Police Siren Circuit Working Using NE555 Timer and Applications

A siren is a loud and a typical sound making device. It is used to alert the persons and to attract the moment various vehicles mounted with sirens used, for example, VIP’s, and in vehicles such as ambulances, police cars and fire trucks. This circuit is used to generate a police siren sound using a Simple NE555 timer and a speaker.

The NE555 timer IC is the popular chip due to its multi-functionality and which finds use in a wide range of applications ranging from industrial, critical power electronics areas and much more.

The flexibility of this NE555 timer IC is that it has a wide range of operating voltage from +5 volt DC to +18 volt DC and it can sink or source 200mA of load current. The 555 timer pin diagram and DIP are shown below.

NE555 Timer DIP and Pin Configuration
NE555 Timer DIP and Pin Configuration

Pin Functionality of IC NE555 Timer as tabulated below.

Pin No. Pin name Input/Output Function
1 GND Input Provides ground
2 Trigger Input Trigger comparator input pin.

Negative trigger (< 1/3 Vcc) is given in monostable operation

3 Output Output Its output pin
4 Reset Input Internal flip-flop reset pin. Necessity is high to enable output
5 Control Input Control voltage input to manage to charge, discharging of external capacitor
6 Threshold Input Threshold comparator input pin. Positive trigger (>2/3 Vcc) is given in bistable operation
7 Discharge Input Discharge pin.

Gives the discharge path to external capacitor

8 Vcc Input For +Ve biasing voltage. Between 4.5V to 16V

The 555 timer can operate in 3 different modes, as explained below.

Monostable Mode

Monostable Multivibrator has only one stable state that is used to generate a single o/p pulse of a specified width either high or low when an external trigger pulse is applied. Please follow the link to know more about Monostable Multivibrator using IC 555 Timer Circuit and Working.

Monostable Mode
Monostable Mode

Astable Mode or Free running mode

It has actually no stable state. It has two quasi-steady state which rapidly alters from one to another and again to the same state. So it’s mainly altered from high to low and from low to high devoid of any input trigger input after pre-settled time. Please follow the link to know more about Astable Multivibrator using IC 555 Timer.

Astable Mode
Astable Mode

Bistable Mode or Schmitt Trigger

A bistable vibrator is a circuit with two stable states: high and low. Generally, a switch is required for toggling between the high and low state of the output. Please follow the link to know more about Bistable Multivibrator using IC 555.

Bistable Mode (or Schmitt Trigger)
Bistable Mode (or Schmitt Trigger)

NE555 Timer IC Based Projects

Home Appliances Control with Time Delay Switch: This project is designed to control home appliances based on a fixed-time delay for each load by using an IC 555 timer to generate switching period intervals to drive a relay to switch ON or OFF for any load.

Hidden Active Cell Phone Detector: The main scope of the project is to sense the presence of an activated mobile phone from a distance of one-and-a-half meters to prevent the use of same in the examination halls.

Touch Controlled Load Switch: The project is designed to develop a touch-sensitive switch to control any load. A 555 timer is used in the monostable mode to drive a relay to switch ON a load for a fixed time duration.

Police Siren: This circuit is used to generate a police siren sound using two NE555 timers and a speaker. In this article, we are going to discuss on the police siren circuit using NE555 Timer circuit.

Police Siren Circuit Explanation

This circuit consists of a breadboard, two NE555 IC timers, two potentiometers also called as presets, LEDs, buzzer and 9V battery for the circuit operation. This circuit can be easily built around the NE555 timer circuit on a breadboard.

This whole circuit derives the power from a 9V battery which can comfortably cater to the power needs of modules or discrete components used. A LED (Red) with a current limiting resistor is used to indicate the power supply on board. The Police Siren Circuit diagram is shown below.

Police Siren Circuit Working Using NE555 Timer
Police Siren Circuit Working Using NE555 Timer

Police Siren Circuit Operation

  • The first NE555 timer IC U1 is wired in Astable/free running mode.
  • In an Astable mode, it delivers a continuous sequence of pulses on its output Pin No. 3, the capacitor C1 charges and discharges continuously at the rate of 1Hz.
  • This pin is further connected to control voltage pin (Pin No. 5) of the second IC U2, whose frequency is modulated depending on the varying voltage from the IC U1.
  • The potentiometers (presets) VR1 and VR2 are used to vary the period of siren repetition and the tone of the siren.
  • The second NE555 IC U2 is also wired in Astable mode, which also delivers a train of varying frequency pulses on its output pin which is connected to an electrolytic capacitor through which a speaker of 8 ohms is driven, resembling exactly the sound similar to the police siren.
  • By varying the preset VR1 you can set how fast the siren changes from high frequency to low frequency and the preset VR2 sets the siren’s frequency.

Applications of IC 555 Timer

Some of the general applications of the NE555 IC are mentioned below.

  • Monostable or Astable Multivibrator.
  • DC-DC Converters.
  • Digital Logic Probes.
  • An oscillator, Waveform generators.
  • Temperature measurements and control devices.
  • Burglar and Toxic Gas alarms.
  • Voltage Regulators.
  • Create some time delays.

Thus, this is all about Police Siren circuit and its working, applications of IC 555 timers. We hope that you have got a better understanding of this concept. Furthermore, any queries regarding this concept or electrical and electronics projects, please give your valuable suggestions by commenting in the comment section below. Here is a question for you, why we use the Potentiometers in Police Siren circuit design?


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