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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 U1as 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.
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Part |
Total Qty. |
Description |
|
R1 |
1 |
10K 1/4W Resistor |
|
R2 |
1 |
2.2K 1/4W Resistor |
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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 |
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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 |
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C2 |
1 |
4.7uF/16V Electrolytic Capacitor |
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C3, C4 |
3 |
0.047uF/50V Metalized plastic-film Capacitor |
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C5 |
1 |
0.1uF/50V Ceramic disc Capacitor |
|
C6, C7 |
2 |
1000uF/16V Electrolytic Capacitor |
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U1
U2, U3
U4
U5 |
1
0
1
1 |
TL072 Low-noise, dual opamp
Not used
741 opamp
LM386 1/4W power amp
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MIC |
1 |
Two-wire Electret Microphone |
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J1 |
1 |
1/8" Stereo Headphones Jack |
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LED |
1 |
Red/green 2-wire LED |
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Batt1, Batt2 |
2 |
9V Alkaline Battery |
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SW |
1 |
2-pole, single throw Power Switch |
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Misc. |
1 |
Stethoscope head or jar lid, Rubber Sleeve
for microphone. |
Assemble the circuit using Veroboard (stripboard)
or a PCB.
Use a shielded cable for the microphone
as shown on the schematic.
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.
The microphone/stethoscope head must not
be moved while listening to heartbeats to avoid friction
noises.
Protect your hearing. Keep the
microphone away from your headphones to avoid acoustical
feedback.
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