FUNDAMENTALS FOR THE BEGINNER
English Police Siren
This circuit lets you simulate the sounds of an
English police siren by creating two different tones
with a pushbutton switch.
1. Build the Circuit.
Using the schematic along with the pictorial diagram, place
the components on a solderless breadboard as shown.
Verify that your wiring is correct.
2. Do the Experiment.
Theory: Here we have a 555 timer IC working as an astable
or free-running multivibrator. As we change the voltage
across pins 6 and 7, we change the frequency of the output
pulses. The time it takes to charge and discharge capacitor
C1 determines the output frequency from pin 3. Pressing
pushbutton switch S1 causes R5, 470K ohm, to be put in parallel with
resistor R2. This, in turn, changes the voltage across these two pins and
that changes the frequency of the output pulses coming from pin 3.
These output pulses are then sent to transistor Q1 and amplified. This
amplified signal is then sent to the speaker.
Procedure: Connect a nine volt battery to the battery snap. You should
hear a tone. If you press the pushbutton switch, you will hear another
tone. As you press and release the pushbutton switch, you’ll be able to
create the sounds of a two-tone English siren.
These experiments are provided
by GSSTechEd at
You can order parts
for this experiment
from their website
3904 NPN Transistor
555 Timer IC
0.01 mfd Ceramic Disc
Pushbutton Switch N/O
8 ohm round Speaker
Seven Wires 4” long