When I was a teenager, I created my first lightshow by connecting some small incandescent light bulbs in parallel with my speakers. Then, I pushed the bulbs through small holes in the center of aluminum pie plates which acted as reflectors. I put the needle of my turntable on the first track of my Grand Funk Railroad record and basked in the glory of my igenuity.
Let's fast forward 35 years (that was quick). Vinyl is dead. The CD may soon be an object of curiosity. Music is digital and if you're like me, you listen to most of it on an iPod.
With this project, you can create a portable light show for your iPod or any other MP3 player.
Using nothing but analog devices, this circuit breaks the audio spectrum into three octaves: octave a, octave b=2*a, and octave c=2*b=4*a. Since the audio spectrum runs from 20 to 20Khz then a+b+c = 20Khz. Or, a+2a+4a = 7a = 20Khz.
So, a = 20Kz/7 = 2857Hz, the b octave runs from a to 3a or 2857Hz to 8571Kz and the c octave covers 8571Khz to 20Khz.
To design the filters, I downloaded Filterpro 2.0 from the T.I. website and selected 2nd order butterworth implementations.
The plot below shows how the Low Pass, Bandpass, and High Pass filter responses overlap each other:
In figure 1, we can see the iPod audio signals are brought into the circuit via J1. J2 provides a headphone output. U2A buffers the audio signal which is then passed to each of the filter circuits.
In figure 2, all of the filtered audio signals are fed into voltage to current converters which drive high intensity LEDS. Figure 2 also contains the bias voltage generator with creates a Vbat/2 voltage for the op-amps.
Figures 3 and 4 contain the circuitry necessary for the right channel.
Notice that the filter audio signals are AC coupled to the inputs of the V to I converters. This means that the inputs to these op-amps will see signals that swing below ground. Fortunately, the OPA422 can handle this as long as the current is limited.
The circuit assumes a 1 volt RMS input signal. This translates to peak voltages of about 1.41 volts on the voltage to current converter inputs. The gain of the V to I converter is I=V/27 so the LEDs will see peak currents of 52mA or an average of 26mA since the duty cycle is 50%. If you want to extend battery life and/or reduce the brightmess, you can increase the value of the feedback resistors R18, R19, R25, R43, R44, and R48.
This design can be built into a small enclosure such as an Altoids Tin. The LEDs can be mounted on the left and right sides with the audio jacks and switch on the bottom.
So, that's all for now. I'm going to plug in my iPod, jam in my earbuds and chill out on some Alison Krauss.