Elenco 130-in-1 Electronics Playground - Manual (Page 106)

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The electronic buzzer we built in the previous circuit
can only make a continuous beep, but we can make
a similar circuit that produces various siren sounds.
Your going to make a siren that gives out a sound
with a variable pitch. When you move the switch, this
siren wails and then creates a continuous high-
pitched noise.
Slide the switch to position B and assemble the
circuit. When you complete the wiring, turn the power
on by sliding the switch to position A. You hear the
speaker produce a sudden, roaring siren sound. At
first the sound is low and becomes higher, then
changes to a steady tone in about 3 to 4 seconds.
When you press the key and release it, the capacitor
discharges and starts the siren sound again.
You can understand how this works by looking at the
schematic. The pitch changes as the 10
μF capacitor
is charged through the 100k
Ω resistor. IC 2 is an
astable multivibrator. IC 1 is a buffer between the
capacitor and IC 2.
Notes:
EXPERIMENT #87: SWEEP OSCILLATOR
Wiring Sequence:
1-29
2-30
3-116
5-84-70-106-114-137-121
89-63-131
64-88-92-115
65-87-105
66-82-83-91
68-67-81
90-69-113-138
119-124
122-132
Schematic