Simple Fog Horn Circuit

Ever wondered how fog horns make that deep, spooky sound?

A fog horn circuit is like a tiny sound machine for fog.

It uses electronic parts to create that classic low moaning sound that rises and falls in pitch.

These circuits are mostly used for fun DIY projects to make a fog horn sound effect, or sometimes to teach people about electronics in a cool way.

Circuit Working:

Parts List:

In this article the circuit is powered by a 9V battery.

The circuit uses two BC547 transistors and one BC557 transistor in a three stage amplifier configuration.

These transistors amplify the signal from the oscillator circuit next component so that it can drive the speaker.

The oscillator circuit is made up of resistors and capacitors.

When the push button is pressed the 100uF capacitor starts to charge through the 1k resistor.

As it charges, the voltage across the capacitor increases which causes the frequency of the oscillation to increase.

This creates a rising pitch effect.

When the button is released the capacitor starts to discharge through the speaker creating a lowering pitch effect until the capacitor is fully discharged and the circuit stops oscillating.

The speaker is an 8Ω speaker that reproduces the sound generated by the oscillator circuit.

The push button acts as a trigger to start the oscillation cycle.

When it is pressed the capacitor starts to charge, creating a rising pitch and volume.

When it is released the capacitor discharges creating a falling pitch and volume.

This creates the characteristic sound of a fog horn.

Formulas:

In order to design a fog horn circuit, an oscillator that powers a loud speaker must be created.

Here is a simple schematic and a list of pertinent formula for creating a circuit with the components listed:

Values of the resistor and capacitor:

One may compute the oscillation frequency f in an astable multivibrator by using:

f = 1.44​ / (R1+2*(R2+R3))*C

where,

• The resistors in the oscillator circuit are R1, R2, R3 and R3.
• The value of the capacitor is C.

To get the proper frequency for the fog horn sound, experiment with different values for the resistor and capacitor.

Calculating the Base Resistor (BC547 and BC557 transistors):

Determine the base resistor RB in order to control the transistors base current IB:

R_B​ = V_in​−V_BE​​ / I_B​

where,

• The input voltage to the transistors base is represented by Vin.
• The base emitter voltage drop, VBE is measured (usually in the region of 0.7V for silicon transistors).
• IB is the intended base current.

Speakers Current Limiting Resistor:

Determine the speakers current limiting resistor Rspeaker in order to safeguard it and guarantee correct operation:

R_speaker ​= ​V_supply​−V_{speaker​​} / I_speaker

where,

• The supply voltage is represented as Vsupply 9V.
• The nominal impedance of the speaker is represented by Vspeaker 8Ω.
• The intended current flowing through the speaker is Ispeaker (compute using ohms law based on the desired loudness and speaker power rating).

Note:

Using BC547 transistors to create an oscillator, driving a BC557 transistor for amplification and sending the sound through an 8Ω speaker are the steps involved in designing a fog horn circuit.

The aforementioned formulas and recommendations offer a place to start when creating a circuit with the designated parts.

To maximize performance and sound quality, adjust component values in accordance with particular design specifications and carry out hands on testing.

How to Build:

To build a Simple Fog Horn Circuit follow the below mentioned steps for components connections:

Transistor Amplifier Stages:

• The base of the first BC547 transistor likely connects to a resistor 68k in series with the output positive terminal of the oscillator circuit.
• The collector of the first transistor connects to another resistor 1k and then to the base of the second BC547 transistor.
• The collector of the second transistor connects to another resistor 68k and then to the base of the BC557 transistor.
• The collector of the BC557 transistor connects to the positive terminal of the speaker.
• The emitters of both the BC547 transistors are connected ground negative terminal of the battery.
• Connect the emitter of BC557 transistor to the one end of speaker and the other end of speaker is connected to positive supply.

Oscillator Circuit:

• One terminal of capacitor C2 100nF connects to the base of the Q1 BC547 transistor through a resistor 68k.
• The other terminal of C1 connects 1k resistor and the base of BC557 Q3 transistor.
• One terminal of capacitor C1 100nF connects to collector of Q1 transistor through a 1k resistor.
• The other terminal of C1 connects to a resistor 68k.

Conclusion:

A fog horn circuit is a fun electronics project that replicates the signature sound of a fog horn.

By cleverly using capacitors and transistors, the circuit creates a rising and falling pitch effect just like a real fog horn.

You can adjust the components to customize the sound and learn some basic electronics concepts along the way.

Remember to prioritize safety while building the circuit.

References:

Foghorn