Instruction manual

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
C2) and the lower critical frequency fC1
(BW = f
C2 - fC1). The selectivity (or Quality) of a band-pass filter is expressed
as the “Q” of the filter. 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 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. In the ready state, the 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.
SOUND GENERATOR
The circuit uses an HT-2810 sound generator IC. Figure 14
shows the internal design of the IC. 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. The Speed
Generator controls the frequency of the output as it is applied to
the output driver.
CIRCUIT DESCRIPTION (see page 16)
Switch SW1 has three positions: LOW / OFF / HIGH. In the LOW mode, the IC outputs a series of pulses at 892Hz,
then a series of pulses at 714Hz to get the ding-dong sound effect. The HIGH mode outputs a series of pulses at
1kHz and a series of pulses at 961Hz. The amplitude of the output of pin 3 is ramped down (see Figure 15) by
placing capacitor C5 from pin 4 to ground. As the voltage from the output decreases, it causes the speaker’s sound
to decrease. This contributes to the ding-dong effect.
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Figure 13
Figure 14
Figure 15