Simple Electronic Stethoscope Circuit

This Simple Electronic Stethoscope Circuit teaches us about how to make one type of stethoscope circuit.

Normal stethoscope uses tube to listen small sound from body like heart and lung but this electronic stethoscope uses small electric parts to make sound more clear and loud.

The circuit also help doctors and nurses hear better what is happening inside the body.

This project is difficult for beginners to build and learn from, hence, only trained medical professionals should use stethoscopes and other medical tools.

Circuit Working:

Parts List:

Components	Values	Quantity
Resistors	470k 1/4 watt	2
	1k 1/4 watt	1
Capacitors	Electrolytic 100μF 16V	1
Semiconductors	Transistor BC547	1
	Headphones 64Ω	1
	Piezo transducer	1
	Battery 4.5V	1

This electronic stethoscope uses piezo sounder like the one from melody card or musical greeting card and it also works like a microphone.

Here, the amplifiers input impedance control the low frequency sound from the piezo and it gives about 100 mV output signal.

Also, to make low frequency sound better we should use emitter follower transistor amplifier because it have high input impedance.

Furthermore, this choice help to catch very low sound from transducer and we can use low impedance headphone just like plug into amplifier output and hear heart beating.

Finally, the circuit uses a Darlington transistor instead of a normal emitter follower to improve input impedance even more.

Formulas:

This part explain simple circuit using Bipolar Junction Transistor BJT in common base CB design.

Voltage Gain (Av):

Voltage gain in this design is almost 1 and input goes to emitter and output come from collector.

Formula is:

Av = RC / re

where:

• RC is collector resistor
• re is emitter resistance about VT / IE
• VT is thermal voltage to about 26 mV at room temperature
• IE is emitter current

Current Gain (Ai):

Formula is:

Ai = β / (1 + β)

where:

• β is transistors current gain

Input Impedance (Zin):

In CB design the input impedance is small which is almost equal to re.

Output Impedance (Zout):

Output impedance is high and mostly because of collector resistor RC and by using these steps and formulas we can build and test simple common base BJT amplifier good for electronic stethoscope.

How to Build:

To build a Simple Electronic Stethoscope Circuit follow the connection steps below:

Choose the Parts:

• First, see what kind of transistor, piezo sounder and other parts we want to use in circuit and choose of good quality.

Transistor Amplifier Circuit:

• Next, connect piezo sounder to base of NPN transistor and make emitter follower setup.
• After that, put needed resistors for biasing and connect emitter to ground and collector to positive side of battery.

Use Capacitors:

• Capacitors help with filtering and signal passing and they block DC and let AC signal go through and connect them in series for this.

Power Supply:

• Now give power to circuit using battery like 4.5V.

Headphone Output:

• Then connect 100μF capacitor from transistor collector to headphone with low impedance and we will hear boosted heartbeat sound here.

Testing Time:

• Next, turn ON the circuit and listen with headphone and if sound is not clear then try changing resistor values.

Darlington Transistor (Optional):

• Also, if want better input then use two transistors together and connect collector of first to base of second and also change resistors for best result.

Be Careful:

• Next, use right voltage for battery but too much power can break the circuit components; then check all wires and connections to avoid short circuit.

Important:

• This is just simple idea but for best working check transistor datasheet and circuit rules and if anyone is new in electronics then ask expert or try easy project first.

Conclusion:

Overall, this type of Simple Electronic Stethoscope Circuit fixes problems found in normal stethoscopes, such as low sound and noise; it uses electronic parts to make the sound louder and clearer, so doctors can hear body sounds better.

Also, in hospitals these stethoscopes help a lot to understand what happens inside a patient body.