Audio Circuit Diagrams

Screaming Siren Circuit

A siren is a device designed to produce a high-volume sound, often used as a means of communication or signaling. Emergency vehicles, including police cars, ambulances, and fire trucks, are equipped with sirens to alert others. Sirens are commonly employed to convey signals or warnings and can produce various types of sounds using different circuit configurations. In this article, we will present a project on constructing a “screaming siren lights” circuit, which generates a siren sound in response to the intensity of light incident on the circuit. This circuit is also referred to as a “Light-Activated Alarm System.”


  • Screaming Siren Lights Circuit Principle:
  • Screaming Siren Lights Circuit Diagram:
    • Screaming Siren Lights Circuit Description:
    • Screaming Siren Lights Circuit Simulation Video:
    • How to Operate this Light Activated Alarm Circuit?
    • Screaming Siren Lights Circuit Applications:
      • Limitations of the Circuit: 
    • Checkout this project output video

Screaming Siren Lights Circuit Principle:

The fundamental concept behind this circuit is to produce sound in accordance with the level of incident light. When the circuit is exposed to increasing amounts of light, it generates extended pulses, resulting in a more pronounced sound. At the heart of this circuit lies the 555 timer IC, which induces oscillations in the Light Dependent Resistor (LDR), influenced by the intensity of light.

Screaming Siren Lights Circuit Diagram:

Screaming Siren Circuit

Screaming Siren Circuit Diagram

Circuit Components:

  • 555 timer IC
  • Light Dependent Resistor
  • Speaker
  • Capacitors C1,C2
  • Resistor R1
  • Breadboard
  • Connecting wires.
  • DC battery.

Screaming Siren Lights Circuit Description:

The central component of this light-activated alarm circuit is the 555 timer IC, primarily employed for timing applications and generating necessary oscillations. The 555 timer IC can be used in three distinct modes: Monostable mode, Astable mode, and Bistable mode. In this circuit, it operates in the Astable mode, obviating the need for external triggering.

In this configuration, the sixth pin of the IC connects to the second pin, while the fourth pin connects to the eighth pin and receives power from a 5V supply. Capacitor C1 links the third pin to the speaker.

In Astable mode, continuous triggering occurs between the second and sixth pins of the IC. As soon as the 5V supply voltage is applied to VCC, capacitor C2 commences charging through both the LDR and R1 resistors. The output at pin 3 switches on when the capacitor reaches 2/3 of VCC, initiating the capacitor’s discharge through R1, generating pulses at the output. When the capacitor reaches 1/3 of VCC, it starts charging again.

The primary means of altering the sound in the circuit is through the light-dependent resistor (LDR), also known as a photoresistor. Typically, LDRs exhibit high resistance in darkness and lower resistance when exposed to light. In this circuit, two mega-ohm resistors, signifying their resistance in the mega-ohm range when dark, are employed. The LDR and a 1K ohm resistor are connected in series, both linked to the 100nf capacitor C2. Both the sixth and second pins are shorted and connected to the same capacitor C2, with the ground connected to the other end of the capacitor.

Another vital component is the speaker, which functions as a transducer by interpreting electrical signals and converting them into physical vibrations, creating audible sounds. The speaker comprises a permanent magnet and a moving magnet to achieve this conversion. In this circuit, an 8-ohm impedance speaker is used, connected to the output pin 3 of the 555 timer IC via an electrolytic capacitor C1. The positive terminal of the capacitor is linked to Pin 3 of the timer, and the negative terminal connects to the positive terminal of the speaker, while the negative terminal of the capacitor connects to the ground.

Screaming Siren Lights Circuit Simulation Video:

How to Operate this Light Activated Alarm Circuit?

  • As shown in the circuit diagram, connect the circuit on the breadboard.
  • Connect the 5V supply voltage to the circuit now.
  • To begin, place the LDR in the dark.
  • The output no longer produces any sound.
  • Place the LDR in the light now. Gradually increase the amount of light falling on the LDR.
  • The increase in sound created can be seen as the intensity of the light gradually increases.
  • Now turn off the power.

Screaming Siren Lights Circuit Applications:

  • This circuit can be utilised in civil defence sirens to provide notice when natural disasters strike.
  • This can be used as a signal in an emergency.

Limitations of the Circuit:

  • This circuit is tested theoretically, to implement practically it may require changes.


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