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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:

CategoryDescriptionQuantity
Resistors
470k 1/4 watt2
1k 1/4 watt1
Capacitors
Electrolytic 100μF 16V1
Semiconductors
Transistor BC5471
Headphones 64Ω1
Piezo transducer1
Battery 4.5V1

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:

Av ​= RC​​ / re​

where,

Current Gain Ai:

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

Ai​ = β​ / 1 + β

where,

Input Impedance Zin:

In a common base design the input impedance is often very low 𝑟𝑒 (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:

Circuit for Transistor Amplifier:

Capacitors:

Source of Power:

Headphone Output:

Testing:

Configuring a Darlington transistor is optional:

Be careful:

Take note:

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

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