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A new “first-aid” tool for your puncture repair kit, that

uses SMD and ultrasonics to detect escaping air.

Every cyclist will sooner or

later need to deal with a flat tire.

After figuring out the intricacies of

getting to the inner-tube, the

puncture must then be located.

Punctures come in all shapes

and sizes, from irreparable

blowouts to slow punctures where

the deflation can take weeks. But

the most common type of punc-

ture is from a thorn or a small

nail. Such punctures can be quite

difficult to find and a basin of wa-

ter is often used to find the hole

(air-bubbles rising to the surface)

and to make certain that it is the

only one.

The ultrasonic detector will not

solve all puncture problems and

can be defeated just like every

other method, particularly when

dealing with very slow leaks.

However, it is another useful

weapon in the cyclist’s armory

and it is easy to use.

Apart from finding punc-

tures, this low cost device will

also be of interest to those in-

vestigating the properties of ul-

trasound.

TURBULENT TIMES

A jet of air ejected under

pressure from an inflated tube

will, in most cases, flow into the

atmosphere in a highly turbulent

manner. Because of the scale

involved in the case of an inner-

tube puncture, a portion of this

turbulent energy results in dis-

turbances in the ultrasound re-

gion.

The commonly available

ultrasound microphones of the

type used in this project operate

at 40kHz. The speed and the

frequency of ultrasound or in-

deed any waveform are inti-

mately related. In all cases the

wavelength can be found by di-

viding the speed by the fre-

quency.

The speed of sound in air is

known to be about 344 meters

per second. So the ultrasound

detector will respond to wave-

lengths of about 8mm.

Prediction of the onset of

turbulence involves very com-

plex fluid mechanics, but an-

other science called common

sense would suggest that such

short wavelength might be as-

sociated with a tiny jet of turbu-

lent air from a leaky inner tube.

We also get a clue from the fact

that several successful compa-

nies market ultrasonic leak de-

tectors world wide for diverse

use such as in the aircraft and

FIND-THE-LEAK

The

Ultrasonic Puncture

Finder

fits in here as a much less

messy way of finding normal

punctures. It will replace the wa-

ter bowl and, with a little skill,

may even help to locate some

slower punctures. In a “find-the-

leak speed test”, the ultrasonic

finder took only a few seconds

whereas the water bowl took sev-

eral minutes including the time to

set it up and clean up.

Other techniques such as

lightly running the hand around

the tube or passing the tube close

the face to feel for the cooling air

stream have their merits. But

again the ultrasonic device

tended to be faster and it is cer-

tainly cleaner.

Maxfield & Montrose Interactive Inc

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Fig.1`. Recovery of audio fre-

quencies from an amplitude

modulated signal. (a) 40kHz

signal with low frequency

amplitude variation (noise),

(b) after diode detection be-

fore smoothing, and (c) re-

covered audio (noise).

EPE Online, August 1999 - www.epemag.com - 777

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the chemical industries.

Turbulence in the jet of air

from a puncture will result in a

complex mix of signals varying

randomly in frequency and am-

plitude. The voltage output from

the ultrasonic microphone will

therefore appear as random

noise. Most ultrasound detec-

tors convert the 40kHz signals

to the audio frequency range by

mixing it with a steady local os-

cillator.

Some experimentation con-

firmed that the amplitude of the

40kHz noise from a puncture air

stream varied at audio fre-

plifier.

STRAIGHT RECEIVER

In a straight radio receiver,

the signal is first separated from

the myriad of signals in the ra-

dio frequency spectrum by a

tuned circuit. This very weak

signal of maybe a few micro-

volts is then amplified to a level

(>100mV) suitable to operate a

diode detector circuit. Before

detection, the signal is balanced

and there is no audible compo-

nent.

The diode acts as a rectifier

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Fig.2. Full circuit diagram for the Ultrasonic Puncture Detector.

quency. This electrical signal

from the detector can, there-

fore, be regarded as an ampli-

tude modulated (AM) radio sig-

nal.

The random ultrasonic

noise signals will also mix in the

detector to give sum and differ-

ence signals, and the latter will

also appear in the audio range.

To put this another way, noise

plus and minus noise equals

noise. The design concept for

the

Ultrasonic Puncture Finder

is therefore along the lines of a

“straight” radio receiver with a

simple detector and audio am-

allowing only positive going sig-

nals to pass. This results in a

series of positive going pulses

at 40kHz, the carrier frequency.

A capacitor smoothes these

pulses to give an average which

varies over time, corresponding

to the low frequency audio com-

ponent of the ultrasonic signal.

Final amplification is at audio

frequency after the detection

process.

In such a line up there is no

mixer as such and no intermedi-

ate frequency amplification.

This is illustrated by the recov-

ery graphs shown in Fig.1.

CIRCUIT

DESCRIPTION

The puncture finder relies

largely on an ultrasonic trans-

ducer to select the input fre-

quency. This is an electrome-

chanical device, which converts

the ultrasound to what is in ef-

fect a VLF (very low frequency)

radio signal. From there on the

processing is the same as a

tuned-in radio signal. The com-

plete circuit diagram for the

Ul-

trasonic Puncture Finder

shown in Fig.2.

The ultrasonic transducer,

Maxfield & Montrose Interactive Inc

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sources of ultrasonic noise can be

traced.

The audio level to the output

amplifier IC1 is controlled by

potentiometer VR1. Although

VR1 is a linear device, the

overall loudness control

seems even over the

track because of the

non-linearity of the

detection system.

Audio amp IC1 is set

to a voltage gain of

200 by capacitor C7 and

it has sufficient power to

drive the small loud-

speaker LS1.

With this high gain circuit it is

essential to remove any ripple on

the supply line to prevent instabil-

ity. The simplest way to achieve

this is with a large electrolytic ca-

pacitor, C12 in this circuit. Capac-

itor C11 is a ceramic device,

which de-couples high frequency

components more effectively

than C12.

COMPONENTS

Resistors

R1, R2 100k (2 off)

R3 47k bead N.T.C. thermistor

R4 680k

R5 2k2

R6 1k5

All 0.25W 1% metal film

(except R3, see text)

RX1,

is a high

impedance de-

vice, and its output is

buffered by the low-noise

field-effect transistor (FET)

TR1, which acts as a voltage

follower. A simple high-pass fil-

ter consisting of capacitor C1,

resistor R3, and capacitor C2

removes any audio frequency

components from the signal.

This is a major requirement to

prevent audio feedback from

the loudspeaker and conse-

quent instability.

Transistors TR2 and TR3

provide a gain block to lift the

signal to sufficient amplitude to

drive the diode detector D1. The

feedback capacitor C4 provides

top cut and improves the RF

stability of the circuit.

A strong noise input signal

will produce a few millivolts

(mV) at most from the trans-

ducer. With a gain of some 600

times before the detector, the

stronger ultrasound sources

therefore produce full audio out-

put.

Unfortunately, silicone

diodes stop conducting much

below about 600mV and sensi-

tivity is lost. The diode detector

is therefore slightly forward bi-

ased to increases the sensitiv-

ity. This is achieved by resistor

R9, which injects about 80mA

into the diode. With this setup,

the increase in sensitivity is

quite dramatic and very weak

Potentiometer

VR1 100k carbon rotary, linear

Capacitors

C1 100u radial electrolytic, 10V

C2, C3 2u2 radial elect. 50V (2 off)

Semiconductors

D1 1N4148 signal diode

TR1 BC549

npn

silicon transistor

IC1 LF441CN low power opamp

Miscellaneous

B1 6V battery pack (4xAA cells

in holder)

S1 s.p.s.t. miniature toggle switch

WD1 6V miniature DC buzzer

Printed circuit board available

from the

EPE Online Store

, code

7000932 (www.epemag.com);

medium size plastic case; PP3

battery connector; control knob;

approx 36s.w.g. (0.19mm) enameled

copper wire; multistrand connecting

wire, solder pins; solder, etc.

MECHANICAL FILTER

An essential component in

the circuit is the home-made me-

chanical filter, which de-couples

the ultrasonic transducer RX1

from the instrument case. This is,

in fact, a small circle of rubber

about 6mm thick, which is about

one wavelength. At full volume,

any distortion in the audio ampli-

fier or the loudspeaker results in

high frequency harmonics, which,

if they reach the transducer, pro-

duce a bad case of “howl-round”.

The quiescent current drawn

by the complete circuit is less

than 9mA and this peaks to about

65mA at full volume. The proto-

type worked with no performance

change down to 7 5V, and went

on working down to 6V with loss

of volume. An alkaline PP3 bat-

tery is therefore well suited to

See also the

SHOP TALK Page!

Approx. Cost

Guidance Only

(Excluding Batteries)

$19

power the puncture finder for

intermittent use.

SURFACE

MOUNTING

This is a surface mount pro-

ject and construction will in-

volve working with some pretty

small devices. The “chip” com-

ponents specified are the most

suitable for hand soldering, but

they are not the smallest SMDs

available. The only require-

Maxfield & Montrose Interactive Inc

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ments for population are a fine

soldering iron of the type used

for normal leaded components

and a pair of tweezers.

Although an SM circuit can

be constructed by the simple

method described below, sev-

eral specialized techniques

have been developed for hand

working. It is important to im-

prove SM skills, especially for

more complex circuits where

reliability can be improved by

more appropriate soldering

methods.

Non-magnetic or demagne-

tized tweezers are required for

handling SM chip components

as their contacts contain nickel

and they are magnetic. Because

they are so small and light, they

will stick to magnetized tweez-

ers making accurate placement

very tedious.

Completed surface mount PCB. Note that components are

mounted directly on the copper pads.

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CONSTRUCTION

The component layout

(twice-size) on the small surface

mount printed circuit board

(PCB), together with the

(approximately) full-size copper

foil master, is shown in Fig.3.

Here the components are

mounted directly on the copper

pads; no holes are drilled in the

PCB. This board is available

from the

EPE Online Store

(code 7000236) at

www.epemag.com

The simplest method of

soldering an SM device is to

hold it on the circuit board

pads with the tweezers and

solder one end with a little sol-

der carried on the iron. Having

fixed one end, the second end

(or the remaining contacts in

the case of active devices)

can be soldered by the more

reliable method of applying

the iron and solder to the joint

at the same time. The anchor-

Fig.3. Layout of components on a twice-size surface mound

PCB and (approximately) full-size copper foil master pattern.

The transistor and diode printouts are also included.

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Fig.4. Positioning of components inside the small handheld

case. Keep the ultrasonic transducer and loudspeaker as far

apart as possible.

Maxfield & Montrose Interactive Inc

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ing end can subsequently be

re-soldered if required.

A suggested method of con-

struction is to place all the resis-

tors and then the capacitors,

finishing off with the transistors

and finally IC1. Ceramic chip

capacitors are delicate and re-

quire the smallest amount of

solder and the shortest heat du-

ration.

In severe cases the con-

tacts can become detached, but

this is an indication of excessive

stress and only occurs with

heavy reworking during fault

finding. More problematic per-

haps is part detachment of the

nickel contact or cracking of the

ceramic where the fault may not

be seen. It is essential that the

tantalum capacitors and the

diode are wired the right way

around.

(For more information

on SM construction see the au-

thor’s web site at

www.billsSMD.mcmail.com

)

soft as possible, a rubber gasket

material about 6mm thick was

used for the prototype. Another

source of this type of soft foam

is the backing from computer

mouse mats.

A small piece of the same

foam should be placed over the

on/off switch contacts to isolate

it from the metal base of the

battery. Make certain that the

foam is not conductive. The po-

sition of the various compo-

nents in the case is shown in

Fig.4.

The small loudspeaker is

held in place by a thin layer of

glue on the magnet. The

speaker type suggested is an

exact fit in the prototype case,

see photographs. A speaker

grille will need to be fashioned

from a series of holes in the box

lid. The prototype sound outlet

consisted of eight equally

spaced 6mm holes and a cen-

tral 8mm hole.

A “knobpot” is suggested for

the volume control VR1. These

are expensive but made to a

very high standard and take up

very little space inside the box.

The populated circuit board

is also glued in place and does

not therefore require any holes

to be drilled. The decoupling

capacitor C12 is adequately

supported by its own leads.

specified comes with a metal

cut-out grille or a wire mesh

guarding the sensing element,

the wire mesh type is marginally

more suitable for this applica-

tion. Finally, it is worth decorat-

ing the finished device with

some labels at least marking

the on/off switch positions. Suit-

able labels can be designed

easily with a computer drawing

package.

TESTING

If all is well, the

Ultrasonic

Puncture Finder

should work

right from switch on as there are

no adjustments to be made to

the circuit. But it is best to carry

out a few spot checks before

applying power and putting the

lid on the box.

The most obvious items to

check are the polarity of the

electrolytic capacitors and the

diodes. Surface mount transis-

tors can come with a lead-out

“joggle”

especially the FET

but the drawing shown is by far

the most popular. If the device

cannot be coaxed to work, it

may be worth just checking the

transistor pins in the usual way,

but this really is a last resort.

Reduce the volume to mini-

mum and measure the quies-

cent current. If this is about

9mA it is very likely that the cir-

cuit is working correctly.

Finally, increase the volume

and see if you can track down

some ultrasound. A low hiss from

the loudspeaker is a good indica-

tion.

IN THE BOX

All the elements fit easily into

a standard 111mm

57mm x

22mm plastic project box. The

layout is not critical except for two

points.

The loudspeaker and ultra-

sonic transducer

must

be as far

apart as possible and orientated

at right angles to each other. The

suggested layout should be fol-

lowed in this respect.

Secondly, the ultrasonic

transducer RX1

must

be me-

chanically isolated from the

case and hence the loud-

speaker. This is simply

achieved by using a small disc

of non-conductive foam rubber

to mount the transducer as

shown. A thin layer of adhesive

such as Evostick holds the

sandwich together.

The foam disc should be as

Maxfield & Montrose Interactive Inc

WIRING-UP

The connecting wires to the

Volume control, loudspeaker,

and transducer is a fine, 0 8mm

diameter and multistrand (7/0 1)

flexible wire rescued from a

section of 25-way computer ca-

ble. This type of wire is ideal for

surface mount projects where a

flexible connection is required.

The interwiring details are

shown in Fig.4.

SOUND SOURCE

There are many sources

that can be used as a test sig-

nal. A portable and very useful

ultrasonic noise generator is

your thumb and forefinger, just

The ultrasonic transducer

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