Electronic Building Blocks
By Julian Edgar
Quick and easy construction
Great results on a low budget
Electronic Stethoscope for Machinery
Here’s a great beginner’s project – easy and low cost to build and very useful in
diagnosing all sorts of noises in machinery and cars.
The author listening to the fuel injectors on a diesel Mercedes.
The electronic stethoscope will quickly show which injectors are
working – and which are not. (Photo: Georgina Edgar)
I
f you work on machinery, you’ve
probably come across a device like
this – an amplified microphone that
uses a steel probe. You touch the end
of the probe to the part of the machine
where you want to hear what is happening and then listen to the sounds
on earphones. But the trouble is, commercial machinery stethoscope units
start at about £50 – and go up rapidly
from that price. On the other hand, this
stethoscope is based around an eBay kit
that will cost you only a few pounds –
and the rest of the components that you
need will probably already be in your
spare parts collection or useful-to-keep
junk box.
Practical Electronics | May | 2022
Starting point
The starting point for this project is a
Banggood product (Fig.1) – dubbed in
the company’s idiosyncratic syntax-free
style, ‘DIY Electronic Kit Set Hearing Aid
Audio Amplification Amplifier Practice
Teaching Competition Electronic DIY
Interest Making’ (item ID 1607893). At the
time of writing, it costs from £6.50 (approx
US$9) including VAT and delivery to your
door. Similar units are available from
AliExpress, Amazon, eBay and the other
usual online suspects, so shop around.
Assembly
The kit comprises a small PCB on which
you build a simple three-transistor
amplifier. A parts overlay diagram is
provided and the PCB itself is soldermasked and well-labelled. Soldering the
parts to the PCB will take you less than 20
minutes, but as always, use a multimeter/
DVM to measure resistor values and
mount the low-profile parts first. Always
double-check components that must be
mounted with the correct orientation –
in this case, the electrolytic capacitors,
the transistors and microphone. (Refer
carefully to the overlay diagram for the
microphone polarity.)
I suggest that when you build the kit,
you mount the microphone, volume
adjustment pot and switch well off the
board. That way you can easily unsolder
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Fig.1. The amplifier – complete with volume control, LED power
indicator and on/off switch – and the stethoscope probe and
earphones. Detect and diagnose mechanical ailments with ease
using this listening system!
Fig.2. The Banggood kit includes the PCB, electronic components,
button cell and earphones. You need to add only a few low-cost
components to produce an effective and easy-to-use electronic
stethoscope. (Courtesy Banggood)
(or cut off) these components later, but still initially complete the
board and thus test that it works. (More on this in a moment.)
All the components are through-hole (oddly, except for the
earphone socket – but that has large surface-mount pads so it’s
easy to solder) and while some parts are rather close together,
this isn’t a kit that will cause any issues, even if you’re not an
experienced ‘solderer’.
In addition to the PCB and on-board components, with the
purchase you also get the earphones and a 3V CR2032 button
cell – amazing value for a few quid, delivered!
marked ‘w’). Connect the new pot in just the same way – lefthand pin on the board to left-hand pin on the new pot, centre
pin to centre pin, and right-hand pin to right-hand pin. Now
do the same for the power switch and microphone socket. You
want to use leads that are long enough that these can all be
mounted in the walls of the box but still connect to the PCB.
Testing, testing… 1, 2, 3
Once you have built the kit, test that it works. Turn the on-board
pot fully anti-clockwise, switch on power (there’s a small switch
on the PCB), plug in the earphones and listen. Then, gradually
wind the pot clockwise. You should hear a startling array of
sounds that you were not aware of when listening normally.
The quality and sensitivity of the amplifier are both very good
– you’ll probably be able to literally hear yourself breathing.
Now, put it in a box
Once you’ve ensured everything is working (and if it isn’t,
check all component orientations and look for solder bridges
or bad solder connections), you can take a few steps backwards.
Backwards? Well, because we want to mount the board in a box;
plus, a new switch, pot and microphone socket will be needed.
Any box will do – the only requirements are that everything
will fit inside it, and it’s suitably durable. As a guide, I used
a ‘jiffy’ box that was 85 x 55 x 35mm.
Remove the pot, switch and microphone from the PCB.
You won’t be using the pot and switch again, but you will be
re-using the microphone, so take care in removing this part.
You’ll now need a 10kΩ pot, a suitable knob for it, an SPST
switch (single-pole, single-throw – ie, a normal on/off switch),
and some type of plug and socket for the microphone cable. I
used an RCA socket and plug for the microphone, but that’s just
because I had them handy. (Technically, you should probably
use a shielded cable for the microphone, and a suitable plug/
socket combination that continues the shield, but it’s probably
not that important unless you will be using the electronic
stethoscope in very electrically noisy environments.)
Working one component at a time, you can now remote
mount the switch, pot and microphone. If you’re unsure how to
do this, look at the pin-outs on the board. The pot, for example,
has three connections. Orient your new pot in the same way
as the original one was wired – you can now see that there are
two ‘end’ pins and a central pin (aka the ‘wiper’, sometimes
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Connecting the microphone
After the power switch, level pot and microphone input have
all been connected on flying leads, it’s time to remote-mount
the microphone. You can decide on how long you want this
wire to be – I used a figure-8 cable length of about 80cm. Solder
the plug that you are using to this lead and then connect the
microphone, ensuring you maintain correct polarity. (If the lead
doesn’t have a trace to indicate which conductor is which, use
your DVM/multimeter, set to the continuity function, to work
it out.) Insulate the microphone connections – you don’t want
them touching each other or anything else.
You should now be able to power-up the board again and
check that the amplifier still works as it did before.
When you are happy that all the wiring has been completed
properly, drill appropriate holes in the box and install the
components. In most cases, the pot and switch will install
from within the box, so none of your new wiring needs to be
broken. However, many sockets insert from outside so you may
need to unsolder the socket connections, insert the socket and
then resolder these connections. Now drill a hole in the box
Fig.3. The completed kit. Note the CR2032 button cell in its holder
– it’s provided with the kit. The current drain of the circuit is fairly
high, so if you expect to use the stethoscope for long periods, you
may wish to replace the button cell with two AA cells (and holder)
to provide longer battery life. However, for normal short periods of
use, the button cell is fine.
Practical Electronics | May | 2022
Fig.4. The pot, microphone and switch should be mounted well
off the PCB, allowing their easy removal after initial testing. The
microphone is subsequently mounted remotely, while the switch
and pot are replaced with new items that are wired to flying leads,
allowing them to be mounted in the walls of the box.
wall for the on-board earphone socket – position the hole so
that the plug can be inserted from outside the box.
The stethoscope probe
You may be wondering why we used a plug and socket for the
microphone, rather than just extending the cable through the
box. The reason is that it’s useful to be able to fit microphones
with different mechanical connections. (The kit comes with
only one microphone, but these electret microphone inserts are
readily and cheaply available.) For example, you can have one
microphone mounted on the end of a metal rod for ‘probing’
duties, and another mounted on a battery clip to be attached
remotely to normally inaccessible machinery parts. Simply
plug in the microphone you want to use to match the test.
Irrespective of the mechanical connection you want to make,
the microphone needs to be firmly mounted. That mounting
approach is up to you – previously, I have glued microphones
to battery clips (it worked very well) but with the system shown
here, I taped the microphone to the end of a 20cm steel rod. In
all cases, ensure that the microphone’s electrical connections
are protected against strain and bending – these connections
can be quite fragile.
In use
To use the stethoscope, place the end of the probe against the
part of the machinery that’s of interest. Start with the volume
control at its lowest and after switching on power, slowly
turn up the volume until the listening level is comfortable.
For example, to determine in a car engine that all injectors are
running, place the tip of the probe against each injector in turn.
You will easily be able to hear the clicking of the injector above
other engine noises. Other uses include detecting worn ball
and roller bearings, plain bearings with excessive clearances,
and even vacuum and compressed air leaks.
Fig.5. The socket for the microphone, the new pot and switch
wired into place before the assembly is mounted in the box. These
components (and the PCB stand-offs and screws) are supplied by
the builder – they’re not in the kit.
Practical Electronics | May | 2022
Fig.6. The assembled electronic stethoscope, minus the sensing
probe. Orient the PCB so that the button cell can be replaced – that
is, have the opening in the holder pointing upwards. Here the optional
power LED can be seen between the volume adjustment pot and the
on/off switch. Finally, note how a hole has been placed in the side of
the box to allow the earphones to be plugged into the PCB socket.
Power LED?
A nice-to-have little extra is a power indicator. If you want to
show visually that the stethoscope is switched on, you can
add a power LED. You don’t want this drawing much current,
and one easy approach is to use an LED pre-wired for 12V.
(The current that then flows at 3V is very small, but modern
LEDs are still quite visible at this voltage.)
But where do you connect this LED? If you look at the
circuit diagram, you can see that when the switch is closed,
battery voltage can be picked up on one side of the switch, and
ground is easiest obtained from one side of the microphone.
Carefully use your multimeter/DVM and measure between
these connections to ensure you can see battery voltage when
the stethoscope is switched on. Then solder the LED in place.
If you don’t have any of these ‘12V’ LEDs available, I
suggest you buy a bunch – they’re very useful in all sorts of
applications and come with appropriate current-limiting and
a useful length of cable. At the time of writing, eBay item
353817519631 offers 50 pieces (white, red, yellow, green and
blue) delivered for under £10. I also like the ones that have
a little extra circuity built in and can flash all sorts of weird
patterns – search eBay for ‘12V led prewired flashing’ – for
example, eBay item 154761929698.
Remember that the LED is polarised – typically with these
pre-wired LEDs, red is positive and black negative (ground),
so connection is straightforward.
Fig.7. The circuit
diagram for the
amplifier and the
parts overlay for
the PCB. (Courtesy
Banggood)
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