Elenco Basic Electronic Experiments - Manual (Page 48)

48
EXPERIMENT #32: SIREN
Connect the circuit 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-
left to rise slowly. As the base current slowly increases, NPN-left’s collector current also increases slowly (though it is
always much higher than the base current). NPN-left 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 by adding a 1k
Ω resistor in series with the base (middle wire) of NPN-right.
+9V
S
P
10k Ω 473
470 Ω
1M Ω
10 μF
+
-