Simple Stethoscope Circuit Diagram

The stethoscope is a medical or veterinary use tool, in which the professional can hear the heartbeat, breath or other bodily sound of his patient. We can say that the operation of the stethoscope is simple, the recorded sound is amplified and taken directly to the ear by a pipe. The stethoscope is also called phonendoscope, it has two different pickups sound the bell and diaphragm.

The traditional stethoscope is an acoustic equipment, and its pickup of sounds, are specific, the bell and one of the pickups that come in contact with the body, and its function is to capture bass. Have the diaphragm is used to capture the treble, all for a physical process, without the use of electronic circuits.
The Electronic Stethoscope

An electronic stethoscope or estetofone , has the ability to overcome low levels of noise, electronically amplifying body sounds. But this electronic system turns out to be limited the microphone audio frequency responses (pickup) and speaker, something that does not occur in acoustic stethoscopes.

Still there are a large number of companies offering electronic stethoscopes that rely on conversion of acoustic sound waves into electrical signals that can be amplified and processed making a faithful sound system, almost equal to the acoustic. Electronic stethoscopes sold commercially using various systems, the most common is a piezoelectric crystal placed in behind a foam rubber membrane as a pickup. 

Simple Stethoscope Circuit Diagram

The evolution of electronic stethoscopes also made possible the emergence of new tests as the phonocar diogram and telemedicine which allowed remote diagnostics.

The circuit simpler and less effective sound detection is achieved by a set of a microphone, an amplifier and a speaker or earphone. This method suffers interference from environmental noise but can be used as a simple electronic stethoscope. Precisely, the simplest is that we show here in this article.

Above the electronic stethoscope circuit, its operation is as follows, one capsule, electret microphone is used as sound pickup, the U1A integrated circuit which is a TL072 has the low noise pre-amplifier function. Your gain is less than 3.9, which is due to the high output impedance of the electret microphone.

The capacitor C2 has a relatively large capacitance, as it filters, leaving only pass the low frequency (20-30 Hz) that are the sounds of a beating heart. The U1B mounted on a Sallen-Key filter low noise with a cutoff frequency of about 103 Hz. R7 and R8 set the gain level of the order of 1.6. The integrated circuit U5 which is a LM386 audio amplifier with an output of 0.25 watts. Part of the U4 integrated circuit can be optional, it is an operational amplifier 741 that controls the two-color LED.

Electronic Stethoscope components list

R1 - 10k ¼ W resistor
R2 - 2.2 ohm ¼ W resistor
R3, R9 - Not used
R4 ¼ W 47 Ohm resistor
R5, R6, R7 - 33K resistor ¼ W
R8 - 56 Ohm resistor ¼ W
R10 - 4k7 ¼ W resistor
R11 - variable resistor of 2.2 logarithmic
R12 - resistor 330 Ohm ¼ W
R13, R15, R16 1K resistor ¼ W
R14 - resistor 3.9 ohm resistor ¼ W
C1, C8 - electrolytic capacitors 470 uF / 16 V
C2 - electrolytic capacitor 4.7 uF / 16 V
C3, C4 - Polyester Capacitors 0.047 uF / 50 V
C5 - ceramic disc capacitor 0.1 uF / 50 V
C6, C7 - electrolytic capacitors 1000 uF / 16 V
U1 TL072
U2, U3 Not used
U4 - 741
U5 - LM386
MIC - electric microphone three terminals

The circuit is very simple and can be mounted on breadboard, on the microphone connection with the circuit should be used shielded cable to prevent noise pickup. You must mount the pickup so that the microphone is placed at a distance from the skin surface but one that is close to the body to protect the microphone from external noise.

Keep the microphone away from the headphones to avoid feedback. Unfortunately, the device offers a very limited application, use it only as a learning circuit, test or demonstration