Automatic lighting - Night Light

8/12/2019 Automatic lighting - Night Light

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This simple nightlight starterproject is switched on simply bytapping on the case, or gentlyclapping ones hands close tothe unit. The low power bulbthen switches on for

approximately five minutes, butit can be switched off at any timeby operating a Reset button onthe top of the case.

The unit was primarilydesigned with children in mind,but it could obviously be a usefulgadget for adults as well,particularly disabled people. It isbattery powered and is thereforesafe for use by small children.The simple circuit and lack ofmains power also makes this

project suitable for constructionby complete beginners.

Reset switch controls this input.

A piezoelectric microphone

is used to provide the triggersignal for the monostable, butthe output signal from the microphone is too low to providereliable triggering. A single stageamplifier is therefore used toboost the signal from themicrophone to a level that givesbetter results.

The sensitivity of the circuitis still quite low, but highsensitivity is not an advantage inthis application. It would simply

result in frequent spurioustriggering of the unit, andoperating the Reset switchwould produce vibrations thatwould immediately re-trigger theunit.

This latest, low-cost starter project is sure to be a turn-

on. Just a gentle tap or clap will trigger it into life.

AUTOMATIC NIGHTLIGHT

by ROBERT PENFOLD

provides an output pulse of acertain duration each time it is

triggered. In this case we requirethe light bulb to be switched onfor about five minutes, and thetiming components in the

AMPLIFIER MONOSTABLEOUTPUTTRANSISTOR

LAMP

RESET

MICROPHONE

Fig.1. Block diagram for the Automatic Nightlight.

SYSTEMOPERATION

The block diagram of Fig.1helps to explain how theAutomatic Nightlight functions. It

is based on a monostable,which is simply a circuit that

monostable are thereforechosen for an output pulse ofthis duration.

The monostable cannotprovide sufficient current topower the bulb, which istherefore controlled by way of asingle transistor output stage.Also, an input of the monostable

permits the output pulse to beterminated prematurely and the


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CIRCUIT OPERATIONThe full circuit diagram for

the Automatic Nightlight projectappears in Fig.2. It is based on alow power version of the 555

timer (IC1) used in the standardmonostable mode. It isimportant to use a low power555 in this circuit, because it willbe left switched on, but in thestandby mode, for long periodsof time.

The standard 555 has acurrent consumption of only afew milliamps, but over a periodof several nights this wouldsignificantly drain the batteries.A low power 555 consumes only

a fraction of a milliamp, andshould not greatly drain thebatteries, even after a fewmonths of use.

ON TIMEThe timing components in

the monostable are resistor R3and capacitor C2. Understandby conditions an internaltransistor of IC1 places a virtualshort circuit across C2 that

prevents it from charging via R3.This short circuit is removedwhen IC1 is triggered, and C2then charges via R3 until the

charge potential equalsapproximately two thirds of thesupply potential. The internaltransistor then switches onagain, and almost instantlydischarges capacitor C2.

Under standby conditionsthe output at pin 3 of IC1 is low,but it goes high while capacitorC2 is charging. The output pulseduration is equal to

approximately 11 CRseconds,with the capacitance value inmicrofarads and the resistancein megohms.

With the specified values thisworks out at 297 seconds, or justunder five minutes in other words.In practice the pulse duration islikely to be somewhat longer thanthis due to slight leakage in C2,and it is not possible to set highlyaccurate output pulse times with asimple circuit of this type.

In the present applicationhighly accurate results are not

required, and any slight lack ofconsistency in the output times isalso of little importance. Differentoutput times can be produced bychanging the value of resistor R3and (or) capacitor C2, but times ofmore than about 20 minutesbecome increasingly unreliabledue to the high values involved.However, longer times are notreally needed in this application,and for most purposes thespecified values will suffice.

LIGHTING UP TIMEAlthough light bulb LP1 is a

low current (60mA) type, it stillrequires more current than IC1can supply. Consequently, it isdriven via common emitterswitching transistor TR2. Whenthe output of IC1 goes high TR2 isbiased into conduction, and LP1 is

Constructional Project

Completed circuit board showing the component layout and

the use of an IC socket for the low power 555 timer IC.

b

c

e

b

c

e

B16V

LAMP

RESET

ON/0FF

MICROPHONE

IC1TS555CN

TR2BC337

TR1BC549

3

R52k2

R110M

R410k

R32M7

R247k

LP16V60mA

4

2

8

1

6

7

C2100

C147

S2

MIC1

+6V

RST +VTHRES.

DIS.

TRIG. GND

OUT

0V

Fig.2. Complete circuit diagram for the Automatic nightlight.


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supplied with virtually the full 6Vsupply. Resistor R5 prevents anexcessive base current fromflowing into TR2.

The circuit can handlehigher output currentsincidentally, but do not use a

bulb having a rating of morethan about 250mA. Of course,using a bulb having a currentrating of more than 60mA willsignificantly shorten the batterylife.

Resistor R4 takes the resetinput of IC1 (pin 4) high so thatthe monostable functionsnormally. Pressing switch S1

pulls pin 4 low and causes theoutput pulse to be terminatedimmediately. Of course, pressingS1 has no effect if LP1 is notswitched on.

INPUT TRIGGERThe trigger input at pin 2 of

IC1 is driven from the output of asimple common emitter amplifierbased on transistor TR1. Totrigger the monostable this inputmust be taken below one third ofthe supply voltage. The high valueof bias resistor R1 ensures thatunder quiescent conditions theoutput voltage at the collector (c)of TR1 is half the supply potential

or more.When the microphone MIC1

picks up a sound the collectorvoltage of TR1 moves either sideof its normal level, and if thesignal is strong enough thevoltage will briefly go below onethird of the supply potential andtrigger IC1. Microphone MIC1 isactually a piezoelectric sounderand not a proper microphone. Inthis application audio quality is nota consideration, and a piezo

sounder used in reverse as acrude crystal microphone givesgood results at low cost.

It is important that the currentconsumption of the amplifier isvery low so that good battery lifeis obtained. The current drawn bytransistor TR1 is typically about60uA, and the quiescent currentconsumption of the circuit as awhole is approximately 250uA.

CONSTRUCTIONThis battery-poweredAutomatic Nightlight, the latest inour special series of low-coststarter projects, is built up on asmall piece of stripboard. Thetopside component layout,underside details and interwiringto off-board components is shownin Fig.3. Containing just 24 holes

by 18 strips the board is not astandard size and a larger boardmust be trimmed down to thissize using a hacksaw or a juniorhacksaw.

The two mounting holes are3mm diameter and will accepteither 6BA or metric M2.5mounting bolts. Most plasticstand-offs do not work well withstripboard and it is better to usemounting bolts plus spacers tokeep the underside of the boardclear of the case.

The seven breaks in thecopper strips can be made usinga special tool, but a handheldtwist drill bit of about 5mmdiameter will do the job just aswell. The circuit board is nowready for the components andlink-wires to be added.

Most low power versions ofthe 555 timer are not static-sensitive, but it is still advisableto mount this component on theboard via a holder. Be careful tofit both capacitors with thecorrect polarity.

Capacitor C2 must be agood quality component if the

unit is to function properly. Usinga low grade capacitor will eithergive greatly extended outputpulses from the monostable, oronce triggered the lamp willsimply stay switched onindefinitely. A tantalum capacitoris ideal, but a good qualityelectrolytic component shouldgive good results.

The five link-wires are madefrom 22s.w.g. or 24s.w.g. (about06mm diameter) tinned copper

wire. Fit single-sided solder pinsto the board at the points whereconnections to the switches, themicrophone, etc. will eventuallybe made.

CASEA medium size plastic or

metal box will comfortably


COMPONENTSResistors

R1 10M

R2 47k

R3 2M7

R4 10k

R5 2k2

See also theSHOP TALK Page!

All 0.25W 5% carbon film

CapacitorsC1 47u radial electrolytic, 16V

C2 100u radial electrolytic, 10V

SemiconductorsTR1 BC549 npngeneral-purpose

transistor

TR2 BC337 npnmedium-power

transistor

IC1 TS555CN low power timer

MiscellaneousMIC1 cased piezoelectric sounder

LP1 6V 60mA bulb, with holder

S1 push-button switch, push-to- make, release-to-break

S2 s.p.s.t. miniature toggle switch

B1 6V battery pack (4 x AA cells

in holder)

Medium size plastic or metal box,

style and size to choice; 0.1 inch

matrix stripboard, 24 holes by

18 copper strips; 8-pin DIL socket;

PP3 type battery connector;

multistrand connecting wire;

solder pins, solder, etc.

$10Approx. Cost

Guidance Only

(Excluding Batts & Case)


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accommodate all thecomponents. It is best to mountReset switch S1 on the toppanel where it will be easy tooperate.

The microphone can bemounted behind the front panel,but a large round cutout will thenbe needed for the body of thecomponent. It is easier to mountit on the front surface of thepanel, and only three smallholes are then required.

Two of these are for themetric M2 or 8BA mountingbolts, which are not normally


supplied with the sounderincidentally. The third holeallows the two flying leads topass through to the interior ofthe case. The sounder can beused as a template whenmarking the positions of themounting holes.

Piezo sounders usually haveone black lead and one red one,which is presumably to indicatetheir phasing. They are notpolarized components though,and can be connected eitherway round in this case.

LIGHT WORKFitting the light bulb to the

case in a reasonably neatfashion is the only awkwardaspect of construction. The

original intention was to mountthe bulb on the top panel using achassis mounting bulb holder.Some form of transparentplastic cover would then havebeen fixed over the bulb andholder.

The problem with thismethod is in finding a suitablecover. The method eventuallyused was to mount the bulbholder beneath the top panel,with spacers holding it 12 7mm

(05 inches) beneath the panel.

This brings the top of the holderroughly flush with the top panelof the case. A 9mm diameterhole drilled in the top panelenables the bulb to be screwedinto the holder.

This gives a reasonablyneat looking finished article, butit is not very childproof. If smallchildren will have access to theunit it is essential to makeaccess to the bulb more difficult.

One solution is to go for themodern approach, and simplymount everything inside atransparent case. Anotherpossibility is to mount the bulbon top of the main case, but tomount a small transparent caseover it. Remember that althoughaccess to the bulb must bedifficult, it must be possible sothat the bulb can be changed.

The small amount of hardwiring is perfectlystraightforward. The bulb holderhas screw terminals, but youmay prefer to remove thescrews and make solderedconnections to the metalterminal plates. Be careful toconnect the battery clip with thepolarity shown in Fig.3.

ON/OFF

S2

TOB1

RED

BLACK

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

9

9

10

10

11

11

13

13

15

15

12

12

R

Q

P

O

N

M

L

K

J

I

H

GF

E

D

C

B

A

AB

C

D

E

F

G

H

I

J

K

L

N

O

P

R

21

21

23

23

17

17

19

19

14

14

22

22

18

18

16

16

24

24

20

20

R2

R4

R5

R3

C1

C2

+

+

R1

TR1 TR2

S1

MIC1

LP1

c c

b b

e e

+6V

Fig.3. Stripboard topside component layout, details of breaksin the underside copper strips and interwiring to off-board

components.


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TESTINGWhen the completed unit is

first switched on it is likely that itwill be triggered and the lampwill switch on. Operating push

switch S1 should switch thelamp off again, but avoid makingtoo much vibration whenoperating S1 or the unit will betriggered again.

With the lamp switched off,try tapping the case or clappinggently near the microphone tocheck that the unit will trigger

properly. If there is any sign of amalfunction, switch off at onceand recheck the entire wiring,etc.

If all is well, check that thelamp remains switched on forapproximately the correctperiod. Due to componenttolerances and slight leakage in

timing capacitor C2 there maybe a substantial error in the timethat the lamp is switched on.Anything in the region of 45 to 7

minutes is acceptable.

If the switch-on time is fartoo long, or the lamp fails toswitch off at all, it is likely thatthe leakage level of capacitor

C2 is too high. It must then bereplaced with a higher qualitycomponent.

Due to its low standbycurrent consumption theAutomatic Nightlight can be leftswitched on all the time, but it isadvisable to switch it off duringthe daytime. Otherwise there isa risk of it being triggeredoccasionally, causing the lampto switch on and run down thebatteries unnecessarily.


The two halves of the case opened out to reveal the general

positioning of the circuit board and the interwiring to the frontand top panel mounted components. Dont forget to leave

plenty of room for the battery.

Using spacers to mount thebulb holder on the undersideof the case lid, above a 9mmdiameter hole to allow thebulb to be screwed in fromthe outside.

Finished Automatic Nightlight showing the piezoelectric micro-phone and on/off switch mounted on the front panel. The bulb

should be protected against breakage.

Automatic lighting - Night Light

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