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 4.5volts 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 R19 and R22 in 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.

Gain is 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 R6 in the upper circuit and R20, R21, and R23 in 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.

Bias involves applying voltage that is above ground to a portion of the circuit to amplify both the positive and negative sides of a signal. Without DC bias, you would lose part of the signal.

C2 and C5 remove any DC bias from the signal coming out of the op amps.

R7 and R24 are potentiometers that allow you to adjust the sensitivity of the circuit in relation to how loud the music is.

C3 and R8 function as a high pass filter, and R25 and C6 function as a low pass filter. These filters are what make higher-frequency sounds light up LEDs 1–8 and lower-frequency sounds light up LEDs 9–16.

IC2 is an op amp that is used to amplify the signal that passes through the filter. 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.

R9 and R10 in the upper circuit and R26 and R27 in the lower circuit set the gain of the op amp. Because R10 is 200 times R9, a signal processed by the op amp is amplified approximately 200 times.

Q1, Q2, Q3, and Q4 in the upper circuit and Q5, Q6, Q7, and Q8 in the lower circuit are 2N3904 transistors whose bases are connected to the output of the op amps in IC2. When the output of the op amp reaches about 0.7 volts, the transistors turn on, and current flows through the LEDs.

Filters

The portion of the circuit made up of C3 and R8 is an RC high pass filter. This pass has nothing to do with a tricky move in football. Rather, with this filter, signals above a certain frequency — determined by the value of C3 and R8 — pass through more easily than signals below that frequency. The strength of signals below this key frequency is therefore reduced. This type of ilter is made up of the capacitor in line with the signal path and the resistor between the output of the signal and ground. Correspondingly, the portion of the circuit made up of R25 and C6 is an RC low pass filter. Here, signals below a certain frequency — determined by the value of R25 and C6 — pass through more easily than signals above that frequency. The strength of signals above this key frequency are reduced. This type of filter has the resistor in line with the signal path and the capacitor between the output of the resistor and ground.

For both types of filters, increasing the value of either or both the resistor or capacitor lowers the value of the key frequency. Lowering the value of either or both the resistor or capacitor increases the value of the key frequency. The circuit won’t mean a thing if you don’t set up the lights for it to control. That’s where the two groups of LEDs and resistors come in. They include

LED1–LED8 to light the display for the high-frequency circuit and LED9–LED18 to light the display for the low-frequency circuit.

R11–R18 in the upper circuit and R28–R35 in the lower circuit are resistors, that in series with LEDs, limit the current running through the LEDs to approximately 10 milliamps.