Biasinvolves 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 C6function 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 R27in 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 Q8in
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.Chapter 5: Making Light Dance to the Music 91
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 fre-
quency. The strength of signals below this key
frequency is therefore reduced. This type of
filter 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.