The following sections explore the schematic elements in detail.Fancy Footwork: Exploring the Dance to the Music Circuit .....................
To make your musical score light up in response to the music, you need to
make a circuit that uses a microphone, as well as two operational amplifier
ICs in combination with some resistors, capacitors, and transistors. Working
together, these control which group of LEDs lights up to a particular frequency
of music.Here’s the overview of the schematic elements that you use to control your
project:The circuit starts with an electret microphone, which transforms sound
into electrical signals.R1 connects the microphone to positive voltage and supplies about 4.5
volts required by the microphone to function.
C1 and C2 are capacitors that block the DC voltage on the input signal
and allow the AC signal to pass.At this point, the circuit splits: The signal processed by the upper half
of the circuit powers the LEDs blinking in response to high-frequency
sound; the signal flowing through the lower half of the circuit powers
the LEDs blinking in response to low-frequency sound.
IC1 is an operational amplifier (op amp) that amplifies the signal from
the microphone. The IC contains two op amps; one half of IC1 is used in
the upper circuit, and one half of IC1 is used in the lower circuit.R2 and R5 in the upper circuit and R19and R22in the lower circuit set
the gain for each side of IC1. Because R5 is 50 times R2, a signal processed
by the op amp is amplified approximately 50 times.
Gainis the amplitude of the voltage out divided by the amplitude of the
voltage in: in other words, how much more juice goes out than comes in.R3, R4, and R6in the upper circuit and R20, R21,and R23in the lower
circuit provide a DC bias to the op amp that allows the full AC signal to be
amplified. If these resistors were not there, the portion of the AC signal
coming into the op amp with voltage less than 0 volts would be lost.90 Part II: Sounding Off!