A new version of the classic color organ where different color lights are triggered by different frequencies of sound, resulting in a display that flashes along to music.
To begin, the required parts were gathered up. This included 49 each 3mm LEDs of various colors, a bunch of 1/8 watt resistors, some tiny salvaged transistors, a microphone, some trimmer potentiometers from a dead CRT, a 4 "AA" cell battery case with power switch, and a few small capacitors and diodes. The plan was to attach the LEDs and electronics to the front of the battery case, forming a compact unit that could be placed on a table or clipped to a shirt or jacket.
The circuit was designed to use common values of parts that are most easily available in smaller sizes. Ceramic and tantalum capacitors, 1/8 watt resistors, and small package semiconductors were chosen to conserve space. Integrated circuits were rejected as being too large for the 6 cm X 6 cm allowable space for circuitry and LEDs on the face of the battery case.
The circuit itself consists of an electret condensor microphone feeding a two stage preamplifier with a sensitivity control. The signal is then split up into three channels with individual level adjustments. Each channel has it's own filter circuit, so that they each respond to a different range of sound frequencies, one each for treble, midrange, and bass sounds. The signal is then sent to an LED bargraph ladder circuit, with red/yellow, green/white, and blue/green leds for each channel respectively. Two LEDs were used for every one shown in the diagram, and the LED resistors were increased to 1K to extend battery life. With high output LEDs there is still lots of brightness even at lower currents, so this works well to lower current consumption. As well, there was a single purple colored UV LED included that is on whenever power is turned on.