This is electronic Stethoscope circuit. Usually the stethoscope is a medical device that listens to the heartbeat. Which we can make it with the least amount of electronic equipment so easy to make.
How it works
We use piezo speakers as a microphone. This is transducer can power output signal up to 100 mV at low frequency response, to use as input impedance for amplifier.
The piezo speaker will detect the heart beats. Then, it is amplified by Q1. In combination with R1-R4, VR1 and C1 are amplifiers, As Common-emitter amplifier: The input and output signals both share a connection to the emitter. It can output a response with low frequencies. So the output is connected to the headset with low impedance. To be able to listen to heartbeat. The VR1 will act as a volume adjuster to loud or low.
We can use Darlington transistor to increase he input impedance of the amplifier.
Parts will you need
R1___________10K__ 1/4W Resistors tolerance: 5%
R2___________330K__ 1/4W Resistors tolerance: 5%
R3___________100K__ 1/4W Resistors tolerance: 5%
R4___________47K__ 1/4W Resistors tolerance: 5%
VR1___________220K Linear Potentiometer
C2_________10µF 25V Electrolytic Capacitors
C1_________0.1uF 50V___ceramic Capacitors
Semiconductor and others
Q1__________BC548.BC547.BC549____45V 100mA NPN Transistor
B1__________9 volts battery