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This circuit will sound a loud

warning when a sensor is sub-

jected to shock or vibration. One

application would be to attach the

sensor to a window. If an attempt

were made to enter the house by

breaking the glass, the alarm

would be triggered.

Readers will no doubt find

other possible uses, such as for

protecting personal belongings,

perhaps. Note, however, that sim-

ply moving the sensor will not -

operate it.

The sensitivity of the circuit is

adjustable and can be set to suit

the application. To get an idea of

the sensitivity, with the prototype

sensor attached to a wooden

table, putting a coffee cup down

about 1m (3ft) away triggered it.

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A multi-purpose vibration-triggered alarm with

remarkable sensitivity.

mounted in a small plastic case.

The main unit is housed in a

larger plastic case (see pho-

tographs). The two sections are

interconnected using a short

piece of light-duty screened

wire.

Inside the main unit is the

circuit panel, the battery pack

and a loud “yelping” car-type

alarm siren. On top, there is a

key-operated switch, which may

be used to switch the unit off or

cancel operation before the nat-

ural time-out period. Of course,

an ordinary switch could be

used with a corresponding re-

duction in security.

The operating time is ad-

justable from about one second

(which will be found useful for

testing) to two minutes. This

could be easily extended if re-

quired.

The circuit requires about

250 A on standby. While actu-

ally operating, it draws a current

which depends on the type of

sounder used (in the prototype it

was 150mA). The prototype unit

was powered using a pack of

eight AA-size alkaline cells. In

normal use these will last for up

to a year.

BI-MORPH ELEMENT

The sensor consists of a bi-

morph (2-layer) element, which

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Fig.1. Simplified analogy us-

ing a wooden ruler to

demonstrate the operation of

the bi-morph element.

is a strip of piezo-electric mate-

rial 15mm long, 1 5mm wide

and 0 6mm thick. Its simplified

operation is illustrated in Fig.1a,

which shows an ordinary

wooden ruler. Normally, this is

not under any stress.

VIBRALARM OVERVIEW

The

Vibralarm

comprises two

parts. The first is the sensor itself

However, if it is bent as

shown in Fig.1b, the lower sur-

face will now be in compression

(the distance between the

molecules slightly reduced) and

the upper one under tension

(the distance between the

molecules increased).

When a bi-morph element

is bent, opposite charges are

developed on the surfaces

which are under compression

and tension due to the piezo-

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Fig.2. Complete circuit diagram for the vibration-triggered Vibralarm.

electric effect. This means that

a voltage difference appears

between them. A current would

flow if there was a conducting

path between the surfaces but

the charges would soon neutral-

ize so this would occur only

briefly.

the record is turning at the cor-

rect speed) and roughly propor-

tional to its amplitude. The sig-

nal is then amplified and fed to

a loudspeaker.

excessive damping. The values

shown worked well. However, this

could be the subject of experi-

ment once the project has been

constructed.

The opamp’s non-inverting

input (pin 3) is connected to the

sliding contact (wiper) of preset

potentiometer VR1. Its track

(outer) connections are con-

nected across Zener diode D1,

which operates in conjunction

with series resistor R1 to provide

a fixed-voltage supply. The volt-

age appearing at IC1 pin 3 may

therefore be adjusted between

zero and the Zener breakdown

voltage.

CIRCUIT DESCRIP-

TION

The full circuit diagram for

the

Vibralarm

is shown in Fig.2.

The bi-morph element is labeled

X1. The nominal 12V battery

supply is applied to the circuit

via key-operated (or other)

switch S1, and diode D3. The

diode prevents possible dam-

age if the supply were to be

connected in the wrong way,

since it would fail to conduct

and nothing would happen.

Any voltage appearing

across the bi-morph element is

applied to the inverting input

(pin 2) of operational amplifier

(opamp), IC1. The network con-

sisting of capacitor C1 and re-

sistor R2 is also connected be-

tween this point and the 0V line.

These components help to pre-

vent high-frequency oscillation

and give a damping effect for

the bi-morph element.

Since the impedance of the

bi-morph is very high, the value

of R2 must accordingly be ex-

tremely high or it would cause

WHAT’S THE DIF-

FERENCE?

With the piezo ceramic ma-

terial used here, the voltage dif-

ference is significant even with

a small amount of bending.

When the bi-morph element is

subjected to shock, the upper

and lower surfaces alternate

briefly between compression

and tension and the polarity of

the voltage will keep changing.

In other words, an alternating

voltage is produced at a fre-

quency equal to that of the vi-

bration.

Bi-morphs were once used

in the crystal-type of record

pick-up but, although still some-

times used, they are not seen

much now. In these pick-ups,

the strip is vibrated by the stylus

running in the groove on the

surface of the record.

An AC output is therefore

obtained proportional to the fre-

quency of the sound (assuming

Maxfield & Montrose Interactive Inc

ZENER VOLTAGE

It is necessary to provide a

stable voltage here, because if it

was derived from the supply di-

rect it would fall as the battery

aged. Since the voltage appear-

ing across the bi-morph element

is independent of the supply volt-

age, the operating characteristics

of the circuit would change as the

batteries ran down. The exact

value of the Zener voltage is not

particularly important, but it

should be close to the range of

specified values.

It will be noted that the value

of R1 is relatively high. With a

supply of 12V it allows less than

EPE Online, November 1999 - www.epemag.com - 1016

90 A to flow through the Zener

diode and this will fall as the

battery ages. The current di-

verted via VR1 is negligible. Us-

ing such a small current here

reduces the requirement of the

circuit as a whole. When using

the specified Zener diode

(which has been designed for

low-current, low-power and low

noise applications) there will be

no problems.

However, if using a different

type of Zener, it might not stabi-

lize and it may be necessary to

increase the current. Readers

using a different Zener will need

to check the stabilization and

reduce the value of R1 if neces-

sary. This procedure is ex-

plained at the setting-up stage.

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lems with the prototype, though,

even under heavy shock.

Depending on the adjustment

of VR1, the positive peaks will

exceed the voltage at IC1 pin 3.

Each time this happens, the

opamp output (pin 6) will go low

instantaneously. The first peak

arriving at IC2 pin 2 will trigger it

and a timing cycle will begin. Fur-

ther trigger pulses applied during

this period will have no effect.

However, any arriving afterwards

will start the timing once again.

While timing, IC2 output pin 3

goes high and allows current to

flow into the base of Darlington

transistor TR1, via current-

limiting resistor R5. Sounder

WD1 then operates due to current

flowing in the collector circuit.

Light-emitting diode D2 is

also turned on at this time, with

its current limited to about 15mA

by resistor R6. The LED will be

useful to check the circuit and ad-

just the time-out period before the

sounder is connected.

state for a short time until the

capacitor has charged suffi-

ciently through R4, so disabling

the IC from responding to any

trigger pulses. After that, pin 4

goes high and the device is en-

abled. This prevents any ten-

dency for the circuit to self-

trigger on powering-up.

Darlington transistor TR1

could operate a sounder of up

to 500mA rating but this would

place an unnecessary load on

the battery. Very loud devices

are available with a current re-

quirement much smaller than

this (say 150mA) and one of

these was used in the prototype.

CONSTRUCTION

On no account experi-

ment by bending the bi-

morph element with the fin-

gers. Anything more than a

minute movement is likely to

destroy it. Also, take extreme

care when handling the end

wires because they are easily

broken off.

All the components, apart

from the sounder, on-off switch,

battery pack, and bi-morph ele-

ment are mounted on the

printed circuit board (PCB)

whose component layout and

full-size copper foil master are

shown in Fig.3. This board is

available from the

EPE Online

Store

(code 7000230) at

www.epemag.com

Supplied attached to the

NO VIBRATION

Imagine that preset VR1 is

adjusted so that 1V appears at

IC1 pin 3. In the absence of any

vibration of the bi-morph, there

will be no voltage across it and

the voltage at the non-inverting

input (pin 3) will exceed that at

the inverting one (pin 2). The

output, pin 6, will therefore be

high (close to the positive sup-

ply voltage) and when applied

to timer IC2 trigger input (pin 2)

there will be no further effect.

This is because a low state is

needed to trigger this type of

device.

SENSITIVITY

The sensitivity of the circuit

may be adjusted at the end by

means of VR1. With this set to a

little above zero volts, the circuit

will be triggered with a relatively

small amount of vibration. How-

ever, if it is adjusted to a higher

value, an increasingly high output

from the bi-morph is needed to

trigger it.

The timing period depends on

the value of capacitor C2, resistor

R3 and preset VR2. With the val-

ues specified, this will be about

one second (with VR1 at mini-

mum) and two minutes (when at

maximum). If the timing needs to

be extended, the easiest way

would be to increase the value of

C2 in proportion.

When the supply is con-

nected, capacitor C3 maintains

IC2’s reset input (pin 4) in a low

IN SHOCK

When the bi-morph element

X1 is subjected to shock, an al-

ternating voltage appears

across it having a peak-to-peak

value relative to the impact

strength. Within limits, the neg-

ative excursions have no effect.

However, they can cause the

inverting input to swing below

the voltage of the 0V rail and

this could damage the IC if high

enough. There were no prob-

Close-up of the bi-morph ele-

ment mounted on its PCB.

Maxfield & Montrose Interactive Inc

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&RQVWUXFWLRQDO 3URMHFW

sive. The blob of adhesive

should be thick enough to allow

the other end of the bi-morph to

remain clear of the board by a

millimeter or so.

This will allow it to bend

slightly when the mounting

board is subjected to shock. If

necessary, use a piece of thin

cardboard underneath the free

end to hold it in position until

the adhesive has thoroughly

hardened.

Decide on the length of wire

needed between the sensor and

the main unit. Pieces up to five

meters long were tested and

worked well. However, very

long runs will introduce prob-

lems of interference pick-up and

poor sensitivity.

Use light-duty single (mono)

screened wire (such as micro-

phone cable). Ordinary wire is

not satisfactory, because it al-

lows the pick-up of random sig-

nals, including AC mains “hum”.

This could cause false trigger-

ing.

A small plastic box is used

to house the sensor, and it is

worth mounting the sensor in

this now before moving on fur-

ther. Mark the mounting hole on

the base and drill this through.

Note that the bolt (which will be

used to attach it) will need to

have a countersunk head. This

will allow the bottom of the box

to make good contact with the

surface on which it will be used.

Drill a hole in one side of

the case for the connecting ca-

ble. Pass this through and apply

a small clamp or tight cable tie

a short distance from the end

(see photograph). Allow enough

cable to reach the sensor pads

plus a little slack. Pull on the

cable to make sure it is secure

and tighten the tie if necessary.

Layout of the components on the finished PCB.

main board is a smaller section

on which the sensor is to be

mounted separately. Begin con-

struction by carefully separating

the two PCB sections, using a

small hacksaw. Then drill the

three marked mounting holes in

the main board, and a mounting

hole in a suitable place on the

small board.

Referring to the main board,

solder the link wire in position

and follow with the IC sockets

(but do not insert the ICs yet) all

resistors (including the presets)

and capacitors (except C2).

Next mount diode D3, Zener

diode D1, LED D2 and capacitor

C2, taking care to solder these

components the correct way

round.

Referring to the wiring dia-

gram in Fig.4, connect up the

power supply. It will be kinder

on the ears if you do not con-

nect the sounder yet, but when

you do, connect it via a piece of

2A screw terminal block to pre-

vent the WD1 wires from short-

circuiting.

Adjust VR1 fully anti-

clockwise, then slightly clock-

wise (as viewed from the bot-

tom edge of the PCB) and VR2

fully clockwise (as viewed from

the right-hand edge of the PCB)

for minimum timing.

SENSOR UNIT

Solder the bi-morph to the

small board and reinforce its

physical stability by using a little

quick-setting epoxy resin adhe-

The bi-morph board installed in a small plastic case.

Maxfield & Montrose Interactive Inc

Solder the cable wires to

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COMPONENTS

9 ,$ 6

;

Resistors

R1 68k

R2 100M cermet film

R3, R5 10k (2 off)

R4 1M

R6 680 ohms

All 0.25W 5% carbon film (except R2)

%²

9 5

72 :'

Potentiometers

VR1 10M sub-miniature vertical

preset

VR2 1M sub-miniature vertical

preset

Capacitors

C1 100p polystyrene

C2 100u radial electrolytic, 16V

C3 47n metalized polyester,

5mm pin spacing

Fig.3. Vibralarm printed circuit board component layout, foil

master, and sensor board master.

the pads on the sensor PCB

with the screening connected to

the large “land” area. Separate

the joints to prevent short-

circuits and secure them in

place using a little quick-setting

epoxy-resin adhesive.

Attach the sensor board to

the bottom of the box using a

small nut and bolt with a short

plastic spacer.

With the new set of batter-

ies installed, switch on. Apply

the meter probes to the outer

tags of VR1 and note the volt-

age. This should be close to the

stated Zener breakdown volt-

age. Now use the run-down bat-

teries or a 9V supply from the

PSU. Note the meter reading

once again.

The second voltage reading

should be the same or only

slightly lower than the first –

less than 0 05V difference. The

stabilization aspect is not very

critical and the circuit should

work well even with a difference

of 0 1V.

Semiconductors

D1 5V9 to 6V2 400mW Zener

diode, Philips PLVA459A or

PLVA462A (see text)

D2 red LED, 3mm

D3 1N4001 50V 1A rectifier diode

TR1 MPSA14

npn

Darlington

transistor

IC1 7611 micropower opamp

IC2 7555IPA low-power timer

Miscellaneous

X1 bi-morph element

WD1 miniature alarm sounder, 12V

150mA (110dB at 1m output)

S1 s.p.s.t. switch (toggle or key-

operated)

B1 alkaline AA-size cell (8 off),

with holder and connector

Printed circuit board available

from the

EPE Online Store

, code

7000230 (www.epemag.com);

8-pin DIL sockets (2 off); 2A

terminal block; plastic case, 158mm

x 95mm x 54mm (external) for main

unit; plastic case, 50mm x 37mm

x 24mm (external) for sensor;

light-duty single screened wire;

small clamps or cable ties (2 off);

connecting wire, solder, etc.

VOLTAGE STABILITY

As stated earlier, for any

Zener diode other than the

specified unit, it will be neces-

sary to check the voltage stabil-

ity. It will be convenient to do

this work before the ICs are in-

serted into their sockets.

You will need a digital volt-

meter (this will generally have

an input impedance of 10M or

more, which will be satisfac-

tory). You will also need two

sets of batteries – one new and

the other run down so that the

terminal voltage is about 9V or

10V. Of course, you could also

use a suitable bench power sup-

ply unit.

However, if it is much more

than this, reduce the value of

R1 until the criterion above is

met. It is thought that a value as

low as 33k would be accept-

able (about 180 A at 12V) but it

would increase the current re-

quirement of the circuit to about

350 A and this would have an

effect on battery life.

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