Speaker Microphone Schematic

Description: This circuit allows the use of a low-cost loudspeaker as a microphone. Sound waves that reach the speaker cone cause fluctuations in the voice coil. The movement of the voice coil within the speaker's magnetic field generates a small electrical signal. The circuit is designed to operate with a voltage between 6 and 12 volts DC. The first transistor operates in common base mode, which effectively matches the low input impedance of the speaker to the common base stage and provides a high voltage gain. The second stage is directly coupled and functions as an emitter follower. While the voltage gain is slightly less than unity, the output impedance is low, making it capable of driving long cables. Although the speech quality is not as high as that of a standard or electret condenser microphone (ECM), acceptable results can be achieved. Loudspeakers with diameters ranging from 1 inch to 3 inches can be used, and speaker impedances can vary from 4 ohms to 64 ohms. The value of the 8.2-ohm resistor can be adjusted to match the impedance of the specific speaker being used.

This circuit effectively repurposes a loudspeaker as a microphone by capitalizing on its inherent properties. The initial stage utilizes a common base configuration, which is particularly advantageous for interfacing with the low impedance of the loudspeaker. This configuration enhances the signal amplification process, allowing for effective signal capture from the sound waves.

In the second stage, the emitter follower configuration provides a means of buffering the signal. This stage ensures that the output can drive longer cable lengths without significant signal degradation, which is critical in practical applications where distance from the microphone to the processing unit may vary. The slightly less than unity voltage gain in this stage is counterbalanced by its low output impedance, facilitating compatibility with various audio processing equipment.

The choice of loudspeaker diameter and impedance is flexible, allowing for a range of applications. The circuit can accommodate speakers with diameters from 1 inch to 3 inches and impedance values from 4 to 64 ohms. This versatility makes it suitable for various audio capture scenarios, though it is important to note that while the quality is acceptable for many applications, it may not reach the fidelity levels of dedicated microphones. Adjusting the 8.2-ohm resistor allows for fine-tuning of the circuit to optimize performance according to the specific characteristics of the loudspeaker used. Overall, this circuit offers a cost-effective solution for audio capture, leveraging common components to achieve satisfactory results in various settings.This circuit is allows you to use a cheap loudspeaker as a microphone. Sound waves reaching the speaker cone cause fluctuations in the voice coil. The voice coil moving in the speakers magnetic field will produce a small electrical signal. The circuit is designed to be used with an operating voltage between 6 and 12 volts dc. The first transistor operates in common base mode. This has the advantage of matching the low input impedance of the speaker to the common base stage, and secondly has a high voltage gain. The second stage is direct coupled and operates in emitter follower. Voltage gain is slightly less than unity, but output impedance is low, and will drive long cables. Speech quality is not as good compared to an ordinary or ECM microphone, but quite acceptable results can be obtained.

Speaker cones with diameters of 1 inch to 3 inches may be used. Speaker impedance may be 4 ohm to 64 ohm. The 8. 2 ohm resistor value may be changed to match the actual speakers own impedance.