Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 48)

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Connect the wires according to the Wiring Checklist and
press the switch. It makes a siren sound.
You saw earlier how you could change the frequency
(pitch) of the oscillator by changing the oscillator’s
resistance. Well this is basically the same oscillator
circuit you’ve been using except that now we are
electronically varying the oscillator’s resistance. The
large 1M
Ω resistor and 10μF capacitor cause the base
voltage (and hence base current) on transistor NPN 1 to
rise slowly. As the base current slowly increases, NPN
1’s collector current also increases slowly (though it is
always much higher than the base current). NPN1 is now
limiting the current just as a resistor does! Similar effects
occur after you release the switch and the 10
μF slowly
discharges.
If you like you can make the sound louder. Move the wire
from spring 18 to spring 40, and add a wire from spring
41 to spring 18.
EXPERIMENT #30: Siren
Wiring Checklist:
o 5-to-21
o 6-to-22
o 25-to-39
o 38-to-19
o 23-to-32
o 18-to-33-to-44
o 45-to-17
o 15-to-34-to-53
o 54-to-55
o 35-to-26-to-20
o 16-to-24-to-27-to-56
Schematic