Saturday, August 18, 2012

Electronic Stethoscope Circuit

This is an electronic stethoscope circuit diagram. The operation of the circuit is very simple. It's work is similar to general home audio system. The heartbeat sounds catched by electret microphone and then the amplifier amplify the signal.

Component List:
R1 = 10K 1/4W
R2 = 2.2K 1/4W
R3, R9 = Not used
R4 = 47K 1/4W
R5, R6, R7 = 33K 1/4W
R8 56K = 1/4W
R10 = 4.7K 1/4W
R11 = 2.2K to 10K audio-taper (logarithmic) volume control
R12 = 330K 1/4W
R13, R15, R16 = 1K 1/4W
R14 = 3.9 Ohm 1/4W
C1, C8 = 470uF/16V Electrolytic Capacitor
C2 = 4.7uF/16V Electrolytic Capacitor
C3, C4 = 0.047uF/50V Metalized plastic-film Capacitor
C5 = 0.1uF/50V Ceramic disc Capacitor
C6, C7 = 1000uF/16V Electrolytic Capacitor
U1 = TL072 Low-noise, dual opamp
U2, U3 = Not used
U4 = 741 opamp
U5 = LM386 1/4W power amp
MIC = Two-wire Electret Microphone
J1 = 1/8" Stereo Headphones Jack
LED = Red/green 2-wire LED
Batt1, Batt2 = 9V Alkaline Battery
SW 2-pole, single throw Power Switch
Misc. Stethoscope head or jar lid, Rubber Sleeve for microphone.

How the Circuit Work:
U1a runs as a low-noise mic pre-amplifier. Its gain is only about 3.9 because the high output impedance of the drain of the FET inside the electret mic causes U1a’s effective input resistor to be about 12.2K. C2 has a fairly high value in order to pass very low frequency (about 20 to 30Hz) heartbeat sounds.

U1b runs as a low-noise Sallen and Key, Butterworth low-pass-filter with a cutoff frequency of about 103Hz. R7 and R8 provide a gain of about 1.6 and allow the use of equal values for C3 and C4 but still producing a sharp Butterworth response. The rolloff rate is 12dB/octave. C3 and C4 can be replaced with 4.7nF to increase the cutoff frequency to 1KHz to hear respiratory or mechanical (automobile engine) sounds.

The U4 circuit is optional and has a gain of 71 to drive the bi-colour LED.

U5 is a 1/4W power amplifier IC with built-in biasing and inputs that are referred to ground. It has a gain of 20. It can drive any type of headphones including low impedance (8 ohms) ones.

Assemble Notes:
  1. Assemble the circuit using Veroboard (stripboard) or a PCB.
  2. Use a shielded cable for the microphone as shown on the schematic.
  3. Fasten the microphone to the stethoscope head with a rubber isolating sleeve or use a short piece of rubber tubing on its nipple. A thick jar lid can be used as a stethoscope head. The microphone must be spaced away from the skin but the stethoscope head must be pressed to the skin, sealing the microphone from background noises and avoiding acoustical feedback with your headphones.
  4. The microphone/stethoscope head must not be moved while listening to heartbeats to avoid friction noises.
  5. Protect your hearing. Keep the microphone away from your headphones to avoid acoustical feedback.

About Audio Amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 - 20 000 Hz, the human range of hearing) to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.

The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification, equalization, tone control, mixing/effects, or audio sources like record players, CD players, and cassette players. Most audio amplifiers require these low-level inputs to adhere to line levels.

While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens, hundreds, or thousands of watts. More explanation about power audio amplifier can be found at

This is a video tutorial about how to a very simple audio amplifier based on the LM386 amplifier chip. It can be built for less than $20 (or might be less than $8 in some countries) and used to amplify any low level audio signal including a guitar, bass or mp3 player.

Watch the video:

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