With this circuit, a well known configuration, small speakers and even low-impedance headphones can be used as microphones. The circuit has a low input impedance and high output impedance. The transistor can be the BC548 or BC549 and consumption is very low allowing the power too be cells. The signal output cable must be shielded. The output level is sufficient to excite most amplifiers. We may also use the circuit to process signals low frequency transducers as magnetic sensors and others. Finally we have the application using a pick-up coil to be coupled to the magnetic type headphones phones. The circuit is in the Mechatronics Sourcebook, book by Newton C. Braga, published in the United States.
