Simple Siren Circuit

This article shows you how to build a circuit that makes a sound like a real siren!

The sound goes up and down in pitch really fast, just like a police car or ambulance siren.

This can be a fun project but remember sirens are also used for emergencies.

Circuit Working:

Parts List:

This circuit produces a tone reminiscent of a siren, employing a generator composed of both PNP and NPN transistors.

Together, these transistors form a free running multivibrator.

If capacitor C2 were linked to the positive power supply line it would function as a stable frequency oscillator.

However, our goal is not a steady frequency oscillator but rather a siren.

To achieve an oscillating tone that rises and falls resistor R2 is connected to an RC circuit.

When switch S1 is pressed, capacitor C1 gradually charges through R1 until it reaches the maximum voltage level of 4V.

This increasing voltage leads to a decreasing time constant at the R2 and C2 junction subsequently causing the multivibrators frequency to rise.

Upon releasing switch S1 capacitor C1 discharges slowly resulting in a decreasing frequency cycle.

The combination of these two time constants produces a sawtooth waveform.

The sound emitted from the speaker will be a tone that either rises or falls depending on whether switch S1 is pressed or released.

Formulas:

The objective of a transistor free running multivibrator siren circuit is to build a circuit that emits a variable frequency output that simulates a siren sound.

Usually, two transistors in a cross coupled arrangement with capacitors and resistors are used in this kind of circuit to adjust the output waveforms frequency and form.

Here are some helpful formulas and a strategy for creating such a circuit:

Frequency of Oscillation (f):

The following formula can be used to approximate the frequency of oscillation in an astable mode, or free running multivibrator:

f = 1 / 2* π *√R1​ *R2 ​* C1 ​* C2​​

where:

• R1and R2 are the resistors connected to the transistors.
• C1 and C2 are the capacitors connected in parallel with R1 and R2

An estimated frequency in hertz Hz is given by this formula.

Duty Cycle:

The output waveforms duty cycle D which influences the sirens audible qualities, may be roughly calculated using the following formula:

D = R2​​ / R1​ + R2​

The duty cycle may be obtained as a fraction using this method and then multiplied by 100 to get a percentage.

Note:

You may develop a transistor free running multivibrator circuit specifically intended to produce a siren sound effect by applying these formulas and concepts.

To get the required siren frequency and sound qualities, adjust component values as needed.

How to Build:

Building the described circuit requires electronic components and basic soldering skills.

Prepare Components:

• Identify the values for resistors R1, R2, capacitors C1, C2 and transistors based on your design requirements.

Place Transistors:

• Insert the PNP and NPN transistors into the breadboard as per the diagram.

Connect Power Supply:

• Connect the positive and negative terminals of the power supply to the breadboard.

Build Multivibrator:

• Connect the transistors to create a free running multivibrator circuit.
• Connect the collectors, emitters and bases as needed.

Capacitor Configuration:

• Connect capacitor C2 in a way that it does not interfere with the free running multivibrator.
• Ensure it is not connected to the positive power supply line.

Add RC Circuit:

• Connect resistor R2 in parallel with capacitor C1 to create an RC circuit.
• This will control the rising and falling of the frequency.

Include Switch:

• Connect switch S1 in a way that when pressed it allows capacitor C1 to charge and when released, it allows C1 to discharge.

Speaker Connection:

• Connect the speaker to the output of the multivibrator circuit.

Test:

• Power up the circuit and test it by pressing and releasing the switch.
• You should hear a siren like tone.

Adjustments:

• If needed, experiment with different component values to achieve the desired sound effect.

Note:

• Remember, this is a basic guide and the specific values and configurations will depend on your design requirements.
• If you are not familiar with electronics it is advisable to seek assistance or consult with someone experienced in electronics.
• Always exercise caution when working with electronic components and follow safety guidelines.

Conclusion:

Siren circuits can be designed in various configurations and their complexity may vary based on the desired features and functionalities.

These circuits are widely used in security systems, emergency vehicles and other applications where an attention grabbing, variable tone alarm is needed.

References:

How does this siren circuit work?

Siren (alarm)