Electronic Stethoscope 2

Circuit Description:

. U1a operates as a low-noise microphone preamp. Its gain is only about 3.9 because the high output impedance of the drain of the FET inside the electret microphone 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 operates 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 reduced to 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.


PRIVATE/Part

Total Qty. Description

R1
1
10K 1/4W Resistor


R2
1
2.2K 1/4W Resistor


R3, R9
0
Not used


R4
1
47K 1/4W Resistor


R5, R6, R7
3
33K 1/4W Resistor


R8
1
56K 1/4W Resistor


R10
1
4.7K 1/4W Resistor


R11
1
2.2K to 10K audio-taper (logarithmic) volume control


R12
1
330K 1/4W Resistor


R13, R15, R16
3
1K 1/4W Resistor


R14
1
3.9 Ohm 1/4W Resistor


C1, C8
2
470uF/16V Electrolytic Capacitor


C2
1
4.7uF/16V Electrolytic Capacitor


C3, C4
3
0.047uF/50V Metalized plastic-film Capacitor


C5
1
0.1uF/50V Ceramic disc Capacitor


C6, C7
2
1000uF/16V Electrolytic Capacitor


U1

U2, U3

U4

U5
1

0

1

1
TL072 Low-noise, dual opamp

Not used

741 opamp

LM386 1/4W power amp

MIC
1
Two-wire Electret Microphone


J1
1
1/8" Stereo Headphones Jack


LED
1
Red/green 2-wire LED


Batt1, Batt2
2
9V Alkaline Battery


SW
1
2-pole, single throw Power Switch


Misc.
1
Stethoscope head or jar lid, Rubber Sleeve for microphone.

Assembly:

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.