This police siren simulated electronic project uses two 555 timers IC to generate a sound similar to the police siren. A single 556 timer IC which consists of two 555 timers can also be used. In this circuit, both of the timers are configured as astable circuit.
The first timer is configured as a square wave close to 1 Hz astable oscillator. The output of this timer is used to feed the control voltage of the second timer where it is subjected to frequency modulation. This frequency modulation will generate a tone similar to the siren used by the police.
The frequency of this tone generator can be varied by changing the value of potentiometer VR1. When set to its maximum value of 220k ohm, it will have a tone frequency of approximately 320 Hz.
When S1 is switched ON, the circuit will be powered ON and U1 will start to oscillate at a frequency given by the formula:
f = 1.44/[(R1 + 2R2)(E1)]
= 1.44/[(10 + 2*82)(10)] Hz
= 0.8 Hz
This output frequency from pin 3 of U1 is fed into pin 5 of U2 where it is subjected to frequency modulation through resistor 10K. The tone generated can be varied by changing the values of potentiometer VR1.
Experiment with the sound and settle with the best sound of your choice. The output of U2 is used to drive a power transistor which in turn drives an 8 ohm speaker. Diode D2 is used to prevent the damage of transistor Q1 due to the back emf generated by the speaker during the ON/OFF driving of the speaker.
A good way to start is to assemble all the components on a breadboard and connecting all of them using jumper wires on the board. Power up the board using 9V DC power supply and test the functionality of the circuit before proceeding to solder them onto the PCB strip board.
If you want to go further, try drawing the schematic diagram followed by the layout of the PCB pattern, etch the board, drill, assemble and solder the parts before testing them.
oin electronics design contest and win prizes. Test your hardware and software skills against the best designers from the rest of the world.
Some engineering free magazine available for you to subscribe today
Free electronics design software - PCB layout, circuit analysis and simulation and other software.
Explore the use of 7-Segment Display, 555 Timer, Decade Counter and Binary Adder. Get the circuit.
Design and build a battery tester to test dry cell and rechargeable battery with a voltage of less than 2V. Check here.
Construct this simple door bell chime and have fun. Find out more here.
If you are looking for electronic kits, electronic gadgets or robotic kits, get them here.
Build this simple home alarm to protect your house from intruders. See the schematic circuit.