Simple Siren Circuit

Building a circuit that sounds like a real siren is made clear in this Simple Siren Circuit article!

Similar to the siren of a police car or ambulance the sound rapidly rises and falls in pitch.

Although this activity can be enjoyable keep in mind that sirens are also used in emergency situations.

Circuit Working:

Parts List:

A generator made up of both PNP and NPN transistors is used in this circuit to generate a siren like tone.

These transistors work together to create a free running multivibrator.

Capacitor C2 can function as a steady frequency oscillator once it connected to the positive power supply line.

However in this article we are looking for a siren not a constant frequency oscillator.

An RC circuit is connected to resistor R2 so as to produce an oscillating tone that rises and falls.

Capacitor C1 slowly charges through R1 when switch S1 is pressed reaching the maximum voltage level of 4V.

As the voltage increases the time constant at the R2 and C2 junction decreases which raises the frequency of the multivibrators.

When switch S1 is released capacitor C1 slowly drains causing the frequency cycle to decrease.

A sawtooth waveform results from the combination of these two time constants.

Depending on whether switch S1 is pressed or released the speaker will produce a tone that either rises or falls.

Formulas:

The main objective of a transistor free running multivibrator siren circuit is to construct a circuit that generates an adjustable frequency output that resembles a siren sound.

This kind of circuit modifies the frequency in the form of the output waveforms by combining two transistors in a cross coupled design with resistors and capacitors.

Below are some helpful formulas and a process for creating such a circuit:

Frequency of oscillation (f):

The following are the below mentioned formula that are used to find out the frequency of oscillation in an astable mode or free running multivibrator:

f = 1 / 2* π *√R1​ *R2 ​* C1 ​* C2​​

where:

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

With this form an estimated frequency in hertz Hz is provided.

Duty Cycle:

The following below formula is sometimes used to calculate the output waveforms duty cycle D which affects the sirens hearing features:

D = R2​​ / R1​ + R2​

This technique is often used to acquire the duty cycle as a fraction which can then be multiplied by 100 to create a percentage.

Note:

By using these formulas and calculations one can develop a transistor free running multivibrator circuit designed to create a siren sound effect.

One can change component values as necessary to achieve the desired siren frequency and sound qualities.

How to Build:

For building a Simple Siren Circuit following steps are required for connections of the circuit:

Prepare Components:

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

Put Transistors in Place:

• According to the circuit diagram place the PNP and NPN transistors on the breadboard.

Connect the power supply:

• Connect the power supplies positive and negative connections to the breadboard.

Build a Multivibrator:

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

Capacitor Configuration:

• Connect capacitor C2 in such 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 such 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.
• By doing so one could hear a siren like tone.

Modifications:

• To get the desired sound effect try by changing the values of the components if necessary.

Take note:

• Keep in mind that this is only a basic advice and requirements for design will decide the exact quantities and settings.
• It is advised to get help or speak with an expert in electronics if one is unfamiliar with them.
• When handling electronic components take precaution and follow to safety rules.

Conclusion:

Based on the desired features and functions siren circuits are created in a number of configurations and with various levels of design.

These circuits are often found in emergency vehicles, security systems and other applications that require an eye catching, adaptable tone alarm.

References:

How does this siren circuit work?

Siren (alarm)