Police Siren Circuit using NE555 Timer







The police siren circuit is made in accordance with the circuit diagram which uses two 555 timer ICs and thus a high pitch frequently varying sound is produced . It can be used in different ways depending upon the requirement.

Working Police Siren Circuit using NE555 Timer

1. The current flows through the diode D1 connected near the positive terminal of the circuit. The P region of the diode is connected to the positive terminal and the N region of the diode is connected to the negative terminal of the battery. Thus, the circuit is in forward Bias mode.
2. It makes use of two 555 timer ICs used as astable multivibrators. The frequency is controlled by the pin 5 of the IC2.
3. The first IC2 (right) is wired so that it works around the frequency of 1Hz.
4. The capacitor C3 which is having the capacitance of 47uF is charged and discharged periodically.
5. Thus the voltage across this capacitor C3 gradually increases and decreases periodically.
6. This varying voltage modulates the frequency of the First IC i.e. IC1(Left).
7. The IC1 feeds the speaker coils and thus a sound is produced whose frequency varies alternatively with time i.e. it produces high pitch sound and then low pitch sound alternatively.
8. This process repeats and what we hear is the sound remarkably similar to the police siren.
9. Two resistors R2 and R5 are provided to vary the siren period of repetition and the tone of the siren.
10. By changing the resistance of the resistor R5 we can set how fast the siren changes from high frequency to low frequency.
11. The resistor R2 sets the siren frequency.

Circuit Diagram of Police Siren Circuit

Uses of Police Siren Circuit

1. It can be used as a siren in the police vehicles, fire brigade vehicles and ambulances as it attracts the attention of the people on the road immediately.
2. It can also be used as a security system in banks and other such places to alert the police.
3. It can also be used as a part of a burglar alarm system as its frequently varying high pitch sound grabs the attention of the nearby people as well as alerts the burglar.
4. It can be used as a call bell.

Components used in Police Siren Circuit

1. I.C. (Integrated Circuit) – Two IC1 555, IC2 555
2. Resistors – Six
R1 10 KW, R2 100 KW, R3 2.7 KW, R4 4.7 KW, R5 4.7 KW, R6 4.7 KW
3. Diode – One
D1 IN400Z
4. PNP-Transistor – One
T1 BC167
5. Capacitances – Three
C1 10 mF
C2 0.1 mF
C3 47 mF
6. Speaker – One
SPKR - 8W


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