How to Convert a Speaker into a MIC Circuit

Instead of the speakers coil, this mic circuit uses the speaker itself to pick up sound waves.

These sounds turn into a very faint electrical signal.

Then, a special circuit with transistors boosts this weak signal to a stronger one we can use.

This microphone will not be the best, but it is a fun way to start learning about how microphones work.

Circuit Working:

Parts List:

Component	Value/Type	Quantity
Resistors(All resistors are 1/4 watt unless specified)
	2.2k	1
	47k	1
Capacitors
Electrolytic	100µF 25V	1
Semiconductors
Transistor	BC547	1
Others
Speaker	8Ω	1

The circuit you sent is indeed a microphone amplifier circuit that can be used with a speaker to function somewhat like an electret microphone.

However, it is important to understand the limitations of this design.

A true electret microphone uses a permanently charged diaphragm and requires a bias voltage to operate.

The circuit here, utilizes the speakers coil as a variable resistor.

Sound waves cause the speaker cone to vibrate which in turn causes the coil resistance to change.

This creates a small voltage signal that is fed through the transistor and amplified.

Here is a breakdown of how the circuit works:

The speaker coil functions as the microphone element.

Sound waves cause the speaker cone to vibrate which changes the coils resistance.

The 47k resistor sets the bias current for the transistor.

The 2.2k resistor along with the 100uF capacitor form a voltage divider that blocks DC voltage from reaching the output.

The BC547 transistor amplifies the weak voltage signal from the speaker coil.

While this circuit can amplify sound the quality will not be comparable to a true electret microphone.

Here is why:

• Speakers are not designed to function as microphones.
• Their frequency response is not optimized for picking up sound and they may introduce distortion.
• The circuit lacks any filtering to eliminate unwanted noise.

If you are looking for a high fidelity microphone solution this circuit is not recommended.

However, it can be a fun experiment for understanding basic microphone amplification principles.

Formulas:

Using the speaker as a microphone element and amplifying the signal to a useful level is the process of converting a speaker into a MIC circuit.

The following fundamental and applicable formula may be used to create such a circuit using the given components:

Base of Transistor the resistor:

To restrict the base current IB , compute the base resistor RB for the BC547 transistor:

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.

Emitter Resistance:

An emitter resistor RE is an optional tool to help stabilize biasing and enhance linearity.

R_E​ = V_supply​−V_CE(sat) / ​​I_E​

where,

• The supply voltage is represented as Vsupply 6V.
• The transistors collector emitter saturation voltage is represented as VCE(sat)
• The emitter current is represented by IE.

Value of Coupling Capacitor:

Select a value for the capacitor to connect the audio signal.

The capacitance value of the capacitor should be such that audio frequencies can travel through it with little to no attenuation:

C = 1​ / 2πfR

where,

• The lowest interest frequency is represented by f.
• R is the resistance connected in series with the capacitor, which is usually the next stages input impedance.

Note:

When designing a speaker to MIC circuit, a transistor BC547 must be properly biased using resistors, and the audio signal must be coupled via a capacitor.

The aforementioned equations and recommendations offer a foundation for creating a circuit with the designated parts.

To maximize performance, test the components in real world scenarios and adjust component values according to particular design specifications.

How to Build:

How to Convert a Speaker into a MIC Circuit following are steps you need to follow for components connections:

Power Supply:

• The positive terminal of the power supply 9V connects to the collector of the BC547 transistor.

Speaker as Microphone:

• The positive terminal of the speaker coil connects to the emitter of the BC547 transistor.
• The negative terminal of the speaker coil connects to ground.

Bias and Amplification:

• A 47k resistor connects from the base of the BC547 transistor to the collector of BC547 transistor.
• Another resistor 2.2k is connected between the collector of BC547 and the positive supply.
• This sets the bias current for the transistor.

Output Coupling:

• A 4.7k resistor connects from the base of the BC547 transistor through 100uF capacitor.
• The negative side of the 100uF capacitor connects to the circuit ground.

Note:

• While this circuit can create a basic microphone function from a speaker, it has limitations in sound quality compared to a true condenser microphone.

Conclusion:

Speakers are not designed for optimal sound pickup leading to potential distortion and uneven frequency response.

The basic circuit lacks filtering, resulting in unwanted noise in the output.

This is more of a science experiment to understand microphone amplification principles.

For high quality audio applications a true microphone is recommended.

References:

Convert speaker level to microphone level