Simple Electronic Stethoscope Circuit

The construction of a circuit for a specific kind of stethoscope is explained in this article.

To increase the bodies sensitive sounds such as heartbeat and lungs the standard stethoscopes uses a tube.

This electronic stethoscope uses tiny electrical components to significantly boost sound quality and the volume.

This can enhance the medical staffs capacity to hear what going on within the body.

Building this project is challenging thus beginners are not advised to try it.

It is also important to remember that stethoscopes and other medical equipment should only be handled by qualified professionals.

Circuit Working:

Parts List:

A piezo sounder derived from a melody generator or musical greeting card is used as a microphone in the electronic stethoscope circuit.

The amplifiers input impedance controls the transducers low frequency response which produces an output signal of about 100 mV.

Because of its high input impedance an emitter follower transistor amplifier is used to maximize the low frequency response.

This decision ensures a very low frequency response from the transducer.

Just plug a set of low impedance headphones into the amplifiers output to hear a beating heart.

A darlington transistor design is used in place of the emitter follower to increase the amplifiers input impedance.

Formulas:

The formulas that follow are used to explain the simple circuit which is mentioned above for an electronic stethoscope that uses a Bipolar Junction Transistor (BJT) in a common base CB configuration:

Av Voltage Gain:

The voltage gain in a usual base design is roughly equal to 1.

This occurs when the emitter receives the input voltage and the collector receives the output voltage.

The formula for voltage gain is:

A_v ​= R_C​​ / _re​

where,

• RC is the collector resistor.
• re​ is the emitter resistance which is approximately VT / IE​​
• VT​ is the thermal voltage (26 mV at room temperature)
• IE is the emitter current.

Current Gain A_i:

The current gain in a common base configuration is given by:

A_i​ = β​ / 1 + β

where,

• 𝛽 is the transistors current gain in the common base configuration.

Input Impedance Z_in:

In a common base design the input impedance is often very low 𝑟_𝑒 (emitter resistance).

Zout the output impedance:

The main cause of the high output impedance is the collector resistor or RC.

One can follow these instructions to build and test a common base BJT amplifier suitable for use with an electronic stethoscope.

By following the basic equations one can also learn them.

How to Build:

To build a Simple Electronic Stethoscope Circuit following are the connection steps to be followed:

Choose the Components:

• Identify the qualities of the transistor, piezo sounder and other electronic parts one is using while designing the circuit.

Circuit for Transistor Amplifier:

• Connect the piezo sounder to the NPN transistors base.
• Configure the emitter follower system.
• Add the required resistors for biasing and connect the emitter to ground and the collector to the positive power source.

Capacitors:

• Capacitors are used for filtering and coupling.
• Capacitors can be connected in series to block DC components and only let AC signals pass through.

Source of Power:

• Connect the power source such as a 4.5V battery to the circuit.

Headphone Output:

• Use a 100μF capacitor to connect the low impedance headphones to the transistors collector.
• The boosted heartbeat signal will be heard here.

Testing:

• Turn on the circuit and listen carefully through the headphones.
• If required change the resistor values for best function.

Configuring a Darlington transistor is optional:

• Connect two transistors in series the collector of the first to the base of the second and alter the resistor values as needed if wished to use a Darlington transistor design in place of the emitter follower for better input impedance.

Be careful:

• Use the power supply voltage carefully to prevent component damage.
• To avoid short circuits verify the electrical connections.

Take note:

• Please be aware that this is only a simple instruction for better operation one needs to check the transistor datasheets and circuit design principles.
• If electronics are new than think about asking an expert for help or looking into other options.

Conclusion:

An electronic stethoscope circuit is designed to get over some of the disadvantages of standard stethoscopes such as for limited volume and poor sound quality.

These stethoscopes use electronic components to filter out background noise, boost weak sounds and give medical personnel a better and more accurate picture of interior body sounds.

In medical contexts electronic stethoscopes are often used for diagnosis and medical examination.

References:

A versatile low cost Electronic Stethoscope design

Digital stethoscope