output of IC2-B is fed to R15, the AR
Envelope output level adjustment
pot. The circuit point “AR” is the wiper
of R15, which is fed to the AR-Gen
switches of the modules.
While contemplating this circuit,
remember that the inverters are
Schmidt triggers and that their inputs
must go lower than a third of the supply
voltage before their output goes high,
and then the input must go to greater
than two-thirds of the supply voltage
before the output goes low. This characteristic is known as hysteresis.
The zener diode on the external
gate is meant to protect against gate
signals greater than nine volts. When
the external gate is high, Q8 will
discharge C2 the same way the switch
S1 does.
NOTE: If you don’t plan to use the
■ FIGURE 3. LFO, Noise Source, and
±9V Battery Power Supply Schematic.
external gate capability of the device,
you can eliminate the following
components: D7, R199, R1, D8, Q8.
Low Frequency
Oscillator, Noise
Source, and ± 9 Volt
Battery Power Supply
The low frequency oscillator is
another modulator used to produce
cyclic voltage changes with which the
unit’s sound generators and sound
modifiers can be controlled (see
Figure 3). By applying the LFOs triangle wave to the control voltage input
of a VCO, you will produce the sound
of a siren. If you increase the frequency of the LFO enough, you will begin
to hear bell-type tones as the oscillator being modulated starts to produce
a characteristic ring-modulated
sound. The noise generator is used to
produce non-pitched sounds like rain,
wind, or steam and, of course, the
batteries power the unit. By using the
recommended low power op-amps,
the batteries will last a long time.
Let’s discuss this part of the
circuit. Two 2N3904s walk into a bar.
One makes a lot of noise and gets
thrown out while the other one sits
and quietly drinks his stout. The
punch line? Some transistors are
noisier than others. So how do we get
noise out of any 2N3904? We do it
with this circuit.
Notice that we do the usual ...
reverse bias a low V(ebo) emitter base
junction, listen to the junction
through C10, and a gain of about
1,000. If this transistor is whispering,
it is still whispering into a lot of gain.
This circuit does not care if this
transistor is in a confessional — you
are going to get at least 100 mV of
noise from the first stage.
We take whatever we are getting
Low Frequency Oscillator
D9 Ramp
a k 2 S16 1 R88
Tri
12
3 SPDT 1K k a on-off-on
D10 Sawtooth
R90
1 100K
2
3 Frequency
Lo Range
(closed)
C13 1 21S17 2
2uF SPST
C14
12
.005uF
-
2
LF444
IC7-A 1
+
3
LFS
LFO
Triangle
2
+9V
R89
12
200K
1 C19
3
Rect
S18 1
SPDT
2 0.1uF
R91
12
1Meg
R92
1 100K
2
R93
12
100K
5+
LF444
IC7-B
6-
3
1 C20
7
2 0.1uF
2
R94
500 ohms
1
-9V
-9V
LF444 Power
Pin 4 to +12V
Pin 11 to -12V
Noise Generator
Power Supply
+9V
.1uF
12
C15 2 2
R97 R98
1M 1K 9
11
+9V
+ B1
2
R96
1M
10 +
IC7-C
LF444
-
8
R95
12 +
21
1M IC7-D
LF444
R99
13 -
21
1M
C17
21
14
1
- 9V
1 + + C16
100uF
2
1
e 2N3904
R101
47K
off
12
S19 on
DPST
34
Power
2
+ B2
Q7
b
c Cut off
collector
R100
21
4.7M
- 9V
Selected for best noise
Q7 can also be a 2N2712
330pF
R102
21
10K
1
+ + C18
100uF
2 -9V
NS
-9V
46
March 2006
