only about 2.5V.  Where the 1.5V goes then?

Any suggestions what could I do to avoid this?

What value of current limiting resistor do you have? And what about the base resistance? Check these values. Also find out Vbe i.e. the emitter base voltage from the FET datasheet. You will get a voltage of 4 - Vbe.