Thebestof Free Segun

8/6/2019 Thebestof Free Segun

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This book is a 75-page free samplecontaining select designs from Books1-4 of Thomas Scarboroughs

electronics series at

http://stores.lulu.com/thomasscarborough

Besides providing free samples, itcontains the Contents pages of all fourbooks in the series. Books 1-3 contain

6-component designs (and less). Book4 is for the mature hobbyist.

This book is exclusive to CircuitExchange International.It is dedicated toAndy Collinson.


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THE BEST OF

THOMAS

SCARBOROUGH

BOOK 1

___________________________________

Printed by Lulu Press860 Aviation ParkwayMorrisville, NC 27560United States of America

www.lulu.com

Publisher ID: 785519http://books.lulu.com/content/785519Copyright: 2007 by Thomas ScarboroughEdition: Second Edition

License: All Rights Reserved. No part of thispublication may be reproduced, stored in aretrieval system, or transmitted in any form or byany means, electronic, mechanical, recording or

otherwise, without the prior written permission ofthe author.

Printed: 123 pages, 4.25" x 6.875" (Pocket), perfectbound, 60# cream interior paper, black and whiteinterior ink , 100# white exterior paper, full-colourexterior ink.


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CONTENTS__________________________

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . v

DEDICATION . . . . . . . . . . . . . . . . . . . . . . . . . . vi

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . 1

AUDIO CIRCUITS . . . . . . . . . . . . . . . . . . . . . . 7

SHORT-RANGE SONAR . . . . . . . . . . . . . . 7

INTERCOM . . . . . . . . . . . . . . . . . . . . . . . . . 13

STEREO 3RD CHANNEL . . . . . . . . . . . . . 21

THEREMIN . . . . . . . . . . . . . . . . . . . . . . . . 27SUPER-SENSITIVE MIC . . . . . . . . . . . . 37

DOUBLE DOORBELL . . . . . . . . . . . . . . . 45

LED CIRCUITS . . . . . . . . . . . . . . . . . . . . . . . . 51

THOUSAND-YEAR FLASHER . . . . . . . . . 51

FREE ENERGY FLASHER . . . . . . . . . . . . 57

BATTERY LED . . . . . . . . . . . . . . . . . . . . . . 67

LED TORCH/FLASHLIGHT . . . . . . . . . . 73

CONJURING TRICK . . . . . . . . . . . . . . . . . 79

OPTICAL ILLUSION . . . . . . . . . . . . . . . . 87


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ESSENTIAL CIRCUITS . . . . . . . . . . . . . . . . . 93

DIODE PUMP . . . . . . . . . . . . . . . . . . . . . . . 93

POWER SWITCH . . . . . . . . . . . . . . . . . . 101AUTO POWER-DOWN . . . . . . . . . . . . . . 109

CIRCUIT SYMBOLS . . . . . . . . . . . . . . . . . . . viii

RESISTOR COLOUR CODES . . . . . . . . . . . . . 6

ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . 36

BASIC FORMULAE . . . . . . . . . . . . . . . . . . . . 86

PLUS: 32 FURTHER IDEAS

VIEW THE WHOLE SERIES AT


PRINTED COPIES $8.95DOWNLOADS $2.95


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SHORT-RANGE SONARNo. of Components: 5

_____________________________

SONAR stands for SOund NAvigation

Ranging, and is a technique for determiningthe distance and direction of objects by acous-tic means. Although SONAR is usually associ-ated with navigation ranging under water, itmay also be used to good effect in air -- forinstance, to detect boats in a fog.

SONAR may be divided into three cate-gories. Active SONAR projects a sound waveand waits for the echo, which these days can

be analysed with great sophistication;passiveSONAR projects no sound, but analyses in-coming sound, e.g. to identify a passing sub-marine; and a third category of SONAR trans-mits and receives messages across an acous-

tic path.

SONAR was first proposed as a meansof detecting icebergs, and the first operationalactive SONAR system was revealed in 1918,having been developed by British and US sci-entists.


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The SONAR described here is an activesystem, with the most basic functionality -- itsimply reports the existence of an object with-

in a 1 metre (1 yard) range. In fact, with justfive components, this has to be the simplestSONAR in the world!

What can be done with such a modestrange? The answer is: Plenty. A simple ex-ample would be guidance for a vehicle whichis reversing in a garage. This circuit would

shriek when the back of the vehicle came within about a metre (a yard) of the wall.Several other suggestions are to be found un-der FURTHER IDEAS ...

CIRCUIT THEORY

The SONAR described here relies onacoustic feedback, which kicks in when so-und bounces off an object -- e.g. the vehicleapproaching a wall.

IC1 is a very sensitive amplifier, withpiezo sounder X2 continually emitting amp-lified sounds, either created by the circuits in-ternal noise, or picked up from the environ-ment.

Ordinarily, the sound emitted by X2dissipates in the air; it does not return to mic-


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rophone X1 with sufficient strength to create afeedback loop. When, however, a barrier ap-pears before X1 and X2, sufficient sound bou-

nces back to create feedback. This naturallychooses the resonant frequency of X1 and X2,producing a loud signal.

Both X1 and X2 arepiezo sounders without in-ternal electronics. A typicalexample is shown on the

right.

Ideally, X1 and X2 will be identical,since this will encourage feedback at their res-onant frequency.

C1 determines gain (amplification) thr-ough positive feedback, while C2 is necessary

to limit output current. Without C2, the cir-cuit would overheat.

Positive feedback, as is used here (thatis, feedback from an output to a non-invertinginput) reinforces a signal, while negative feed-

back (from an output to an inverting input)reduces gain. Negative feedback is the most

common of the two, and is best known for itsuse in amplifiers.

IC1 is a veteran amplifier IC which hassuitably biased inputs and a balanced output.These two characteristics make it useful for agreat many things. In particular, notice how


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no additional biasing components are require-ed at the inputs.

This is explained in greater detail whenI introduce the INTERCOM, which follows.

GETTING GOING

Ordinarily, this circuit will have a range

of about 1 metre (1 yard), although I obtaineda range of 2 metres (2 yards) by replacing thepiezo sounders with piezo cone tweeters, and

by tweaking the value of C1.

Using the 470p value for C1 as shown,the circuit is quite sensitive, and X1 and X2


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may need to be mounted more than a metre (a yard) apart, on the same surface (e.g. thesame garage wall).

If C1 is omitted, then the circuit is farless sensitive, and X1 and X2 need be movedonly a few centimetres (a few inches) apart.

When an object (e.g. a car bumper) moves within about 10 cm (4) of X1 and X2, X2screeches. The value of C1 may also be red-ucedto less than 470p, for reduced sensitivity,

so that sensitivity may be adjusted as desired.Note that since the LM380N's current

consumption is not the most modest (about12mA in this application), this circuit wouldideally be run off a 12V DC plugpack powersupply.

FURTHER IDEAS ...

OPEN DOOR ALARM. X1 and X2 may be mounted on opposite sides of a closeddoor. In this case, feedback is blocked untilthe door is opened. Thus the circuit could be

used as a simple alarm, or as a reminder toclose the door. With 470p for C1, the circuit

will be particularly sensitive. Of course, a window or a cupboard door would work justas well.

ANTI-PILFER ALARM. Alternatively,


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X1 could be placed underneath an item, togive warning if someone picks it up. For in-stance, if one were to place a wallet on top of

X1, X2 would squeal loudly if the wallet werestolen. This would serve as an excellent anti-pilfer alarm at e.g. a hospital bedside.

BLIND GUIDE. The circuit could con-ceivably be used as a guide for the blind, to

warn them of objects in front of them, or ofsteps going up. Regrettably, it would not in-

form them of steps going down!

COMPONENTS

C1 . . . . . . . . . . 470p ceramic capacitor

C2 . . . . . . . . . . 220 16V electrolytic

capacitor

IC1 . . . . . . . . LM380N amplifier IC

X1, X2 . . . . . . Piezo sounders (without

integral electronics), or

try piezo cone tweeters

S1 . . . . . . . . . . On-off switch

B1 . . . . . . . . . . 12V battery or battery pack

Battery clip to suit battery or battery pack

Suitable plastic case

Connecting wire

Solder


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THOUSAND-YEARFLASHERNo. of Components: 5

____________________________

Thats right -- this LED flasher will fla-sh for one-thousand years off high-capacity

AA batteries. At least, it would do so if high-capacity batteries could ever last that long(they would surely die of internal leakage farsooner).

While I have not, of course, been able

to test whether the circuit really does last thislong, there are formulae with

which one can quite reliablycalculate it. Anyway, if you canfind a good enough battery,

your very distant ancestors co-uld still be enjoying the LED

Flasher that you made. (Of course, they might

replace the battery, but you cant be sure youcan trust your ancestors).

In order to last a thousand years, a cir-cuit needs to draw a miniscule amount ofpower. Very few integrated circuits will workoff such extremely low power -- in fact, IC1 is


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one such integrated circuit which ordinarily will not. Ordinarily it draws more than 500A at 9 Volts. However -- it may be persuaded

to use as little as 0.3 A. Also, part of what makes this circuit

possible is major advances in LED technology. While in the 1970s one could obtain onlysmall, dull red LEDs, many LEDs, by now,come with warnings that they could pose arisk to ones eyesight.

Not only this, but every conceivable co-lour is available -- including infrared and ul-traviolet -- as well as various shapes and view-ing angles. A narrow viewing angle usuallytranslates into greater intensity.

CIRCUIT THEORY

IC1 can be persuaded to use much lesspower than it normally would, simply by rest-ricting the current flow through R1. The res-ulting flash is not too bright to be sure, yet it

will be clearly visible at night.

This circuit is unorthodox, in that IC1requires a minimum 3V to operate -- yet whenLED D1 flashes, the voltage across IC1 dropsto 2V.

At this point, the circuit is theoretically


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non-functional -- yet it does permit C1 to recharge through R1 and R2. As the voltageacross IC1 again approaches 3V, so IC1 kicks

into life again, and the discharge of C1 is againpermitted, through D1. Unused gates aretied high to conserve power.

This circuit is what is commonly calleda relaxation oscillator, or a clock generator.

As input pins 1 and 2 go high, so output pin3 goes low, and vice versa. High meanstwo-thirds of supply voltage, while low me-ans one-third of supply voltage. With this inmind, see if you can work out how the THOU-SAND-YEAR FLASHER flashes. The IC iscalled a Schmitt device because its gatesswitch very decisively. The circuit will flash atabout 0.5 Hz.

I used the Motorola version of IC1 (the

MC14093 BCP). While other CMOS 4093 ICsshould work in this position, this has not beentested. D1 should be an ultrabright red LEDonly. C1 should be a quality, low-leakage co-mponent.

With the component values shown inthe table on the right, this

circuit will flash much morebrightly at 0.5Hz for -- alas --a mere twenty years, drawing12 A off six high capacity AA

batteries. Even so, this couldmake a very nice gift that keeps on giving.For instance, encapsulate it in polyester resin


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and give it to a young child, and it could stillbe flashing when they enter college one day!

CONSTRUCTIONAL NOTES

IC1 is a CMOS component, whichmeans that it is sensitive to static. Immedi-ately before handling IC1, touch your body to

earth -- e.g. by touching a metal tap, or themetal frame of your PC. Notethat ultrabright LEDs mayalso, sometimes, be sensitiveto static.

Ultrabright LEDs canbe confusing with their polarity. The best wayto judge this is by the flat on the side of theplastic encapsulation as shown. This is thecathode (k).

FURTHER IDEAS ...

MINIATURISATION. Since currentconsumption is very small, this circuit wouldlast for a long time off a very small battery.Thus the circuit could be miniaturised.

MICROPOWER PENDULUM. For thepatient experimenter, this circuit could be us-


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ed to swing a small magnet on a pendulum fora very, very long time. In this case, an electro-magnet would need to be wired in series with

ultrabright LED D1 -- or could replace D1. Ahand-wound, coreless electromagnet might

work best here.

The rate of flash (controlled by R1, R2,and C1) would need to be matched perfectly tothe period of the pendulum, and this couldrequire patience. To aid this task, R2 could be

replaced with a preset potentiometer for easyadjustment.

COMPONENTS LIST

R1 . . . . . . . . . 3M3 Watt resistor

R2 . . . . . . . . . 470k Watt resistorC1 . . . . . . . . . 1 high grade electrolytic

(e.g. tantalum) capacitor

D1 . . . . . . . . . Ultrabright red LED only

IC1 . . . . . . . . MC14093BCP Schmitt

quad NAND IC

B1 . . . . . . . . . 9V battery or battery pack



Connecting wire

Solder


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THE BEST OF

THOMASSCARBOROUGH

BOOK 2

___________________________________


www.lulu.com

Publisher ID: 785564http://books.lulu.com/content/785564

Copyright: 2007 by Thomas ScarboroughEdition: Second Edition

License: All Rights Reserved. No part of thispublication may be reproduced, stored in aretrieval system, or transmitted in any form or by

any means, electronic, mechanical, recording orotherwise, without the prior written permission ofthe author.



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CONTENTS__________________________

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . v

DEDICATION . . . . . . . . . . . . . . . . . . . . . . . . . . vi

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . 1

DETECTION CIRCUITS . . . . . . . . . . . . . . . . . . 7

BFO METAL DETECTOR . . . . . . . . . . . . . . 7

BB METAL DETECTOR . . . . . . . . . . . . . . . 19

THUNDERSTORM MONITOR . . . . . . . . . 31

CRYSTAL AMP RADIO . . . . . . . . . . . . . . . 41DAY-NIGHT SWITCH . . . . . . . . . . . . . . . . 51

HELIX THERMOSTAT . . . . . . . . . . . . . . . 60

SECURITY CIRCUITS . . . . . . . . . . . . . . . . . . . 68

SUPER VIBRATION SWITCH . . . . . . . . . 68

SUPER STATIC SNOOP . . . . . . . . . . . . . . . 79

MAGNETIC KEY . . . . . . . . . . . . . . . . . . . . 84

CODE LOCK . . . . . . . . . . . . . . . . . . . . . . . . 95

PRESSURE PAD SWITCH . . . . . . . . . . . . 101

BIRD SCARER . . . . . . . . . . . . . . . . . . . . . . 107


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LATCHING CIRCUIT . . . . . . . . . . . . . . . . 113

SIMPLE TIMER . . . . . . . . . . . . . . . . . . . . 119SENSITIVE SWITCH . . . . . . . . . . . . . . . . 123


RESISTOR COLOUR CODE . . . . . . . . . . . . . . 6

ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . 18

BASIC FORMULAE . . . . . . . . . . . . . . . . . . . . 59


IEW THE WHOLE SERIES AT




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BFO METAL DETECTORNo. of Components: 2_____________________________

Beat frequency operation (BFO) metal

detectors were very popular in the 60s and70s, soon after the advent of the first commer-cial transistors. Some models sold thousandsof times over. But these quickly went out offashion as superior induction balance (IB) andpulse induction (PI) designs appeared on themarket.

This having been said, BFO metal det-ectors still have significant advantages in theareas of cost and ease of construction. Theymay also be better suited to certain applicati-ons, such as pipe-finding or probing. Also,they are particularly well suited to miniatur-isation.

This BFO metal detector surely has to be the simplest in the world. Not only this, but by BFO standards, it is particularly sen-sitive and stable, and equals the performanceof some commercial models of the 60s and70s! It will potentially pick up a 25 mm (1")


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diameter coin at 90 mm (3").

It suffers hardly any drift, as manybudget designs do, and this makes it particu-

larly pleasant to use. This is not strictly a self-contained design, however -- it requires aMedium Wave (MW) radio to operate.

CIRCUIT THEORY

A BFO metal detector usually incorpo-rates two oscillators -- a search oscillator anda reference oscillator. These run at almost thesame frequency. When the search oscillator"beats against" the reference oscillator, thisproduces an audible heterodyne or beat freq-

uency.

Recently, in the USA, this same prin-ciple was used to project two ultrasonic (in-audible) beams towards a person, over severalmetres (yards), so that an audible beat frequ-ency was heard only by the intended recipient.I tried to reproduce this myself, and it worked

-- yet I discovered that standard ultrasonictransmitters are too weak to transmit an aud-ible signal over any useful distance.

This circuit only has a search oscillator,with the reference oscillator being provided bythe MW radio. The difference frequency is


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then heard in the radio speaker. Although aMW radio's frequency (about 500-1600 kHz)is much higher than that of the search oscil-lator (about 200 kHz), it will clearly pick up

harmonics of this frequency.

The design is based on a simple inver-ter oscillator. In fact, I might well be the inve-ntor of this oscillator.

Since an inductor resists rapid changesin voltage (called reactance), any change in

the logic level at IC1 output pin 6 is delayedduring transfer to inverting input pin 2. Also,there is a propagation delay within the IC its-elf. This sets up a rapid oscillation, which isfed to the aerial of a MW radio. The oppositeend of the search coil is wired to non-inverting


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input pin 3, which stabilises operation.

Since the presence of metal changes theinductance of L1, this brings about a change inthe oscillator frequency when metal is detect-ed. This in turn shifts the difference frequ-ency which is heard in the MW radio speaker.


The sear-ch coil is 70 tur-ns of 30 swg (22awg) enamelledcopper wire on a120 mm (4")

diameter former. A former which was used for a

smaller coil is shown on the left. Winding thecoil is not critical -- nor is the wire gauge.This design is very forgiving.

Wind the coil around the former, and

temporarily hold it together by passing stubsof insulating tape under it and pressing themtogether over the top. Then bind it tightly

with insulating tape all round. Scrape the en-amel off the ends of the coil's enamelled cop-per wires.


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A Faraday electrostatic shield is essen-tial in this circuit, for stability. This preventscapacitive coupling and ground effect. To put

it simply, without the Faraday shield, the BFODETECTOR would pick up both your bodyand the ground as though these containedmetal.

With theFaraday shield inmind, prepare

some long, thinstrips of alumini-um- or tin-foil.Twist a 100 mm(4") length of

bare wire aroundthe coil, over the insulating tape. This wire

provides electrical contact with the foil (seephoto).

Wind the foil around the circumferenceof the coil -- beginning at the base of the bare

wire -- yet the foil should not quite complete afull 360. Leave a small gap (see over thepage), so that the foil does not meet up after

having done most of the round. Now tightlybind the whole Faraday shield with insulatingtape.

Attach the coil to IC1 by means of ba-lanced (figure-8) microphone cable. Each ofthe cables two cores has a screen, and both of


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these screens are soldered to the coils Fara-day shield at the one end, and to -6V(negative) at the other. The scraped ends of

the enamelled copper wire are soldered to thetwo microphone cable cores, which are taken

to IC1 as shown inthe circuit diag-ram. If balancedmicrophone cablecannot be found,

use two separatescreened micro-phone cables.

The coilmay be mounted on a rigid plate, and a suit-able handle attached. A lazy persons solutionfor the handle is to saw the end off a length of

PVC piping at a slight angle, and to drill holesas required to bind the pipe to the plate with acable tie (see the BB METAL DETECTOR inthis book for more detail).

Attach an ae-rial wire for the MWradio. This is ideally

a length of screenedmicrophone cable,

with the screen being wired to -6V. The core may terminate with asmall crocodile clip, as shown in the photo.


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GETTING GOING

Clip the aerial wire to the aerial of aMW radio, switch on both the metal detectorand the radio. Now tune the radio until aclear heterodyne (a whistle) is heard in theradio speaker. Some heterodynes will work

better than others.

You will notice that there is a "band ofsilence" at the centre of most heterodynes --that is, a zero beat frequency. Depending

which side of this zero beat frequency is tunedin, the metal detector's tone will either rise orfall at the presence of metal.

You will find that the detector will

discriminate between small ferrous and non-ferrous items (e.g. coins) by giving an oppositereaction in the difference frequency. This isparticularly useful for distinguishing uselessiron from noble metals during treasurehunting.

FURTHER IDEAS ...

1. COIL SIZE. A question I am quiteoften asked is whether the size of a metaldetectors coil may be changed. Usually the


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answer is yes -- as is the case here. In fact, thecoil in this circuit may be replaced with e.g. atiny 100 H inductor. The calculations are

complex -- however, as a rule of thumb, if thecoils diameter is halved, the number of turnsneeds to be doubled, and vice versa.

2. SIGNALLING. Since metal detectorswill work e.g. through walls, this circuit couldbe used to signalthrough walls. If a MW rad-io is suitably tuned -- that is, to just one side

of a beat frequency -- and if the search coil ismounted on one side of a wall, then this wouldproduce a tone in the MW radio speaker whena larger piece of metal is held to the othersideof the wall.

3. PINPOINTER. A pinpointer can be a

very useful device formetal detecting enthusi-asts. One might havemade a small find, anddug up a pile of earth --

yet where exactly is it? A pinpointer helps one

to snuffle out the object in question. Forthis purpose, one would reduce the size of thesearch coil, as described above, and mount iton the end of a shaft, so that one can poke atthe soil or sand without needing to bend over.I built such a pinpointer (see the photo) by


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placing a coil inside a large nylon end plug,and binding this to a PVC shaft with clearepoxy resin.

COMPONENTS LIST

IC1 . . . . . . . . . LF351N j-fet op-amp IC

(or TL071CN)

S1 . . . . . . . . . . On-off switch


L1 . . . . . . . . . . 30 yds (30 m) 22awg

(30 swg) enamelled copper

wire for the search coil


1.5m (1 yds) balanced figure-8 screened

microphone cable to attach search coil

1 m (1 yd) single-core screened microphone

cable to connect to a MW radio aerial

100 mm (4") bare wire to twist under the

Faraday shield

Aluminium- or tin-foil for the Faraday shield

Search plate to which to fix coil and handle


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SUPERVIBRATION SWITCH

No. of Components: 6

_____________________________

The old-fashioned vibration switch is well known. This consists of a flexible lever,fixed at one end, with a small weight attachedto the other. When there is vibration, the

weight "bobs" slightly, knocking a terminalbeneath it. Electrical contact is made, and analarm is triggered.

This type of switch usually has an adj-ustment screw, to adjust the gap between the

weight and the terminal, thus adjusting itssensitivity. This project simulates this old-fashioned vibration switch. As with the oldswitch, it also has an adjustment screw (VR1).

The real difference, however, lies in theSUPER VIBRATION SWITCHs sensitivity. Assimple as it is, it may justifiably be describedas being "super-sensitive". While the old-fashioned vibration switch is best suited todetecting noticeable motion, such as a bicycle

being disturbed, or valuables being lifted, the


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SUPER VIBRATION SWITCH is capable ofpicking up very subtle vibrations indeed.

It will easily pick up a person walkingacross a wooden floor at virtually any distance(e.g. at the far side of a hall). My prototype

was capable of picking up a pin striking awooden floor at 2 metres' (2 yards) distance,or the vibration of the neighbours' car doorsclosing at 20 metres (20 yards).

Having said this, however, the circuitmay be adjusted to whatever level of sensitiv-ity would best suit your purposes.

MECHANICAL

The circuits principle of operation is well known. It uses a cheap, standard piezo


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element which flexes slightly in response to vi-brations, thus creating a minute electrical out-put. In short, it converts mechanical energy to

electrical energy.Bi-morph elements are also available

for this purpose - however, these are morecostly, and are not as robust as the piezoelement used here. Such bi-morph elementsmay by all means be tried in this circuit.

To mountthe piezo elem-ent, I carefullydrilled a 3 mmhole at one edgeof the element. Isoldered a wash-er to this hole to

reinforce it, theninserted a 25 mm(1) long 3 mmdiameter bolt

through the hole, and mounted the wholecontraption on a base as shown. An arran-gement I once used for a similar circuit is

shown above left.The "far end" of the piezo element,

which is suspended in the air, ideally needs to be weighted in such a way that most of theelement will be free to flex. A weight of a fewgrams would be suitable, and this may be


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glued, clipped, or screwed to the edge of thepiezo element. A greater weight is likely to in-crease sensitivity a little.

CIRCUIT THEORY

IC1 is wired as a standard monostabletimer. It is based on the very well known 555

timer IC -- but note that this is a moresensitive CMOS version of the IC, the 7555.

The trigger input, pin 2, is biased closeto triggering through the adjustment of VR1.R1 and C1 are timing components, and switchTR1 for a period determined by the followingformula:

t = 0.69 R C

where R is Ohms, C is Farads, and the result(t) is given in seconds. With the component

values shown, the circuit will switch for 6.9seconds.

TR1 is an n-channel power MOSFET. I

use these transistors throughout this series.Since they are voltage controlled, they areeaser to use than bipolar transistors -- at leastfor switching applications. They generallyrequire fewer peripheral components (inthis case none), and have an extremely low


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on resistance (typically much less than 1Ohm). This MOSFET easily conducts 10 Wat-ts, or up to 43 Watts with a suitable heat sink.

The voltage of the load will of course need tomatch the supply voltage.

GETTING GOING

Switching S1 may of course cause thecircuit to vibrate. To give you time to getclear, you may wire a 100k Watt resistor

between reset pin 4 and +12V, and a 100 16Velectrolytic capacitor between pin 4 and 0V(the negative terminal of the capacitor is takento 0V). This will let the circuit settle down

after it is switched on, then it will quietlyactivate.

Carefully adjust VR1 to the point whereTR1 just switches, then adjust further for thedesired sensitivity.

The SUPER VIBRATION SWITCH is well suited to battery operation, as it uses a

mere 1.5 mA on standby. Thus AA batteriesshould power it for more than a month -- if itis not triggered too often. However, for moresensitive applications it would make sense touse a regulated power supply, since this willkeep the voltage steady.


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FURTHER IDEAS ...

STANDARD TIMER. This circuit is based of course on a classic timer circuit. When IC1 trigger pin 2 goes low (belowabout one-third of the supply voltage, thetimer begins to time. The formula forcalculating the timing period s given on theprevious page.

So, for instance, one might take triggerpin 2 high (to +12V, or positive) by means ofa 10k resistor, and low (to 0V, or negative)

by means of a push-to-make (PTM) pushbutton switch. Now, on pressing the button,TR1 conducts for a timed period.

BICYCLE ALARM. The circuit may beused as a very sensitive bicycle or motorbikealarm. No one will be able to move yourwheels in the slightest without triggering thecircuit.

However, you will need to provide aswitch whereby, on returning to your bicycle

or motorbike, you can instantly switch offtheSUPER VIBRATION SENSOR, or you yourself

will trigger the circuit.

While there are ways of delaying trig-gering, this requires more than six com-ponents. See the SIMPLE TIMER elsewhere


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in this book.

PORTABLE ITEM ALARM. The circuitmay also be secured to an item such as a com-puter, so that it will trigger as soon as the itemis picked up.

EARTHQUAKE ALARM. Earthquakesare preceded by P-waves and S-waves (shortfor primary and secondary waves). These tra-

vel faster than surface waves, and are far less

severe -- perhaps rattling windows or vibrat-ing lights just seconds before a major quake(the photo depicts Los Angeles in 1906).

At 30 km (about 20 miles) from theepicentre of a quake, one would typically rec-eive 6 seconds' warning from P-waves. Con-sidering that it may realistically take less than10 seconds to get out of doors, and perhaps


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only 3 seconds to get under a table or otherstable structure, this could be a life-saving

warning.

The SUPER VIBRATION SENSOR isentirely capable of picking up even fairly smallP-waves and S-waves.

AIR PRESSURE ALARM. If e.g. e pieceof stiff card is glued to piezo element X1, thenthe circuit will pick up relatively gentle puffs

of wind -- even a door or a window opening atseveral metres (yards) distance.

MAGNETIC SWITCH. If the weight which is fixed to X1 is a small neodymiummagnet, the circuit will pick up another,moving magnet in the vicinity. In this case,the circuit will need to be mounted so that it

will not pick up vibrations from other sources,e.g. a floor or a door.

In some situations, this may offer agood alternative to a Hall effect sensor, andoutperform it significantly.

Hall effect sensors typically have amaximum range less than 10 cm (4), whilethis circuit should be capable of responding attwice this distance.


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COMPONENTS

R1 . . . . . . . . . . 100k -Watt resistor

VR1 . . . . . . . . 1M multi-turn preset

potentiometer

C1 . . . . . . . . . . 100 16V electrolytic

capacitor

TR1 . . . . . . . . IRF510 n-channel power

MOSFET (or equivalent)

IC1 . . . . . . . . . ICM7555IPA timer (notthe

NE555N) IC

X1 . . . . . . . . . Large piezo element, approx.

30 mm (1) dia.S1 . . . . . . . . . . On-off switch



25mm (1) long 3 mm bolt, 3 nuts, 1 washerSuitable plastic case

Connecting wire

Solder


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Page 42

THE BEST OF

THOMASSCARBOROUGH

BOOK 3

___________________________________


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License: All Rights Reserved. No part of thispublication may be reproduced, stored in aretrieval system, or transmitted in any form or by

any means, electronic, mechanical, recording orotherwise, without the prior written permission ofthe author.



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CONTENTS__________________________

FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . v

DEDICATION . . . . . . . . . . . . . . . . . . . . . . . . . . vi

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 1

NOVELTY CIRCUITS . . . . . . . . . . . . . . . . . . . . 7

ELECTRIC SHOCK MACHINE! . . . . . . . . 7

JUMPING SPIDER . . . . . . . . . . . . . . . . . . 13

CHRISTMAS CHEEKS . . . . . . . . . . . . . . . 25

NOVEL ANEMOMETER . . . . . . . . . . . . . . 31

ROBOTIC VOICE . . . . . . . . . . . . . . . . . . . . 37

POT-PLANT POWER . . . . . . . . . . . . . . . . 43

SPECIAL CIRCUITS . . . . . . . . . . . . . . . . . . . . 49

TENS UNIT . . . . . . . . . . . . . . . . . . . . . . . . 49

WART REMOVER . . . . . . . . . . . . . . . . . . . 55CAT/DOG SCARER . . . . . . . . . . . . . . . . . . 63

CCO METAL DETECTOR . . . . . . . . . . . . . 69

HAND GENERATOR . . . . . . . . . . . . . . . . 83

VERSATILE TIMER . . . . . . . . . . . . . . . . . 91


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LEADING EDGE TRIGGER . . . . . . . . . . . 97

TRAILING EDGE TRIGGER . . . . . . . . . 105REGULATED SUPPLY . . . . . . . . . . . . . . 113


RESISTOR COLOUR CODE . . . . . . . . . . . . . . 6

ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . 12

BASIC FORMULAE . . . . . . . . . . . . . . . . . . . . 90


IEW THE WHOLE SERIES AT




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ELECTRICSHOCK MACHINE!

No. of Components: 1

_____________________________

I cannot claim any originality for thisidea. It has been around for decades in ex-actly this form. However, it is a fun design toinclude, and represents perhaps the mostexciting thing one can do with a single com-ponent! It may generate a voltage of around100V, yet it is a miniscule current -- givinganything from a light tingle to a sharp bite

when the probes are touched.

The circuit is basically harmless. Theskin, even when moist, will have a resistanceof around 200k, so that the body will typicallyconduct no more than about half a milliamp at

100V.This having been said, even small

currents can do strange things to the body, sothat this circuit should on no account be used

by anyone who suffers epilepsy or uses a pace-maker. Also, do not use anything larger than

AA batteries for this circuit, and on no acco-


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continual back-EMF is generated. In fact, thisis the basic principle which is used for step-ping up a voltage.

Note that a relay with an internal diode(for suppressing back-EMF) is not suitable inthis design -- it is precisely the back-EMF

which is required here.


The grips may be any metal items which may be held in the hands, e.g. 15 cm(6) stubs of copper piping.

It could be awkward trying to solderleads to the two metal grips. However, one


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could solder the leads to tags, and screw thetags to holes in the metal grips with small nutsand bolts.

GETTING GOING

Turn on the power -- the circuit will buzz -- and hold the grips. If the effect is

slight, or cannot be felt at all, wet your handsand try again. Or try another relay, since thecharacteristics of relays may differ consider-ably.

FURTHER IDEAS ...

SHARED SHOCK. Special pleasuresare there to be shared. Ask someone else tohold one grip with their left hand. Hold theother grip with your own left hand. Switch onthe circuit -- then shake hands!

NEON LIGHT. Since the voltage gene-rated is likely to be above 80V, this circuit willilluminate a small neon indicator (this is asmall glass tube with two metal rods inside) --or even (depending on the relay) a larger tube,say a 100mm (4) tube.


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PARTY TRICK. If the grips are dippedin a saline solution (water with some salt ad-ded) this may cause a tingling sensation if one

dips ones hand into the water. The effect-iveness of this trick will again depend on therelay used. Now add coins to the water, andinvite someone to retrieve them ...

COMPONENTS

S1 . . . . . . . . . . On-off switch

RLA1 . . . . . . . 12V single-pole-double-throw

(SPDT) relay


(nota DC plugpack power

supply)

Metal grips

Small metal tags and nuts and bolts to suit

Battery clip to suit battery or battery packSuitable plastic case

Connecting wire

Solder


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TENS UNITNo. of Components: 6_____________________________

TENS stands for Transcutaneous Ele-ctrical Nerve Stimulation. TENS units, inmany cases, are extraordinarily effective incontrolling pain. This is not to say that aTENS unit will always have the desired effect-- however, they have been shown to have asignificant effect in 70% of cases.

Interestingly, TENS units have be-

come popular for, shall we say, stimulation ofother kinds. For this reason, a major maga-zine recently pulled their TENS unit, toovercome a run on their website!

As with all TENS units, this one com-es with a caution. While it should be perfectlysafe to use, it is strongly advised that a med-

ical doctor's advice should first be obtained.Its use is advised against during pregnancy,or where a patient has a heart condition, orany history of epilepsy.

TENS units may ordinarily be quiteexpensive. However, all that is required for


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The required pulse width is providedin this case by a unijunction transistor (UJT),

wired as a relaxation oscillator. When emitter

potential reaches the UJTs peak point. anavalanche effect is created, so that it conductssimultaneously across its emitter to base 1

junction, and across bases 1 and 2, shootingvery short pulses through transformer T1.

With T1 being a mains to 6-0-6V tra-nsformer wired in reverse, the voltage is step-

ped up to the required level.

The unit allows some adjustment ofthe pulse frequency through VR1, for experim-entation.

Ordinarily, the current which the TE-NS unit pulses through the body is hardly

discernible. One might be tempted to askwhether anything is happening at all -- but trythis experiment: if two fingers of one handare pressed onto the two separate electrodesfor half a minute, one may feel an enduringtingling sensation afterwards -- proof thatsomething significant is happening.

The TENS unit should give about onemonth of all-night service before the batteryneeds replacing. For safety reasons, the unitshould always be run off AA batteries. Itshould not under any circumstances be pow-ered off a mains transformer.


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The two output leads may be solderedto large metal washers, to serve as electrodes.These are then thinly coated with moisturisingcream and taped to each side of the painfularea.

Better still, purpose-made electrodesmay be used. Although these items are some-

what hard to obtain, they are routinely used inhospitals, and are indeed to be found. Oneoften sees them used, for instance, with ECGapparatus.

FURTHER IDEAS ...

INCREASED BITE. If the pulse isthought to be disappointing (but it probablyneed not be more), you may increase battery

voltage to 12V.

Also, the value of R2 may be reducedto as little as 10 Ohms, and the transformerssecondary voltage may be reduced to 3-0-3V.If all of these recommendations are followedtogether, you could be in for a shock!

A pushbutton and miniature neon in-dicator may be wired across T1s 230V prim-


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ary coil -- these two components being wiredin series. This would serve to test the func-tioning of the unit, pulsing the indicator if it is

working. A neon indicator requires in excessof 70-90V to illuminate at all, therefore this is"proof of the pudding" that the unit is func-tional.

BATTERY STATE INDICATOR. Theabove would serve as a rough and ready bat-tery state indicator -- and not only for the

TENS UNIT itself. With suitable adjustmentof R2, the circuit could test any 9V or 12V bat-tery.

NEON TORCH/FLASHLIGHT. Be-fore White LEDs became available, I designeda torch/flashlight (which I used in the Pacific),

which flashed a green fluorescent glow lamp

-- a small fluorescent tube, smaller than thetip of ones little finger. A torch/flashlightreflector is required to focus the light. Thelight was soft, but power consumption wasexcellent, and the bulb was virtually indest-ructible.

NOVEL FLASHER. Flashing LEDs

today are ubiquitous. So why not be different,and use a flashing neon indicator instead?

Also, one may substitute electroluminescentsheets, and these can be quite impressive.

METRONOME. T1s 6-0-6V second-ary coils could be replaced with virtually any


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inductor, e.g. a suitably rated loudspeaker. Inthis way the circuit could serve as a metro-nome. The tempo could be slowed by inc-

reasing the value of C1, as well as, of course,adjusting potentiometer VR2.

COMPONENTS

R1 . . . . . . . . . 10k -Watt resistor

R2 . . . . . . . . . 47R 2-Watt resistorVR1 . . . . . . . . 20k carbon track

potentiometer

C1 . . . . . . . . . . 10 16V radial electrolytic

capacitor

TR1 . . . . . . . . 2N2646 unijunction

transistor (UJT)S1 . . . . . . . . . On-off switch

T1 . . . . . . . . . 230V to 6-0-6V 100mA mains

transformer



Twin-flex wire to take to the electrodes

Large washers for purpose-made electrodes


Connecting wire

Solder


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THE BEST OF

THOMASSCARBOROUGH

BOOK 4

FOR THE MATURE HOBBYIST

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retrieval system, or transmitted in any form or byany means, electronic, mechanical, recording orotherwise, without the prior written permission ofthe author.

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LIGHT BULBS CONJURING TRICK . . . . . . . . . . . . . . . . 41

MAGIC WAND CONJURING TRICK . . . . . . . . . . . . . . . . 44

GYM AGILITY CIRCUIT . . . . . . . . . . . . . . . . . . . . . . . . . . 47

DAY-NIGHT SWITCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

BODY DETECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

SQUASH SWITCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

OMNIDIRECTIONAL PENDULUM . . . . . . . . . . . . . . . . . 66

EVERYTHING THAT MOVES ALARM . . . . . . . . . . . . . . 70

FLUID FINDER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

FREQUENCY SWITCH . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

JAZZY HEART . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

64-LED SEQUENCER . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

SHOESTRING RADIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

LOOP AERIAL RADIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

LM380N RADIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

MICRO RADIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

TRANSISTOR CCO METAL DETECTOR . . . . . . . . . . . . 102

INTEGRATED CIRCUIT CCO METAL DETECTOR . . . 105

TRI-COIL METAL DETECTOR . . . . . . . . . . . . . . . . . . . . 109

MATCHLESS METAL LOCATOR . . . . . . . . . . . . . . . . . . 113

555 BFO METAL DETECTOR . . . . . . . . . . . . . . . . . . . . . 123


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PIPEFINDER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

BB METAL DETECTOR . . . . . . . . . . . . . . . . . . . . . . . . . 127

MINIMAL METAL DETECTOR . . . . . . . . . . . . . . . . . . . 130

2 METAL DETECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . 132

MIRJAMS ROOM RECORDER . . . . . . . . . . . . . . . . . . . . 134

SEEING EYE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

DOG YAP INHIBITER . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

FREQUENCY METER . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

SIMPLE SIREN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

STEPPER MOTOR DRIVER . . . . . . . . . . . . . . . . . . . . . . 147

MAGNETOMETER-SEISMOMETER . . . . . . . . . . . . . . . 149

SIMPLE STYLOPHONE . . . . . . . . . . . . . . . . . . . . . . . . . . 158

VISUAL CAPACITANCE GAUGE . . . . . . . . . . . . . . . . . . 160

4060 YES-NO INDICATOR . . . . . . . . . . . . . . . . . . . . . . . 162

4047 YES-NO INDICATOR . . . . . . . . . . . . . . . . . . . . . . . 164

SUPERIOR HEADS-TAILS INDICATOR . . . . . . . . . . . . 166

60p DPDT RELAY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

MAGNETIC GUN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

INTERCOM/BABY LISTENER . . . . . . . . . . . . . . . . . . . . 173

DUMMY BOMB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

DISCO LIGHT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179


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ELECTROMAGNETIC LOCK . . . . . . . . . . .. . . . . . . . . . . 182

PATTERN FLASHER . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

SPACE CASE ALARM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

MODEL TRAIN DETECTOR . . . . . . . . . . . . . . . . . . . . . . 188

NEON TORCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

LED TORCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

NEON DESKLAMP . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 194

WORLD LAMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

LED TOUCH LIGHT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

MULTI-LEVEL LOCK . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

ONE-WAY BROKEN BEAM ALARM . . . . . . . . . . . . . . . 213

PLUG-PACK CHECKER . . . . . . . . . . . . . . . . . . . . . . . . . . 216

MULTI-LEVEL LOCK . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

LINE-O-LIGHT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

WART ELIMINATOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

PROGRAMMABLE ROBOT . . . . . . . . . . . . . . . . . . . . . . . 241

SHORT-RANGE RADIO CONTROL . . . . . . . . . . . . . . . 257

ALTERNATE LED FLASHER . . . . . . . . . . . . . . . . . . . . . 260

INTRUDER DELUDER . . . . . . . . . . . . . . . . . . . . . . . . . . 262


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FLUID FINDER__________________________

RETROSPECTIVE COMMENTS: 1.This circuits sensitivity was compromised alittle by frequency lock. It is hard to see how

this could be worked out of the systemwithout significant re-design. 2. This circuitwon me a valuable prize -- a dual trace storageoscilloscope.

__________________________

The circuit of Fig.1 is capable ofdistinguishing between different liquids, or,alternatively, of determining their purity. Itmay also be used to measure the level ofhighly corrosive liquids in a tank where these

would devour an ordinary probe. With slightmodification it would distinguish certainsolids, e.g. one sheet of paper as opposed to

two.

A critical element of the formula usedto determine the capacitance of a parallel-plate capacitor is the relative permittivity of

the dielectric, or the dielectric constant (r).Relative permittivity is defined as


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"capacitance with material as dielectric" over"capacitance with air as dielectric". Althoughthe circuit is relatively crude, it will determine

relative permittivity to within a fraction of onepercent. This means that the Fluid Finder ofFig.1 distinguishes unfailingly between e.g.distilled water, tap water, and milk, andreveals vast differences between fluids such as

water, alcohol, and oil.

At the heart of the circuit is custom-

built parallel-plate capacitor C1, whose platesare sealed in epoxy resin, and separated by anair space. This capacitor is dipped into a liquidto determine its relative permittivity, which inturn is deduced from the capacitancemeasured by means of VR1.

The frequency of RC oscillator IC1a-

IC1b is fed to digital bandpass filter IC2a-IC2b, then VR1 is adjusted until l.e.d. D4illuminates. The relative permittivity ofdistilled water (which we shall designate x) ismeasured as 100,000-VR1 in Ohms, then therelative permittivity of any other fluid iscalculated as (100,000-VR1 in Ohms) * ( 78 /x ) -- assuming that the relative permittivity ofdistilled water is 78. Needless to say, aquantity of distilled water is first required todetermine the value of x. This should beavailable from most garages.

The digital filter IC2 goes either "high"or "low" at ouput pin 9 until the desired


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frequency (about 100kHz) is presented at theinput. In this case a square wave at output pin9 is detected by means of simple diode pump

D2-D3 and C4-C5. When C5 charges, theinput of IC1c goes "high", and l.e.d. D4illuminates. L.E.D. D1 is included for easieradjustment, as this shows whether the outputis "high or "low". S1 is provided so that IC1and IC2 are not overdriven in the absence of aliquid. With IC1a-IC1b typically oscillating ataround 100kHz, this would be multiplied by

78 if distilled water were replaced with air,which has a relative permittivity of about 1.

Custom-built capacitor C1 was madefrom two plates of copper-clad boardmeasuring 30mm x 40mm, with the platesfacing inwards. These were soldered to wires,then sandwiched in 80gsm paper, which was

coated twice with epoxy resin. The preparedplates were mounted 7mm apart with nylonspacers.

The accuracy of the Fluid Finder isdetermined by R4. This may be decreased for

greater accuracy, and vice versa. 220 was

found to be a practical value here. Use amultimeter to ensure that the values of R2-R4are the actual values used. Note that capacitorC1 is affected by body capacitance, thereforeneeds to be kept a few centimetres away fromany part of the body for an accurate reading.


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THEREMIN DOORBELL_____________________________

Install a doorbell with a difference. TheTheremin Doorbell shown in Fig.1 uses ametal sensor for the "doorbell", playing aTheremin as a hand approaches. Therefore, a

slight tremolo effect might indicate thepresence of Aunt Agatha, while a moreauthoritative swoop in pitch might indicatethe presence of Brother Joe.

Not only this, but the sensor plate maybe placed at the foot of a doorway instead, toreport people walking in and out. This would

make an interesting alternative to the moreusual broken beam.

The range of the Theremin Doorbell isup to 20cm. That is, a hand will induce a shiftin frequency of one tone at a maximum 20cm.For everyday use, however, this lies morerealistically around 8cm. This is still sufficientto play e.g. Happy Birthday with carefulcontrol of one's hand.

Relaxation oscillator IC1a employs avery small value for C1, so that the presence ofa human body at the sensor plate increases itseffective capacitance. This in turn decreases


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the frequency of oscillator IC1a. As thenumber of pulses generated at IC1b output pin4 decreases, so the charge on capacitor C2

falls. If the rate of discharge of capacitor C2 iscritically adjusted through VR1, a smallvariation of the voltage across C2 causes alarge variation of potential at power MOSFETTR1's drain. This is used in turn to control thefrequency of a voltage controlled oscillator(v.c.o.) IC2. TR1 may be virtually any powerMOSFET.

The v.c.o. selected for this task is surelythe simplest and most versatile available.Strictly, it is a phase-locked loop i.c., of whichonly the oscillator section is put to use. It hasthe great advantage of falling completelysilent as the voltage at control pin 9 falls toabout 1V, as well as having an easily

adjustable top frequency limit, which isdetermined by R6. Thus the ThereminDoorbell is silent until a hand approaches,and will not exceed a specified frequency even

when a hand touches the sensor plate directly.

The Theremin Doorbell draws just over3mA current on standby. A regulated power

supply is recommended for stability. Initiallytry a metal plate or sheet of tin foil measuringabout 20cm by 20cm for the sensor,connected firmly to the circuit. To set up theDoorbell, turn preset potentiometer VR1across its range until the critical point isreached where frequency varies vastly over a


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few turns. Then adjust for silence, so that ahand at about 8cm begins to cause a crackle inpiezo sounder WD1.

The picture shows inventor LeonTheremin with his musical instrument.


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system. It may be placed within the line-of-sight of almost any light source, including

vague ambient light, and simply switched on

without any adjustment.In daylight, the Seeing Eye will

typically detect a single finger moving at adistance of 2 metres - withoutthe use of anylenses. It will detect a person crossing a pathat 10 metres' distance - withoutlenses. Under

AC lighting, as an "active" system, it willtypically detect a person walking in front of an

ordinary light source at more than 10 metres -withoutthe use of lenses. This range isachieved by sliding a a black tube over thelight dependent resistor (LDR) as shown inFigure 2.

The LDR is so wired in conjunctionwith R1 to R3 that, between darkness and full

sunlight, it offers a potential at point X ofbetween roughly one-quarter and three-quarters of supply voltage. The present circuitdiffers from the more usual "passive" lightsensor in that it uses the offset-adjust featureof comparator IC1 to balance the inputsinstead of a potential divider. This makes for amore sensitive and reliable circuit. A wide

variety of sensors may be used in place of thespecified LDR, including photo-transistors,photo-diodes, and infra-red and ultra-violetdevices.

The potential at point X is presentedsimultaneously, through R4 and R5, to the


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inputs of comparator IC1. As the potentialfluctuates at point X, changes in potential aredelayed at the comparator's non-inverting

input through C2. Thus an imbalance occurs,causing the output of the comparator goes"low". Thus monostable timer IC2 is triggered,switching relay RLA. IC2 may be adjusted bymeans of VR2, to hold the relay closed

between about three and thirty seconds.

As with all such circuits, the Seeing Eyemay not work as well under AC lighting as

opposed to natural lighting. If AC lightingshould prove to be particularly problematic, acapacitor (say 1 16VW) may be added

between point X and 0V, to smooth thepotential presented to comparator IC1'sinputs, and holes are provided on the printedcircuit board (PCB) for this purpose.

Because the circuit is very sensitive, aspecial problem presents itself in the form ofrelay RLA, which carries a relatively heavycurrent when switched by monostable IC2.This would ordinarily upset the circuit andreduce its sensitivity. Besides the use ofsupply decoupling, this problem is overcome

by "blanking" the relay's action through TR1,

which disables the trigger input of timer IC2,thus allowing the circuit to settle after relayRLA has disengaged. The "blanking" alsomakes it possible to run external circuits offthe same power supply as the Seeing Eye.Current consumption is nearly 20mA onstandby, so that unless the circuit is run off a


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car battery, a 12V "plugpack" adapter isrecommended.


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