Electronic Stethoscope
By Audioguru
Parts
List:
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.
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.
copied from Audioguru Article
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