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Need to do a bit of selfdiagnosis? Make sure your
heart is still beating or
check other body sounds?
Maybe you would like to
sort out some unusual
rattles or other noises in
your car’s engine or other
machinery?
This electronic stethoscope
will do the job – and you
can listen via headphones
or a loudspeaker. It has
switchable frequency
shaping in four bands so
you can hone in on sounds
which might otherwise
be masked out.
Features
• Portable
• Battery supply
• Volume control
• Boost or cut control
• Selectable tone frequency
• Reverse supply protection
• Power and battery condition
indicator
Electronic
Stethoscope
By
JOHN CLARKE
S
o why have an electronic stethoscope when a traditional cheap and cheerful stethoscope might be all
you need?
Well, a conventional stethoscope is OK if you have keen
hearing and you are listening in a quiet environment but its
sound levels are quite low, particularly at low frequencies.
Secondly, on a cheap stethoscope there is no way of
tailoring the frequency response of sound heard at the
earpiece (apart from choosing the diaphragm or bell on
the chestpiece of a medical stethoscope).
The SILICON CHIP Electronic Stethoscope has plenty of
gain – you can adjust the volume level to suit and you can
use switchable filtering to cut or boost a particular band of
frequencies. As well, it can be connected to headphones
siliconchip.com.au
or a loudspeaker, in which case more than one person can
hear the sounds.
If you wanted to, you could record the monitored sounds
and display the waveforms on computer screen.
Our Electronic Stethoscope comprises a chestpiece
(sound pickup) that connects to a small amplifier box via
a shielded cable. It has a headphone socket, knobs for
volume and equaliser (EQ) and switches for power and
for frequency band.
The equaliser provides boost or cut in the frequency
band selected by the 4-position slide switch.
These bands are centred on 63Hz, 250Hz, 1kHz and 4kHz
and are labelled Low, Mid1, Mid2 and High respectively.
To simulate the bell sensor (of a medical stethoscope)
August 2011 21
1k
4.7k
ELECTRET
BIAS
LK1
+8.6V
100nF
10
10
33pF
9
IC1c
VR3
100k
+8.6V
K
K
470 F
VOLUME
VR1
10k
LOG
100nF
5
6
4
IC1b
10k
7
3
1nF
1k
LED1
A
10 F
8
100k
IC1a
2
68pF
1M
A
ZD1
4.7V
A
2.7k
100 F
IC1: TL074
INPUT
CON1
K
D2
1N4148
1k
150
100 F
1
220nF
10k
100k
GAIN
47
47
1nF
VR2 50k
10 F
10k
CUT
10 F
BOOST
+4.3V
820nF
10k
10 F
LOW BAND: 63Hz
MID1 BAND: 250Hz
MID2 BAND: 1kHz
HIGH BAND: 4kHz
220nF
56nF
15nF
S2
220k
LOW
MID1
MID2
1.8k
13
HIGH
'BAND SELECT'
12
IC1d
11
SC
2011
ELECTRONIC STETHOSCOPE
18nF
4.7nF
1nF
270pF
Fig. 1: the circuit is based on two low-cost ICs – an amplifier (IC1a), buffer (IC1b), frequency band selection (IC1c and d)
and finally, a power amplifier capable of driving a set of headphones or ear buds (IC2). It’s all powered by a 9V battery.
where low frequency sounds are more prominent, the Mid2
band can be selected and the equaliser control set for an
amount of signal cut. Or the low band could be selected
with the equaliser control set in the boost position. To
simulate the chestpiece diaphragm, the high band can be
selected and the “EQ” pot set to the cut position.
Alternatively, any one of the bands can be selected by
the switch and the equaliser pot can be set to boost or
cut. Boosting the frequency band selected will make more
prominent any sounds of interest within that band.
Conversely, the cut position will remove prominent
sounds in that band that may otherwise mask out the
sounds of interest.
The chestpiece is adapted from a low cost stethoscope
but with a piezo transducer fitted inside.
For use with car engines or other machinery, the chestpiece is further modified to provide a more direct contact
with the piezo transducer element.
Circuit details
The Electronic Stethoscope
is based on two low-cost ICs:
a TL074 quad op amp IC and
an LM386 power amplifier IC.
The op amps are used for amplification and filtering of the
signal from the piezo element
in the chestpiece. The power
amplifier drives the head22 Silicon Chip
phones or a loudspeaker. The full circuit is shown in Fig.1.
Signal from the piezo element is applied via CON1,
3.5mm socket and a 100nF capacitor to op amp IC1c. The
associated 33pF capacitor and 10resistor attenuate high
frequencies and thereby reduce the possibility of picking up
radio signals. IC1c is biased at +4.3V via the 1Mresistor
connected to the 10kvoltage divider resistors across the
8.6V supply. The 1Mresistor also sets the input impedance of the amplifier.
Note that bias for an optional electret microphone is
included and is fed via link LK1 and 4.7kresistor to the
8.6V supply via a 1kresistor and 100F bypass capacitor.
(Electret bias is included so that the stethoscope can be
used in a different application. See the section entitled
“Using the stethoscope as an audio eavesdropper”).
IC1c is connected as a non-inverting amplifier with a
gain that can range from about two when trimpot VR3 is
set at 100kup to 101 when VR3 is set to its minimum
resistance. IC1c’s output is coupled to the volume control
potentiometer, VR1, via a
10F capacitor.
SPECIFICATIONS
The output of VR1
(wiper)
is fed to IC1b
Supply voltage:......... 9V <at> 12mA quiescent current
which is connected as
Battery life: ...............Typically 30 hours
a unity gain buffer. The
output from IC1b drives
(with alkaline battery)
the equaliser (EQ) stage
Selectable bands:......63Hz, 250Hz, 1kHz and 4kHz
consisting of op amps
Boost or cut range:...±15dB
IC1a and IC1d.
siliconchip.com.au
14
4
10 F
100k
4.7k
1M
VR1
10k
VOLUME
47
BAND
SELECT
18nF
S2
10
Fig,4: here’s how it all goes together on the PCB. Note how the twin wires
from the battery snap pass through the front of the board and back out again
– that’s for strain relieve on the solder joints (they could otherwise snap off).
Watch the IC, LED and electrolytic capacitor orientation; also make sure the
two pots aren’t mixed up. Finally, the jumper for the link (LK1) is only placed
in position for use with an electret microphone – it is not used at all for the
“normal” piezo version.
ZD1
A
K
1N4148
LED
A
K
1N5819
K
A
A
K
Switchable single band equaliser
These two op amps form a single band equaliser which
can boost or cut the signals in a defined frequency band
selected by the 4-position slide switch, S2.
The concept for the single band equaliser can be seen in
Fig.2. In essence, we have an op amp (IC1a) connected as
a non-inverting amplifier and a feedback network with a
potentiometer (VR2) which sets the amount of boost or cut.
The frequency band is defined by the resonant frequency
of the series-connected capacitor C1 and inductor L1.
With VR2 wound fully to the left, the tuned series LC
circuit is connected to IC1a’s input via a 47 resistor. At
SIGNAL
IN
CHESTPIECE
1nF
47nF
270pF
15nF
7
CON2
OUTPUT
1 1 1 8OUTPUT
0140
CON1
56nF
470 F
220k
4.7nF
5
LK1
A
K
1nF
220nF
470 F
10
47nF
820nF
OUTPUT
CON2
10
1.8k
IC2
LM386N
8
9V
BATTERY
100nF
1
LED1
47
IC1 TL074
33pF
1k
VR3
100nF
1nF
68pF
100 F
10 F
S1
POWER
10k
10k
100 F
+
2
6
ZD1
100k
100 F
3
4.7V
VR2
50k
EQ
220nF
10 F
D2
1k
10 F
10 F
IC2
LM386
D1
4148
EP O C S O HT E T S
9V
BATTERY
100 F
5819
2.7k
150
10 F
1k
470 F
POWER
10k
+
+
A
–
K
10k
D1 1N5819
(–)
S1
+8.6V
10k
SIGNAL
OUT
IC1a
IC1c
10k
47
47
Fig.2: the essence
VR2 50k
of an equaliser. A
series-resonant
CUT
BOOST
LC network
C1
(comprising L and
C1) and potentiometer (VR2) is
L
connected within
the IC1c op amp
feedback network.
siliconchip.com.au
Vin
Iin
C2
the resonant frequency, the impedance of the LC network
is at a minimum. Thus, the signal applied to IC1a will be
shunted to ground, reducing the signal at the IC1a output.
When VR2 is rotated to its boost setting, the LC network
is connected directly to the inverting (-) input of the op amp
via another 47resistor, shunting the negative feedback to
ground. At the resonant frequency, the low impedance of
the LC network reduces the feedback and the gain of IC1a
will increase.
The centre frequency of the circuit can be obtained from
the formula:
F0 = 1
2 L C1
In fact, our circuit does not use an inductor in the
equaliser as it would be very large and bulky. Instead we
have replaced the inductor with a gyrator. A “gyrator” is
a pseudo-inductor using an op amp and a capacitor, as
shown in Fig.3.
In an inductor, the current lags or is delayed by 90° with
respect to the voltage waveform. With a capacitor, however,
the voltage lags the current by 90°.
To simulate the inductor, the voltage lag of the capacitor
must be converted to a leading voltage compared to the current. With an AC signal applied to the input of the circuit
(Vin) of Fig.3, current will flow through capacitor C2 and
the resistor R2.
Because it is connected as a voltage follower,
I
out
R1 1.8k
the op amp will reproduce the voltage across
R2 at its output.
IC1d
R2
220k
Fig.3 (left): circuitry of a gyrator. The op amp
IC1d simulates an inductor by a phase
transformation of the current through C2. The
resulting inductance is equal to the product of
R1, R2 and C2.
August 2011 23
The three basic components of our new Electronic Stethoscope.
At left is a pair of standard heaphones – it will also work
with ear buds but we find ear-covering headphones best, as
they mask more external noise. Top right is the “works” while
at lower right is the chest-piece, itself made by modifying a
low-cost medical (acoustic) stethoscope. Inset top left is the
“mechanic’s” attachment we made to listen into machinery etc.
This voltage will now cause a current to flow in R1 and
this adds to the input current. The resulting total current
lags the input voltage by 90°. So as far as the signal source
is concerned, the circuit behaves like an inductor.
The value of simulated inductance is given by the equation: L = R1 x R2 x C2. By substituting the gyrator for the
inductor in the circuit of Fig.2, we have the basis for a
complete equaliser.
The 4-position slide switch, S1, selects different values
for C2 and C1 for each of the frequency bands.
+20
Stethoscope Frequency Response
06/22/11 10:29:09
+17.5
+15
+12.5
Amplitude Variation (dBr)
+10
+7.5
+5
+2.5
+0
-2.5
-5
-7.5
-10
-12.5
-15
-17.5
-20
20
LOW
50
MID1
100
200
MID2
500
1k
HIGH
2k
5k
10k
20k
Frequency (Hz)
Fig.5: boost and cut graph for
each band as set for maximum
boost and cut. Note that only one single band can be used at
one time in either boost or cut position. Boost or cut can be
set to any level between the two extreme boost or cut levels.
24 Silicon Chip
Following the equaliser stage, the signal is fed via a 220nF
capacitor to the non-inverting input (pin 3) of IC2, the LM386
audio power amplifier. IC2 can provide about 500mW into
8with a 9V supply and distortion is typically 0.2%. When
using stereo 32headphones (with the earpieces connected
in parallel to give a 16load), the power is about 250mW;
more than enough to provide sufficient listening volume.
IC2 drives the output load via a 470F capacitor and a
Zobel network, comprising a 10resistor and 47nF capacitor, which helps prevents amplifier instability.
The power for the stethoscope comes from a 9V alkaline
battery, with diode D1 providing protection against a reverse
polarity connection. A Schottky diode is used due to its
low forward voltage loss (about 0.3V compared to a normal
silicon diode’s 0.6V).
LED1 has two functions: to show power ‘on’ and to show
battery condition. It operates as follows. When power is first
applied, current for the LED flows through the 4.7V zener
diode (ZD1), the 1kresistor and the discharged 470F
capacitor. If the battery is fresh, the 9V battery provides
8.7V at the anode of LED1. This voltage is reduced by about
1.8V by LED1 and 4.7V by ZD1, leaving 2.2V across the 1k
resistor. LED1 lights with 2.2mA.
At lower battery voltages, there is less voltage across the
1k resistor so the LED is dimmer. At a battery voltage of
7V, there is about 0.2V across the 1kresistor and the LED
barely lights.
As the 470F capacitor charges up, the LED current is reduced to much lower level, set by the 2.7kresistor across the
capacitor. This indicates that the Electronic Stethoscope is
switched on without wasting significant power. When power
is switched off, diode D2 discharges the 470F capacitor so
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Inside the assembled Electronic Stethoscope
“works”, reproduced here close to same size.
Use this in conjunction with the component
overlay (fig.4).
the circuit is again ready to indicate the battery charge state
when it is turned back on.
The 8.7V supply is connected directly to IC2 but it is fed
to IC1 via a 150resistor. A 100F capacitor decouples this
supply and removes any supply modulation from IC2 which
could otherwise cause instability. This would take the form
of audible “motor-boating”.
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Construction
With the exception of the piezo mounted on the chestpiece,
all the Electronic Stethoscope components are accommodated on a single PCB, coded 01108111 and measuring 65 x
86mm. In turn, the PCB is housed in a black plastic “remote
control” case measuring 135 x 70 x 24mm.
The PCB is designed to mount onto the integral mounting
bushes within the box. Make sure the front edge of the PCB is
shaped to the correct outline so it fits into the box. It can be
filed to shape if necessary using the PCB outline as a guide.
Begin construction by checking the PCB for breaks in
tracks or shorts between tracks or pads. Repair any defects,
if necessary. Check the sizes for the PCB mounting holes and
for the battery leads. These are 3mm in diameter.
siliconchip.com.au
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Kickboard lighting
Colour changing & effects via remote control.
Sets the mood & atmosphere for your venue.
Website: www.tenrod.com.au
E-mail: sales<at>tenrod.com.au
Sydney:
Melbourne:
Brisbane:
Auckland:
Tel. 02 9748 0655
Tel. 03 9886 7800
Tel. 07 3879 2133
Tel 09 298 4346
Fax. 02 9748 0258
Fax. 03 9886 7799
Fax. 07 3879 2188
Fax. 09 353 1317
August 2011 25
End-on view showing the three controls (Eq, Volume and
Power) on the end panel and the four-way filter band
selection switch on the front panel.
The component overlay for the PCB is shown in Fig.4
You can start assembly by the inserting the resistors. Check
each resistor value against the colour code table as you go
and double-check with a digital multimeter. Next, install the
two PC stakes followed by the diodes, mounted as shown.
IC1 & IC2 can be directly mounted on the PCB, or if you
wish can be mounted on DIP8 sockets. When installing ICs
(and sockets if you use them), take care to orient them correctly. Orientation is with the notch positioned as shown.
Switch S2 does not mount directly onto the PCB but is
raised off the PCB using a 6-way dual row pin header. Remove
a pair of pins so that there is a row of three pins, then a gap
then two pins on each side of the DIL header.
The header pins are longer at the top than the bottom.
Push them down so that the tops are 5mm above the bottom
of the plastic section and solder it in the switch mounting
position. The switch is mounted by soldering its pins to the
top of the header pins. The switch must be oriented correctly
with the row of three pins toward the volume pot (VR1).
The top of the switch body should be 12mm above the PCB.
The capacitors can be mounted now. The electrolytic
types must be oriented correctly – the polarity is shown
on the component overlay. Make sure these capacitors are
placed in the PCB so their height above the board surface
is no more than 12.5mm otherwise the lid of the case will
not fit correctly.
The potentiometer (VR2) and the PCB mounted switch
S1 can also be fitted now, along with the 3.5mm sockets.
LED1 mounts horizontally but at a height of 6mm above
the PCB. Bend its leads at 7mm back from the base of the
LEDs at 90° making sure the anode lead is to the left.
When assembled, the PCB is secured to the base of the
case using four M3 x 6mm screws that screw into the integral
mounting bushes in the box. Before putting this in place, drill
out the small front panel for the LEDs, potentiometer and
switch. A drill guide is provided with the front panel label.
Holes are also required in the base and case lid for the
3.5mm sockets. A rat-tail file can be used to make these
cut outs.
The panel label for this project can be downloaded from
the SILICON CHIP website (www.siliconchip.com.au). Go to
the downloads section and select the month and year of
publication.
When downloaded, you can print onto paper, sticky
backed photo paper or onto plastic film. Paper labels need
protection, so cover them with self-adhesive clear plastic
or, best of all, hot laminate film.
When using clear plastic film (overhead projector film)
you can print the label as a mirror image so that the ink is
behind the film when placed onto the panel. Once the ink
is dry, cut the label to size.
The paper or plastic film is glued to the panel using an
even smear of neutral cure silicone sealant or spray contact
adhesive. If you are glueing a clear plastic film label to a
black coloured panel, use coloured silicone such as grey
or white so the label can be seen against the black.
A rectangular hole in the panel is required directly above
the slider switch S2. The positioning for this is shown on
the label. This shape can be first drilled in the plastic lid
and then once the panel label is affixed, the cut the panel
hole out using a sharp hobby knife. The top of the switch
can be coloured black using a permanent marker pen to
improve the appearance through the switch hole.
If you require the Stethoscope to be secured to a belt, a
suitable belt clip is available from Altronics (cat no H0349).
Contact www.altronics.com.au
Chestpiece
The chestpiece for the Electronic Stethoscope is cannibalised from a commonly-available (and low cost) acoustic
medical stethoscope. Ours came from Jaycar Electronics
(www.jaycar.com.au), cat no QM7255 <at> $14.95 but most
chemists and medical supply houses have them.
You can pay a lot more for a stethoscope – for example the
Littman Cardiology III, manufactured by 3M, sells for more
Fig.6: we used a
commercially available
stethoscope to obtain the chestpiece
for our electronic version, then fitted it
with a piezo transducer and a cable with
3.5mm jack plug to the amplifier.
26 Silicon Chip
siliconchip.com.au
PROBE STEM
MADE FROM
2mm DIAM
BRASS ROD
43mm DIAMETER
DISC OF 1mm
ALUMINIUM
Parts List – Electronic Stethoscope
PROBE TIP MADE
FROM 6mm LENGTH
OF 6.5mm DIAM.
BRASS ROD
12mm LONG
M3 TAPPED
SPACER
6mm LONG
M3 SCREW
Fig. 7: to listen in to engines and other
equipment you’ll need something like
this probe. It transmits vibrations etc
direct to the piezo transducer of the
chestpiece.
than $150. But we’re not interested
in specialised models, the common
or garden-variety stethoscope is what
we’re after.
The following applies specifically
to the Jaycar model but you will probably find that most of the low-cost
stethoscopes use a similar method of
construction.
The diaphragm section is removed
from the chestpiece to access the
inside of the casting. Unscrewing the
outer annulus from the rear casting
does this.
The piezo element from a piezo
transducer is used as the detector and
is placed within the chestpiece diecast
housing.
The piezo transducer is available
from Jaycar, cat. no AB-3440 or from
Altronics, cat no S 6140.
We did test the stethoscope using an
electret microphone (Jaycar AM4008)
for the chestpiece pickup sensor. This
was a very small microphone at 6mm
in diameter and 3.5mm deep that fits
within the back of the diecast moulding.
This proved to be unsatisfactory for
this application, although there was
nothing wrong with the microphone
itself.
The main problem was that it would
detect far more than was required for
a stethoscope including detection of
noises from adjacent rooms. The use
of an electret microphone, however, is
ideal for use as an eavesdropper. See
the separate section concerning its use.
A piezo element proved to produce a
much better result. The piezo element
is removed from its plastic transducer
housing.
To do this, firstly remove the backing
siliconchip.com.au
1 PCB, coded 01108111, 65 x 86mm
1 remote control case 135 x 70 x 24mm (Jaycar HB5610 or equivalent)
1 panel label 50 x 114mm
1 9V battery
1 9V battery clip lead
1 low cost stethoscope (Jaycar QM7255) used for parts
1 miniature PC mount SPDT toggle switch (Altronics S 1421 or equivalent)
(S1)
1 DP4T switch (Tyco Electronics STS2400PC04) (Element14
Cat.1291137) (S2)
1 10k log potentiometer, 9mm square, PCB mount (VR1)
1 50k linear potentiometer, 9mm square, PCB mount (VR2)
2 knobs to suit potentiometers
2 PC mount 3.5mm stereo sockets (CON1,CON2)
1 3.5mm mono line jack plug
1 DIP8 IC socket (optional)
1 DIP14 IC socket (optional)
1 piezo transducer (Jaycar AB-3440, Altronics S 6140)
1 PAL (Belling Lee) line plug with plastic housing (Jaycar PP0600)
(required for the metal crimp shield connector)
4 M3 x 6mm screws
1 M2 x 3mm screw (or a cut down M2 x 8mm screw)
1 6-way DIL pin header
1 2-way pin header with 2.54mm spacing (with jumper shunt)
2 PC stakes
1 60mm length of 10mm diameter heatshrink tubing
1 750mm length of single core shielded cable
Semiconductors
1 TL074 quad op amp (IC1)
1 LM386N amplifier (IC2)
1 1N5819 1A Schottky diode (D1)
1 1N4148 switching diode (D2)
1 4.7V 1W zener diode (ZD1)
1 3mm high intensity red LED (LED1)
Capacitors
2 470F 16V PC electrolytic
3 100F 16V PC electrolytic
5 10F 16V PC electrolytic
1 820nF MKT polyester
2 220nF MKT polyester
2 100nF MKT polyester
1 56nF MKT polyester
1 47nF MKT polyester
1 18nF MKT polyester
1 15nF MKT polyester
1 4.7nF MKT polyester
3 1nF MKT polyester
1 270pF ceramic
1 68pF ceramic
1 33pF ceramic
Mechanic’s
Stethoscope –
Optional Parts
1 43mm diameter circle of 1mm
aluminium
1 M3 x 12mm tapped brass spacer
1 M3 x 6mm countersunk screw
1 brass rod 2mm diameter x 40mm
long with a 6mm brass spacer tip or
1 top end from a telescopic antenna
Audio Eavesdropper
– Optional Parts
1 electret microphone insert (9.5mm
diameter) (Jaycar AM-4010 or sim)
1 300mm length of single-core
shielded cable
1 3.5mm mono line plug
1 IP68 waterproof gland for 4-8mm
diameter cable
1 plastic cylinder 125ID x 157mm long
(eg empty 100 x CD container)
or similar (see text)
1 timber handle 65 x 115mm
(eg, pine off cuts or similar)
2 wood screws (to secure handle)
Resistors (0.25W, 1%)
1 1M
1 220k 2 100k
4 10k
1 4.7k
1 2.7k
1 1.8k
3 1k
1 150
2 47
2 10
1 100kmultiturn top adjust trimpot (VR3)
Miscellaneous
Earphones or headphones, neutral cure silicone sealant, solder.
August 2011 27
These three photos show, respectively (from left) the disassembled chest piece
with the piezo fitted; the reassembled chestpiece with the cable going off to the
amplifier fitted into a short length of the tubing from the original stethoscope
and finally a close-up of the “clamp” (actually the wire clamp from a TV cable
plug) used to hold it all together.
piece from the housing to expose the
transducer. The transducer is easily
prised out as it is glued to the housing
with a soft rubber-based adhesive. Take
care not to crack the piezo element.
Wires connected to the transducer
are removed by melting the solder
from the metal disk and piezo element
itself. Remove the solder with some
solder wick.
The piezo element is attached to the
chestpiece using an M2 x 3mm screw
that is tapped into the chestpiece casting. Drill a 2mm hole in the edge of the
transducer but away from the piezo
material itself and align the transducer
central to the chestpiece housing.
Mark out where the mounting hole
is required. Drill a 1.5mm hole (1/16”)
and screw the M2 screw into the hole.
You may need to file a small notch
along the M2 screw thread to act as
a makeshift thread cutter if the screw
does not enter the hole easily. (Of
course, if you happen to have an M2
tap, use that!)
Once the hole is ‘tapped’, remove
the screw. The piezo sensor is placed
onto the chestpiece housing with the
piezo element facing inward.
The core wire of the shielded cable
passes through the metal tubing of the
chestpiece and is soldered to the centre
of the piezo element.
The shielding wire is soldered to
the end of the metal tube after firstly
filing out a small flat landing on the
side of the tubing to allow for a solder
joint. Secure the transducer with the
M2 screw.
A smear of neutral cure silicone
sealant (eg, roof and gutter sealant)
is applied around the outside of the
transducer to form an air seal to the
chestpiece housing.
A short (60mm) length of the tubing
from the low-cost stethoscope is cut
and slid over the shielded cable and
onto the metal tubing of the chestpiece.
The tubing is crimped to the shielded
cable wire – we used the crimp section
of a PAL (Belling Lee connector) placed
over the tubing.
This is squeezed down over the tubing to grip the shielded cable in place
within the tubing. A 20mm length of
10mm diameter heatshrink tubing is
shrunk down over the section to hold
the crimp fingers closed.
The diaphragm and annulus can
now be reattached to the chestpiece
housing by screwing this back together.
RESISTOR COLOUR CODES
1
1
1
1
1
1
1
1
1
1
1
No.
1
1
2
1
1
1
1
3
1
2
2
Value
1MΩ
220kΩ
100kΩ
10kΩ
4.7kΩ
2.7kΩ
1.8kΩ
1kΩ
150Ω
47Ω
10Ω
28 Silicon Chip
4-Band Code (1%)
brown black green brown
red red yellow brown
brown black yellow brown
brown black orange brown
yellow purple red brown
red purple red brown
brown grey red brown
brown black red brown
brown green brown brown
yellow purple black brown
brown black black brown
5-Band Code (1%)
brown black black yellow brown
red red black orange brown
brown black black orange brown
brown black black red brown
yellow purple black brown brown
red purple black brown brown
brown grey black brown brown
brown black black brown brown
brown green black black brown
yellow purple black gold brown
brown black black gold brown
The opposite end of the shielded cable
is terminated to a 3.5mm mono jack
plug.
Mechanic’s stethoscope
attachment
For the mechanic’s attachment, we
used a 43mm diameter disk of 1mm
aluminium to replace the flexible diaphragm of the chestpiece.
This means that the annulus is unscrewed and the flexible diaphragm
removed by pressing it out with using
your fingers.
A rod attaches through the centre of
this aluminium disk to provide contact
with the machinery. We supported our
rod using an M3 tapped brass spacer
secured to the disk with an M3 x 6mm
screw. To this spacer is soldered a brass
rod with a tipped end.
We used the end from a discarded
telescopic antenna for the rod and
soldered this to the 12mm spacer. The
rod is 60mm long but it could be longer
than that if you need it to be.
An alternative tip could be made
from a length of 2mm diameter brass
rod and a 6mm long brass standoff.
These parts are then soldered together.
The aluminium disk is held in place
Capacitor Codes
Value
820nF
220nF
100nF
56nF
47nF
18nF
15nF
4.7nF
1nF
270pF
68pF
33pF
F Value IEC Code EIA Code
0.82F
820n
824
0.22F
220n
224
0.1F
100n
104
0.056F
56n
563
0.047F
47n
473
0.018F
18n
183
0.015F
15n
153
0.0047F
4n7
472
0.001F
1n
102
270p
271
68p
68
33p
33
siliconchip.com.au
using the anulus in the same way as
the diaphragm.
Testing
Testing can be done with the 9V
battery connected. Apply power and
check that the power LED momentarily
lights brightly when switched on and
then dims.
Wind the VR1 volume control fully
anticlockwise and set the tone control
to mid position. This will prevent IC2
from producing large signal levels
with no input connected. This allows
DC voltages to be tested without being
masked by a large AC signal.
For a 9V battery supply, we measured
8.7V at the cathode of D1, 7.7V at pin 4
of IC1 and 8.7V at pin 6 of IC2 with the
multimeter’s negative probe connected
to the casing of one of the 3.5mm jack
sockets. A half supply voltage of around
4.3V should be at pins 1, 7, 8, and 14
of IC1 and at pin 5 of IC2.
Make sure the jumper link (LK1)
for the electret microphone bias is not
inserted for the piezo element of the
chestpiece.
Connect the chestpiece and headphones or earpieces to the Stethoscope.
Set the volume about mid way and adjust VR3 for a suitable level of volume
when monitoring the heart beat on
the left side of your chest. Rotate VR3
clockwise for more gain and anticlockwise for less gain.
Check that the tone can be adjusted
to boost and cut the selected band of
frequencies. This will be evident on the
low setting as the heart beat thump is
boosted or cut. On the high band more
hissing sound will be produced on
boost but reduced on cut.
For an idea of what various body
sounds make, log onto www.easyauscultation.com. You can then try and
find those sounds using your SILICON
CHIP Electronic Stethoscope.
Using the stethoscope as an
audio eavesdropper
The stethoscope can be used to
monitor sounds from a distance using
an electret microphone mounted in an
open ended container instead of the
piezo element within the chestpiece.
With this setup you can listen to bird
or animal calls (or virtually anything
else) at a distance.
The container provides directional
sound response, where sound enters
the open ended container to be received
by the microphone.
siliconchip.com.au
Designing your own single-band equaliser
While most users will be satisfied
with the four frequency bands selected
for the equaliser, there may be some
who require different bands. To satisfy
this, we have included a method to
design your own equaliser section.
Fig.9 shows the typical bandwidth
of an equaliser section under boost.
The centre of the band is designated
F0 while the frequencies where the
response is 3dB down from the F0 level
are shown as F1 and F2.
To design for a particular frequency
band you can use the equation:
AMPLITUDE
1.00
–3dB
0.707
0
F1
F0
F2
FREQUENCY
Fig.9: typical bandwidth of an
equaliser section under boost.
1
L=
42 C1 F02
This is to obtain a value for the inductance L, using selected values for C1
and F0. The equation is just a rearrangement of the standard
F0 = 1
2 L C1
Knowing the inductance enables us to calculate the required value for CL.
Use the equation
C =
L
L
R1 R2
For our circuit we used a 1.8kresistance for R1 and 220kresistance for R2.
The Q of the circuit determines the two frequencies either side of F0 where
the signal drops off in level by 3dB. You can calculate the Q using this equation:
Q=
2 F0 L
R1
The Q is also found using the equation
Q=
F0
F2-F1
although the equations to find F1 and F2 are more difficult. A useful calculator to find F1 and F2 is at:
www.sengpielaudio.com/calculator-cutoffFrequencies.htm
The tables below show the components used in the stethoscope, the inductance, Q and F1 and F2 for the four bands. You can use these values to
practice calculating the values for L and C2.
F0
63Hz
250Hz
1kHz
4kHz
Required
F0
63Hz
250Hz
1kHz
4kHz
CL
18nF
4.7nF
1nF
270pF
R1
1.8k
1.8k
1.8k
1.8k
Calculated
F0
65Hz
248Hz
1068Hz
3.974kHz
R2
220k
220k
220k
220k
C1
820nF
220nF
56nF
15nF
Q F1 F2 L
1.62
1.61
1.49
1.53
48Hz
188Hz
768Hz
2.882kHz
88Hz
337Hz
1485Hz
5.479kHz
7.13H
1.86H
396mH
107mH
August 2011 29
into the Electronic Stethoscope and
note that for this application (or any
other using the electret microphone),
the electret bias needs to be selected
by inserting LK1.
A parabola?
For maximum concentration of
sound at the microphone, a parabolic
dish should be used with the microphone mounted at the focal point.
We’ve used a number of different
parabolic or near-parabolic shaped
dishes in the past.
Design of the pickup using a ‘parabolic’ shaped dish and electret microphone is shown in the Ultrasonic
Eavesdropper article from August 2006
by Jim Rowe. While that design was to
receive ultrasonic sounds and convert
them to the normal audio band, the
pickup arrangement is the same for
the audio sound band.
One alternative parabola which we
haven’t tried (but should be near perfect!) is a metal cooking wok, available
quite cheaply from oriental food suppliers. To find the focal point, shine a
single-point light source (eg, a LED)
into the wok along its centre line. As
you move it in and out, at one point
the light will appear to “fill the dish” –
that’s the focal point. (See “Ask Silicon
Chip” November 1994, page 93).
We’ll leave the mechanical arrangement for mounting the microphone
up to you.
SC
To eavesdrop on birds and animals,
we made this “sound gun” from an
old CD stack pack – but just about
any cylinder would do. The idea is to
prevent sound entering from the sides.
Ideally, for maximum sound pickup the
shape should be a parabola with the
mic insert at the focus but in practice
we found it really doesn’t make a great
deal of difference.
Sounds coming from the side and
rear of the container are reduced in
level before reaching the microphone.
Our CD pack sound gun
Construction is straightforward and
is shown the photo and in Fig.8. It’s not
at all critical – you can use whatever
is available.
The closed end of the cylinder is
drilled out to accept the IP68 gland.
This gland neatly houses the electret
while the clamp end secures the
shielded cable.
Strip the ends of the shielded cable
and solder the wires of one end to the
microphone insert connection pads.
The shielded wires connected to the
insert’s case pad and the inner wire to
the other pad.
The opposite wire end passes
through the gland and is clamped
down with the electret inserted into
the open gland end. The wire is then
terminated to a 3.5mm mono jack plug.
A handle was fashioned from an
off cut of timber (we reshaped the
decorative top section of a cyprus pine
picket) and secured this to the side of
the cylinder with self tapping screws.
The shape of the handle is not criti30 Silicon Chip
cal so long as it is comfortable to hold.
The handle can be finished with olive
oil rubbed into the timber before wiping off the excess.
The unit is now ready to test. Plug
CABLE GLAND
OPEN END
OF CYLINDER
TOWARDS
SOUND
SOURCE
ELECTRET
MICROPHONE
INSERT
SCREWS
CYLINDER
Fig.8: our “sound gun” fashioned
from an old blank CD bulk case and
a wooden handle. In this case, the
electret microphone is used rather
than the piezo – but make sure that
LK1 is in place on the PCB to provide
bias voltage for the electret. It won’t
work otherwise!
HANDLE
SINGLE CORED
SHIELDED CABLE
TO 3.5mm
JACK PLUG
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