Simple Electronic Stethoscope Circuit

This post explains how to build a circuit for a special type of stethoscope.

Regular stethoscopes use a tube to amplify faint sounds from your body, like your heartbeat and lungs.

This electronic stethoscope uses tiny electrical parts to make those sounds much louder and clearer.

This can be helpful for doctors and nurses to hear what is going on inside your body better.

Building this project is complex and not recommended for beginners.

It is also important to remember that medical devices like stethoscopes should be used by qualified professionals.

Circuit Working:

Parts List:

The electronic stethoscope circuit utilizes a piezo sounder sourced from a musical greeting card or melody generator as a microphone.

This transducer generates an output signal of approximately 100mV, and its low frequency response is determined by the input impedance of the amplifier.

To optimize the low frequency response, an emitter follower transistor amplifier is selected for its high input impedance.

This choice ensures that the transducer exhibits a very low frequency response.

To hear your heartbeat, simply connect a pair of low impedance headphones to the amplifier output.

Enhancing the input impedance of the amplifier can be achieved by substituting the emitter follower with a darlington transistor configuration.

Formulas:

The fundamental circuit above for an electronic stethoscope that uses a Bipolar Junction Transistor BJT in common base CB configuration can be explained by the following formulas :

Voltage Gain Av:

In a common base arrangement, the voltage gain is about equal to 1.

This is as a result of applying the input voltage to the emitter and removing the output voltage from the collector.

The voltage gain formula is:

A_v ​= R_C​​ / _re​

where,

• R_C is the collector resistor.
• r_e​ is the emitter resistance, which is approximately V_T / _IE​​
• V_T​ is the thermal voltage (26 mV at room temperature)
• I_E 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).

Output impedanceZ_out: The collector resistor R_C is the primary factor determining the high output impedance.

These instructions will help you construct and evaluate a common base BJT amplifier that is appropriate for an electronic stethoscope application.

You may also learn the fundamental equations by following them.

How to Build:

Building an electronic stethoscope using an emitter follower transistor amplifier involves several steps.

Identify Components:

• Understand the specifications of your piezo sounder, transistor and other electronic components.

Transistor Amplifier Circuit:

• Connect the piezo sounder to the base of the NPN transistor.
• Set up the emitter follower configuration.
• Connect the emitter to the ground, the collector to the positive power supply and add appropriate resistors for biasing.

Capacitors:

• Use capacitors for coupling and filtering purposes.
• Connect capacitors in series to block DC components and allow only AC signals to pass.

Power Supply:

• Connect the power supply e.g. 4.5V battery to the circuit.

Headphone Output:

• Connect the low impedance headphones to the collector of the transistor through 100μF capacitor.
• This is where you will listen to the amplified heartbeat signal.

Testing:

• Power up the circuit and carefully listen to the headphones.
• Adjust resistor values if needed to optimize the performance.

Optional: Darlington Transistor Configuration:

• If you want to replace the emitter follower with a darlington transistor configuration for increased input impedance, connect two transistors in series collector of the first to the base of the second and make necessary adjustments to the resistor values.

Caution:

• Be cautious with the power supply voltage to avoid damaging the components.
• Double check your circuit connections to prevent short circuits.

Note:

Please note that this is a basic guide, and you may need to refer to transistor datasheets and circuit design principles for a more precise implementation.

If you are unfamiliar with electronics consider seeking help from someone experienced or consulting additional resources.

Conclusion:

The purpose of an electronic stethoscope circuit is to overcome some of the limitations of traditional stethoscopes such as low volume and poor sound quality.

By using electronic components, these stethoscopes can amplify faint sounds, filter out background noise, and provide healthcare professionals with a more detailed and accurate representation of internal body sounds.

Electronic stethoscopes are widely used in medical settings for auscultation and diagnosis.

References:

A versatile low cost Electronic Stethoscope design

Digital stethoscope