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BAND PASS FILTER
The combination of a low and high pass filter create
what is called a Band Pass Filter. The frequencies
passed by each filter overlap and create a bandwidth
(range), passing all signals within the bandwidth and
reducing all others. Figure 13 illustrates the general
band-pass response curve. A critical frequency is
defined as the point where the voltage is reduced to
.707 (the square root of ½ is used because it
represents the point where power has been reduced
to ½).
The bandwidth can be defined as the
difference between the upper critical frequency (f
and the lower critical frequency f
selectivity (or Quality) of a band-pass filter is
expressed as the "Q" of the filter.
Gain
100%
70.7%
f
c1
Figure 13
SOUND GENERATOR
The circuit uses the single sound generator HT2812G
IC. The HT2812G is a CMOS LSI chip designed for
use in sound effects products. Figure 14 shows the
internal design of the IC.
Osc1
Oscillator
Divider
Osc2
Tone
Key
Generator
Input
Key
Noise
Logic
Generator
Figure 14
Key
Output
(BW = f
- f
). The
C1
C2
C1
BW
f
f
f
r
c2
Speed
Generator
Selector
Output
Output
and
Driver
Envelope
Output
Circuit
Env
It is the ratio of the center (or Resonant) frequency to
the bandwidth (Q = fr/BW). A filter with a higher value
of Q has a narrower bandwidth, thus passing fewer
frequencies than one with a lower value. Bandpass
filters can be classified as either a narrow-band (Q >
10) or a wide-band (Q < 10).
CIRCUIT DESCRIPTION (See page 16)
The op-amp IC1D shapes the frequency response
to amplify those frequencies produced when motion
is detected and rejects all others, such as those due
)
to noise or slow temperature changes. Frequencies
C2
above 20Hz and below 1Hz are beyond the
bandwidth of the circuit and thus are rejected. The
output at pin 14 is about 1.6V when no motion is
detected. As motion is detected, the voltage at the
output will change and trigger either IC1C or IC1B.
The op-amps IC1A, IC1B and IC1C are configured
as voltage comparators.
output of IC1A is high and IC1B and IC1C are low.
When IC1D outputs a voltage lower than 1.41V, it will
force pin 2 of IC1 high. When IC1D outputs a
voltage higher than 1.67V, it forces pin 8 and pin 2 of
IC1 to go high. A high in with one of these cases
causes the output to go low and allows C9 to
discharge through IC1A. The discharging of C9 will
pull pin 6 of IC2 low and trigger the sound generator.
CIRCUIT DESCRIPTION (see page 16)
As the Key Input is brought low, the Oscillator, Speed
Generator, Tone Generator, Noise Generator and
Envelope Sections are all enabled. The Oscillator
Section begins to oscillate at a frequency determined
by the voltage across pins 7 and 8. This frequency is
then divided down and applied to the Speed
Generator.
frequency of the output as it is applied to the output
driver. The output consists of 15 pulses. Appling the
pulses to the base of transistor Q1, turns it on and off
rapidly, causing the speaker to sound. You can select
between a high and a low tone using switch SW1.
Figure 15
-8-
In the ready state, the
The Speed Generator controls the
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