0200 - find it - dont lose it.pdf

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EPE Online, February 2000 - www.epemag.com - 109

A fuse blows and the lights goout. Everything goes dark. Youfumble for the torch you knowits there somewhere butwhere?

Let this little gadget show youwhere!

FIND ITWith the battery-powered

Find It circuit, you will always beable to locate a torch (flashlight),bunch of keys, door lock justabout anything in darkness!

While sufficient light reachesa sensor (light dependent resistor LDR) on the unit, nothinghappens. However, when it isdark enough, a light-emitting

fitted to the box so that, say, abunch of keys or a torch couldbe hung from it (seephotograph).

It would also be possible toattach the unit to a portableitem. In some cases, it mighteven be possible to build thecircuit panel inside a piece ofequipment but the reader willneed to make certain that he orshe is totally aware of anysafety implications and must becompetent at doing the jobcorrectly.

If you are going to use thedevice to locate a door lock, itmay be convenient to have onlythe LED showing next to thelock and connect it to the unitmounted on the inside of thedoor.

Locates almost anything in the dark!

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diode (LED) begins to flashbriefly about once every fiveseconds. This helps to locatethe item.

If preferred, you couldincrease or reduce the flashrate. However, any increasewould reduce the life of thebattery.

LIGHTING THE WAYThere are many ways of

using this circuit and readerswill, no doubt, have their ownideas. One method would be toattach the unit to a wall close tothe object to be found.Alternatively, a hook could be

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Fig.1. Complete circuit diagram for Find it. Left, Find Itbeing used as an illuminated keyring hook.

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As well as householdapplications, this circuit will befound handy in many outdoorpursuits. Campers and anglerswill certainly find uses for it. Notethat, in some cases, it will benecessary to waterproof the boxand this is left up to theconstructor.

The circuit draws power fromtwo AA-size alkaline cells, whichshould give some one or twoyears of service. Since it requiresmore current while the LED isflashing, the actual life of thebatteries will depend on thenumber of hours of darkness in agiven 24 hour period. It alsodepends on what degree ofillumination is set for the unit tobegin operating.

While sufficient light reachesthe sensor (so that the LED isoff), the current requirement ofthe prototype circuit is only 5uAwhich may be regarded asnegligible. While the LED isflashing, this rises to an average250uA approximately.

This small operating currentis achieved by using a short dutycycle that is, the LED is off formuch longer than it is on about65 times longer. Thus, in eachfive second cycle the LED is onlyactually operating for some 0

08s(80ms).

Although while glowing theLED draws about 10mA, theaverage requirement is thereforeonly 150uA approximately. This isadded to the 100uAapproximately required by therest of the circuit, giving a total of250uA. If it is assumed that thereare eight hours of operation in a24 hour period, the averageoverall current requirement istherefore only 80uAapproximately.

While the battery voltageexceeds about 2

5V, the LED willflash brightly. It will become

correspondingly dimmer downto about 2V which is thepractical end point.CIRCUITDESCRIPTION

The complete circuitdiagram for the Find It project isshown in Fig.1. This may beconsidered to comprise twosections. The first is the lightsensing part based on IC1 andassociated components and thesecond, the LED flashercentered on IC2.

Integrated circuit IC1 is anoperational amplifier (opamp).This has been specially selectedfor its ability to operate from alow supply voltage combinedwith an exceptionally smallstandby operating current.

Looking at the light sensorstage first, the opamp invertinginput (pin 2) is maintained at avoltage equal to one-half that ofthe supply (nominally 1

5V), dueto the effect of equal valueresistors R3 and R4 connectedas a potential divider across thepower supply. Since these have

a very high resistance, thecontinuous current flowingthrough them is only a fraction ofa microamp.

The opamps non-invertinginput, pin 3, is connected to afurther potential divider. The toparm of this comprises presetpotentiometer VR1 connected inseries with fixed resistor R1. Thelower one is simply light-dependent resistor (LDR) R2.

As the intensity of lightreaching the LDRs sensitivesurface falls, its resistance risesand so does the voltage across itand hence at the non-invertinginput, pin 3. Depending on theadjustment of VR1, this voltagewill exceed that at the invertinginput, pin 2, at the operating lightlevel.

A simple rule about opampsis this. When the voltage appliedto the non-inverting input exceedsthat at the inverting one (as willhappen here in dim light), theoutput (pin 6) will be high. Whenit is less (bright light), it will below.

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Fig.2. Printed circuit board component layout and(approximately) full size copper foil master

pattern. Fig.3 (inset). How to make up R5 bywiring two resistors in series.

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full output swing between thesupply voltage, and its outputwill therefore go from 0V to 3Vnominal as the light level falls tothe required operating point.

LIGHT FLASHERNow let us look at the LED

flasher based on IC2. Thisconsists of an astable (free-running pulse generator). Itsfrequency is related to the valueof resistor R5 (in the prototype,this consisted of two resistorsconnected in series to make up

the required value), resistor R6and capacitor C1.

The on times (during whichthe output, pin 3, is high) areprovided when C1 chargesthrough resistors R5 and R6 totwo-thirds of supply voltage (2Vapprox.). After that, the capacitordischarges via internal circuitrythrough resistor R6 alone to one-third of supply voltage (1Vapprox.) and this gives the offperiod during which pin 3 is low.

This cycle repeats indefinitelyas long as a supply exists to pin 8and the reset input at pin 4 ishigh. With the values ofcomponents specified, each cycletakes about five seconds.

Since resistor R5 has a muchhigher value than R6, capacitorC1 charging time is much longerthan the discharge time. Thus,the time during which output pin 3is high is much greater than whenit is low. If a LED was connectedbetween pin 3 and the 0V rail, thiswould give the opposite effect tothat which was required it wouldbe on for longer than it was off!

To overcome this, thecurrent-sinking capability of IC2 isexploited. That is, current is ableto flow from supply positivethrough the LED and into theoutput.

With the LED connected likethis, current will flow through itwhen pin 3 is low rather thanwhen it is high. The result is thatthe on transitions are muchshorter than the off ones. Notethat there is no need to use acurrent-limiting resistor connectedin series with the LED, becausethe operating current is limited toa suitable level by the chip itself.

Referring back to theoperation of IC1, its output (pin 6)is connected to IC2 pin 8 (supplypositive) and pin 4 (reset) so,while IC1 output is high (that is,when the LDR is sufficiently dark)

the criteria are met for theastable to operate and the LEDflashes.

In the original version of thecircuit, the LED D1 anode (a)was connected direct to supplypositive so relieving IC1 of itsload. However, even with IC1pin 6 low (and so apparently nosupply existing for IC2), theLED continued to flash dimly!

It seems that current sinkingthrough the LED provided aweak supply for IC2 whichallowed it to oscillate. In thefinal version of the circuit theanode of D1 is connected to IC1output and this solves theabove problem. When the LDRreceives sufficient light, there isno power supply for IC2 andnothing happens.

Using IC1 to switch on thepower supply for IC2 has aparticular advantage in that IC2draws no current at all while theLDR receives sufficient light,and this greatly reduces thestandby current requirement ofthe circuit as a whole.

CONSTRUCTIONAll components, except the

cell holder, are mounted on asmall single-sided printed circuitboard (PCB). The topsidecomponent layout and full sizeunderside copper foil trackmaster are shown in Fig.2. Thisboard is available from the EPEOnline Store (code 7000252) atwww.epemag.com

Begin construction bydrilling the fixing holes thensolder the IC sockets and singlelink wire in position. Do notinsert the ICs themselves yet,however. Follow with all othercomponents except the LED D1and LDR R2.

The suggested value forresistor R5 (14

7M:) may be


COMPONENTSResistors

R1 1M (or as required - see text)R2 sub-miniature light-dependent resistor (5mm diameter, 5 megohms dark - see text)R3, R4 2M2 (2 off)R5 14M7 (10M and 4M7 in series or as required - see text)R6 220k

See also theSHOP TALK Page!

All 0.25W 5% carbon film except R2

CapacitorC1 470n miniature metalized polyester - 5mm pin spacing

SemiconductorsD1 3mm red high-brightness LED (see text)IC1 ICL7611 micopower opampIC2 ICM75551PA low-power CMOS timer

MiscellaneousB1 3V battery pack (2 x AA cells with holder).

Printed circuit board availablefrom the EPE Online Store, code7000252 (www.epemag.com);plastic case, size 102mm x 76mmx 38mm external; 8-pin DIL socket(2 off); PP3-type battery connector(or as appropriate for the holder);connecting wire, solder, etc.

$16Approx. CostGuidance Only(Excluding Batts)

PotentiometerVR1 1M miniature enclosed carbon preset, vertical

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made up using a 10M: unitconnected in series with a4

7M: one. These are arrangedas shown in Fig.3 with the freeends soldered to the R5 padson the PCB.

Raising the value of thecombination would reduce theflash rate and vice-versa. A 10megohm, resistor alone wouldgive a rate of about one flashevery three seconds.

If you are using thespecified miniature LDR havinga dark resistance of about5M:, then the suggested valueof resistor R1 will probably befound to work well. If you use adifferent LDR having a lowerdark resistance (say, thecommon ORP12 type), you mayneed to reduce the value of R1to, say, 100k:.

FINAL ASSEMBLYHold the PCB a small

distance above the base of thebox and decide how long theLED and LDR leads need to be.The LED should be soldered sothat its tip will eventually stand

slightly higher than the face of thelid. Take care over its polarity(the slightly shorter lead is thecathode (k).

Note that the specified LEDused in the prototype is the high-brightness type, and this wasfound to give better results thanthe standard variety. However,beware of any LED that has anarrow viewing angle. This couldprevent it from being seen if theuser is too far off-axis.

The LDR leads should be ofsuch a length that its windowwill take up a position level witheither the top face or side of thebox depending on the layoutdecided on. Solder it in placeusing as little heat as possible toprevent possible damage.

If you wish to mount the LEDremotely from the PCB, use apiece of light-duty twin-strandedwire soldered to its copper padson the PCB. When soldering theLED to the other end, take careover the polarity.

Also, be careful to avoidshort-circuits at the joints.Insulate and waterproof them asnecessary using heat-shrinkable

sleeving. Solder the end wiresof the PP3-type batteryconnector (or as appropriate forthe battery holder being used)to the +V (red) and 0V(black) points on the PCB.

Insert the ICs, with thecorrect orientation, into theirsockets. Since they are bothCMOS components, they couldpossibly be damaged by staticcharge, which might exist on thebody. It would therefore be wiseto touch something which isearthed (such as a water tap) toremove any such charge beforehandling the pins.

TESTINGA check may be carried out

before mounting the PCB in thebox. In that way, any faults willbe more easily rectified.

Adjust preset VR1 fully anti-clockwise (as viewed from thetop edge of the PCB). This willallow the circuit to respondwithout having to cover the LDRcompletely and this will maketesting easier. Insert the cellsinto their holder taking care withthe polarity and connect it up.

With the LDR covered withthe hand, the LED D1 shouldflash about once every fiveseconds. Be patient becauseyou will have to wait longer thanthis for the first flash. The actualrate is not particularly importantbut it could be made faster orslower by reducing or increasingthe value of R5 respectively.

Now, uncover the LDR sothat light falls on it. The LEDshould stop flashing. If you finddifficulty making it work, tryagain with the LED coveredmore carefully or take the unitinto a dark cupboard.

Adjust VR1 so that thecircuit operates at the requireddegree of illumination. You may


Completed unit showing positioning of the circuitboard and the two-cell holder. Note the lightdependent resistor (LDR) mounted in one

side panel.

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find that you need to makefurther small adjustments whenthe circuit panel is mounted inposition.

BOX ITIf a hook or something

similar is to be attached to thecase, take account of the PCBposition so that any fixings willnot cause a short-circuit.

Remove the connector fromthe cell holder. Position thePCB on the bottom of the boxand mark through the fixingholes. Remove the PCB againand drill them through.

Decide where the hole is tobe drilled to allow light to reachthe LDR. It must not beobscured too much during useor this would result in the LEDflashing more than necessarywith a consequent increase inthe current requirement.

In the prototype, the LDRleads were bent through right-angles (see photograph) and thehole was made in the side of thebox. However, the exactarrangements will depend onthe application.


Carefully measure thepositions of the LED and LDRand drill the holes for thesecomponents. The hole for theLED should be of such adiameter that its tip will protrudethrough it only slightly. That forthe LDR should be a littlesmaller than its window so thatthis will lie just behind the holewhen the PCB is in position.

If required, you could drill asmall hole to allow preset VR1to be adjusted from outside thecase using a small screwdriveror trimming tool. However, thiswas not done in the prototype.

FINISHING OFFAttach the PCB using

plastic spacers on the boltshanks so that the LED andLDR take up their correctpositions. Attach the cell holderto the bottom of the box using asmall fixing.

Secure the lid of the casetaking care that the LEDengages with its hole and testthe circuit under real conditions.Make further adjustments topreset VR1 if necessary so that

the LED begins to flash at therequired light level.

Clockwise rotation of thesliding contact (as viewed fromthe top edge of the PCB) allowsoperation with less light. If youwould like the LED to startflashing under dimmerconditions and this is notpossible with VR1 adjusted fullyclockwise, increase the value ofresistor R1 2

2 megohmswould be a good starting point.

Put the Find It into service.When the LED begins to flashtoo dimly to be seen effectively,it is time to replace thebatteries.

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0200 - find it - dont lose it.pdf

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