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Although digital cameras havebeen progressing rapidly in recentyears, and most of thephotographs in this magazine arenow produced without the aid offilm, most people are still usingtraditional cameras. It seemslikely that they will continue to doso for some years yet.
The quality of the finishedresult with traditional photographyis dependent on a number offactors. One of these is theaccuracy of the shutter, which isparticularly important when usingcertain types of film. There is verylittle exposure latitude with anyform of transparency film, andalso with certain negative films.
Modern SLR cameras mostlyhave electronically timed shutters,and in terms of accuracy they aregenerally superior to the purelymechanically shutters of olderdesigns. Electronically timedshutters are also less prone to
However, these longer shuttertimes of around 8 seconds to aminute are easily tested using astopwatch, and do not reallyneed a shutter timer.
Although this unit isprimarily designed for testingshutters, it should be possible tomodify it for use in some otherautomatic timing applications.The light level on a sensor goingabove a certain threshold levelactivates the timer, but it canalso be controlled by switchcontacts.
INTEGRATORThe circuit is based on an
operational amplifier (opamp)connected as an integrator. Thebasic integrator circuit is shownin Fig.1. Like most basicoperational amplifier buildingblocks an integrator requiresdual (positive and negative)supply rails with a central (zerovolt) earth rail. The inputs andoutputs can therefore havenegative as well as positivevoltage levels.
An operational amplifieramplifies the voltage difference
An easy-to-build project that will check your camerashutter speeds. Four switched ranges up to: 5ms;50ms; 500ms and 5 seconds.
CAMERA SHUTTER TIMER by ROBERT PENFOLD
creeping out of adjustment overa period of time, but things canobviously go awry with shuttersof either variety.
ON TIMEIt is not essential to use a
complex digital timer to testcamera shutters, and a verybasic analogue timer circuit iscapable of providing accuratemeasurements in thisapplication. The simple designfeatured here has fourmeasuring ranges having full-scale values of 5ms, 50ms,500ms, and 5 seconds. It cantherefore be used to measurethe full range of speeds coveredby most cameras.
Some of the more moderncameras have manual shutterspeeds of up to around 30seconds or so, which takes thelonger shutter speeds beyondthe maximum range of this unit.
at its two inputs, and the outputvoltage is positive if the non-inverting (+) input is at thehigher potential. The outputvoltage is negative if theinverting (–) input is at thehigher voltage. The voltage gainis extremely high at DC and lowfrequencies, with a figure ofaround 200,000 beingunexceptional.
Under standby conditions,the non-inverting (+) input isbiased to the 0V rail, and thereis no charge on capacitor Ca. Anegative feedback actionstabilizes the inverting input atthe 0V bias level fed to the non-inverting input.
If the output was at apositive potential, the couplingthrough Ca (which has nocharge voltage) would take theinverting input positive,unbalancing the input voltagesand taking the output negative.With the output at a negativevoltage the feedback through Caagain unbalances the inputlevels, and this time sends theoutput positive. Any drift in theoutput voltage is thereforecorrected by the feedback.
With a real world opamp theoutput will not be stabilized atprecisely 0V, since the gain ofthe amplifier is very high ratherthan infinite. Also, imperfectionsin the opamp’s circuit canproduce small offset voltages atthe output, but in practice anyerrors are normally very small.
ON CHARGESo far we have only
considered the circuit with theinput “floating” or at 0V.Suppose that the input is takena few volts positive. CapacitorCa then charges throughresistor Ra, and the voltage atthe inverting input starts to rise.
This sends the output
Constructional Project
LIGHTSOURCE
ELECTRONICSWITCH INTEGRATOR
PHOTODIODE METER
CAMERA
Fig.2. Block schematic diagram for the Camera Shutter Timer.
MiscellaneousS1 4-way 3-pole rotary switch (only one pole used)S2 pushbutton switch, push-to make release-to-breakS3 d.p.s.t. miniature toggle or rotary switchME1 50uA FSD moving coil panel meterB1, B2 9V batteries, PP3 size (2 off)
Metal insrument case, size about200mmwide; 0.1 inch matrix stripboard having33holes by 18 strips; 8-pin DIL socket;battery connector (2 off); control knob(3 off); multistrand connecting wire;screened lead; single-ended solder pins;solder, M2.5 or 6BA fixings, etc.
See also theSHOP TALK Page!
$19Approx. CostGuidance Only (Excluding meter, case, batts)
SemiconductorsD1 SFH309/5 phototransistor or other small phototransistor/diode (see text)TR1 BC559 pnp transistorIC1 78L05 +5V 100mA voltage regulatorIC2 LF351N biFET opamp
negative in order to keep theinverting input at 0V. As Cacharges, the inverting inputremains at 0V and the output ofthe circuit goes steadily morenegative.
The important point to notehere is that the voltage acrossresistor Ra remains constant,and is equal to the input voltage.The current flow through Ra,and therefore the charge currentfor Ca, is also constant.
If a capacitor is simplycharged from a resistor via avoltage source, the voltage risesquickly initially, but thengradually slows down. This isdue to the fact that there isinitially the full voltage acrossthe resistor, giving a high chargecurrent. As the voltage acrossthe capacitor rises, the voltageacross the resistor falls, givingan ever-decreasing charge rate.
The constant current flowinto Ca results in the output ofthe circuit going negative at alinear rate. If the output is at –1Vafter one second, it will be at –2V after two seconds, –3V afterthree seconds, and so on.
An integrator is thereforeideal as the basis of a simpletimer. It is just a matter of usinga voltmeter circuit at the outputand adjusting the scaling so thatthe meter readings are easilyconverted into times.SPEED AND SIZE
The block diagram of Fig.2shows the general scheme ofthings used in the CameraShutter Timer project. Theintegrator drives a moving coilpanel meter, with the laterdirectly indicating shutter timersover the four ranges statedpreviously. Using four inputresistors, giving four differentcharge rates for the capacitor,provides the four ranges.
The integrator is driven from
a photodiode or a phototransistorvia an electronic switch. It isessential to use a photocell thathas a fast response time sincemost cameras have a fastershutter time of one millisecond orless. Photo-resistors are too slow,but phototransistors andphotodiodes have suitably fastresponse times.
In theory there is someadvantage in using a photodiodedue to its faster response time. Inpractice a photodiode seems toprovide little improvement inaccuracy in this application. Also,a photodiode has the drawback ofbeing far less sensitive than aphototransistor. This designincorporates a simple transistorswitching stage to ensure that theintegrator receives a properswitching signal whether a photo-diode or a phototransistor is used.
On the face of it, the size ofthe sensor is unimportant, but inthis application it is a case of“small is beautiful”. The design ofmechanical shutters has to takeinto account the fact that it takes acertain amount of time for theshutter to open and close.
At the slow speeds this factordoes not matter because theopening and closing times arevery short relative to the shuttertime. At the fastest shutter speedsthe opening and closing times canactually be longer than thenominal shutter speed. This mayseem to be impossible, but allbecomes clear when the action ofa focal-plane shutter is examined.
CURTAIN CALLThere are two shutter curtains
that are made from metal orrubberized cloth. One of thecurtains blocks light from the filmuntil the shutter is fired, and thenit slides out of the way to one sideto expose the film. After theappropriate time, the secondcurtain moves across the film
gate and blocks light fromreaching the film again. The twocurtains are then pulled back totheir original positions, and theymove together in such away thatlight is always blocked from thefilm. The shutter is then ready tooperate again.
As it takes severalmilliseconds for each curtain totraverse the film gate, the onlyway of achieving fast shutterspeeds is to start the secondshutter curtain before the firstone has completely moved outof the way. This effectively givesa slit that travels across the film,exposing it a bit at a time. Thefaster the shutter speed, thenarrower the slit is made.Although it may take somethinglike 10 milliseconds for theshutter to complete anexposure, by making the slitnarrow enough each part of thefilm can be exposed for amillisecond or even less.
Here we are measuring theshutter time by having a lightsource in front of the shutter,and the sensor positionedbehind it and quite close to thefilm plane, see Fig.2. The unitresponds to the pulse of lightproduced when the shutter isfired. If the sensor has a largediameter its size effectivelyincreases the size of the slitwhen the shutter is used at fastspeeds, giving elongated shuttertimings. For this type ofmeasurement the sensor wouldideally be only a fraction of amillimeter in diameter, but it alsoneeds to be reasonablysensitive.
In practice a diameter ofabout one millimeter is a goodcompromise. Photodiodes andphototransistors as small as thisseem to be unobtainable thesedays, but a larger device iseasily masked to produce aneffective diameter of about onemillimeter.
the Camera Shutter Timer isshown in Fig.3. Operationalamplifier IC2 is used in theintegrator mode, and capacitorC3 is the integration capacitor.Resistors R3 to R6 are the inputRange resistors, and therequired resistor is selectedusing Range switch S1.
Meter ME1 is driven fromthe output of IC2 (pin 6) via theseries resistance of R8 andcalibration preset potentiometerVR2. Operating Reset switch S2discharges capacitor C3 throughcurrent limiter resistor R7 so thatthe meter is zeroed and a newreading can be taken. ResistorR7 prevents excessivedischarge currents that wouldotherwise shorten the operatinglife of switch S2.
Under dark conditions,photodiode D1 operates muchlike any other semiconductordiode. When reverse biased, asit is here, only a minute leakagecurrent flows. Under brightconditions the leakage levelincreases, and the leakagecurrent is roughly proportional tothe received light level.
If the leakage current is highenough, transistor TR1 is biasedinto conduction and it applies+5V to the input of theintegrator. In practice, sensitivityor Level control VR1 is adjustedso that TR1 is switched off whenthe shutter is closed, and turnedon when the shutter is open.Potentiometer VR1 enables theunit to function properly under awide range of ambient lightlevels.
Operation using aphototransistor is much thesame, and Fig.3 shows thecorrect method of connection fora component of this type. Underdark conditions the collector (c)to emitter (e) leakage current is
Constructional Projectthe low level associated withordinary silicon transistors, butat high light intensities it rises toa milliamp or more. This againgives the required on/offswitching action from TR1.
It is important that theintegrator is fed with a stableinput potential since anychanges in this voltage will alterthe charge current of theintegration capacitor anddegrade the accuracy of theunit. A monolithic voltageregulator (IC1) is therefore usedto produce a well-stabilized +5Vsupply for the input circuitry.
The integrator, IC2, doesnot require a stabilized supplyand is powered direct from twosmall (PP3 size) 9V batteries.The current consumption is onlya few milliamps from eachsupply rail.
CONSTRUCTIONMost of the components are
assembled on a piece ofstripboard that has 33 holes by18 copper strips. Thecomponent layout, interwiringand details of the breaksrequired in the underside copperstrips are shown in Fig.4.
Construction of the boardfollows along normal lines, witha board being trimmed down tothe correct size using ahacksaw, the two 3mm diametermounting holes then beingdrilled, after which the 12 breaksin the copper strips are cut. Thebreaks can be made using ahandheld twist drill bit of about5mm diameter or so. The boardis now ready for the componentsand link wires to be added. TheLF351N specified for IC2 is nota static-sensitive component,but it is still a good idea to use aholder for this component. Donot overlook any of the eight linkwires.
The shorter links can bemade from the wire trimmedfrom resistors, but some22s.w.g. or 24s.w.g. tinnedcopper wire is needed for thelonger links. At this stage only fitsingle-sided solder pins to theboard at the positions whereconnections to the meter,controls, etc. will eventually bemade.
BOXING-UPWhen choosing a case for
this project, bear in mind that afair amount of front panel spaceis required for the meter andfour controls. This precludes theuse of small and most mediumsize cases. A metal instrumentcase about 200mm or so wide isprobably the best choice andwas used for the prototype, butone of the larger plastic boxeswould also be suitable.
The exact component layoutused is not important from theelectrical point of view, since thecircuit operates at DC and is notprone to problems with strayfeedback or noise pick up.However, try to use a sensiblelayout that makes the unit easyto use.
PANEL METERFitting the meter on the front
panel is potentially difficult, as itrequires a large round mountinghole. This is 38mm in diameterfor most of the smaller meters,but it is clearly advisable tocheck this before cutting thehole.
The quickest way of makingthis cutout is to use a hole-cutterof the appropriate diameter, orto use one of the adjustablecutters that are available frommost do-it-yourself superstores.Alternatively, cutting carefullyusing a coping saw, Abrafile, orany similar tool will produce the
mounting holes are required forthe threaded rods built into themeter. These are normally at thecorners of a square having32mm sides and the samecenter as the main mountinghole. Again, it is advisable tomake some measurements onthe meter itself to check itsmounting arrangements prior todrilling any holes.
LIGHT SENSORPhotodiode D1 can be
connected to the main unit via atwo-way plug and socket suchas a 35mm jack type, but it ischeaper and easier to simplyhard wire it to the circuit board.A hole for the connecting cablemust be drilled in the case, andthe hole should be fitted with agrommet to protect the cable ifthe case is a metal type.
A piece of two-way cable
about 05 meters long is used toconnect light sensor D1 to thecircuit board, and low costscreened cable is probably thebest choice. Use the outerbraiding to connect the anode(or emitter) to the 0V rail, andthe inner lead to connect thecathode (or collector) to resistorR2.
The unit worked when tried
with various photodiodes andphototransistors, but asexplained previously, results willbe more accurate at fastershutter speeds if a sensorhaving a diameter of 3mm orless is used. Results will beeven better if the sensor ismasked down to a diameter ofabout one millimeter, and theeasiest way of doing this is to
Constructional Project
Finished unit showing front panel layout and lettering. Note thephototransistor/diode light sensor is attached to the free end of
the screened cable.
+
c
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b
c
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B19V
B29V
R32k7
R427k
R8120k
R5270k
R62M7
R11k
5ms
5s
50ms
500ms
TR1BC559
R22k2
R722Ω
VR1220k
+SFH309/5
S1
RANGE
RESET
ON/OFF
LEVEL C1100n
C4100n
C31µ
C2100n
OUT IC178L05
COM
IN
+IC2LF359
S2
ME150 Aµ
S3a
VR2100k
ON/OFF
S3b
*
*
*
SEE TEXT
a
kD1
A
3
2
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4
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Fig.3. Complete circuit diagram for the Camera Shutter Timer. Note S3 is a “ganged” double-pole switch.
apply a ring of black paintaround the front of the diode.
FINAL ASSEMBLYThe circuit board is mounted
on the base panel of the case,well towards the right-hand side,as viewed from the rear, so thatthere is sufficient space for thebatteries to its left, seephotograph. Mounting bolts arepreferable to plastic stand-offswhen using stripboard, and both6BA and metric M3 bolts aresuitable. Include spacers atleast 6mm long to hold theunderside of the board well clearof the case.
All the hard wiring isincluded in Fig.4 and is largelystraightforward. Be careful toconnect meter ME1 and the two
battery connectors with thecorrect polarity. If sensor D1 hasa standard light-emitting diode(LED) style case, the shorterleadout wire will be the cathodeor the collector. For diodeshaving other case styles themanufacturer’s or retailer’sliterature should be consulted.
The base terminal is notnormally accessible on smallphototransistors, but if there is abase lead it is simply ignored.
The four range resistors (R3to R6) should be mounteddirectly on the rotary Rangeselector switch S1. It is best tosolder the resistors on theswitch tags before it is fitted inthe case front panel.
Try to complete thesoldering fairly swiftly so that the
resistors do not overheat. Ittakes quite a lot of heat todestroy them, but relatively
small amounts to impairtheir accuracy.
CALIBRATIONAND TESTING
Start calibration/testing with Range switchS1 set to the five-secondrange (R6 switched intocircuit) and preset VR2adjusted to roughly middlesetting. When the unit is
Constructional Project
+
ME1
VR1
A
S1
1234
5
67
8 910
11
12
B2
B1
RED
RED
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 25
2515
12
12
RQPONMLKJI
HGFEDCBA
ABCDE FG H
JK L
NO
Q
21 31
3121
23 33
3323
17 27
2717
19 29
2919
14
14
22 32
3222
18 28
2818
16 26
2616
24
24
20 30
3020
R2
R1
R8
R7
R4R
5
R 6
R3
C2
C3
C4
C3
VR1
D1
a (e)
S2
BLACK
BLACK
S3
k (c)
SCREEN
IN COM
OUT bc
e
Fig.4. Camera Shutter Timer stripboard component layout, inter-wiring and details of breaks required in the underside copper
strips. (Right) Mounting the Range resistors directly on the switchtags before it is fitted in the case will help with the final wiring.
first switched on the meter ME1may read other than zero, butbriefly pressing pushswitch S2should zero the meter.
With the sensor (D1) aimedtowards a light source, whichcan simply be bright daylightcoming through a window orlight from a table lamp, themeter reading should slowlyadvance. However, it mayrequire some adjustment toLevel control VR1 before thishappens. Clockwise adjustmentof VR1 reduces the light levelneeded to activate the unit. If thesensor is a photodiode areasonably strong light sourcewill be required.
With a little trial and error itshould be possible to find asetting that results in the meterreading advancing when thesensor is aimed towards thelight source, and stopping whenit is not. If the unit seems to be
malfunctioning in any way,switch off immediately andrecheck the entire wiring, etc.
It might be possible to checkautomatic exposure times withsome cameras, but the unit wasonly designed to check manualspeeds. Obviously the timer isonly usable with cameras thathave removable or hingedbacks, which means the vastmajority of 35mm models.
The main exceptions aresome of the older Leica modelsand copies of these such as theearly Fed and Zorki cameras.The timer was not designed totest “”leaf” shutters, but itseemed to work quite well whenused with shutters for largeformat lenses and when triedwith some “golden oldies”, suchas an Agfa Rangefinder model.
Testing is easier if the lensis removed from the camera, butit is possible to make accurate
measurements with camerasthat have fixed lenses. A largelight source such as a windowis then preferable. Thisminimizes any dark areaswhere there will be insufficientlight to activate the sensor.
Results are best with thesensor positioned quite closeto the shutter curtains, butgreat care must be taken toavoid getting the sensor incontact with any part of theshutter. These days the costof having damaged shuttersrepaired is so high that thesums involved are higher thanthe value of the repairedcameras. Therefore, it isbetter to err on the side ofcaution and have a gap ofseveral millimeters betweenthe shutter and the sensor.
The best way ofcalibrating the unit is to use arecently manufactured orserviced camera as thecalibration source. Set thetimer to the 500-millisecond
range and the camera for ashutter speed of 05 seconds(500 milliseconds).
Press Reset switch S2 tozero the meter, take a reading,and then adjust preset VR2 forprecisely full-scale reading onthe meter. In the absence of asuitably reliable camera,calibrate the unit on the five-second range using the cameraset to “bulb” or “B”, and astopwatch to help generate afive second shutter time.
The 0 to 50 scaling of thespecified meter producesreadings that are easilyconverted into shutter times,and it is not worthwhile doingany recalibration. Cameras useshutter times expressed asfractions of a second rather thanin milliseconds, which slightlycomplicates matters. However,Table 1 shows the equivalent
Constructional Project
Layout of components inside the prototype metal case. The circuitboard is bolted to one side of the case to make space for the batter-ies. The board should be bolted on 6mm spacers to keep the cop-
times in milliseconds for thestandard shutter speeds.IN USE
It is only fair to point out thatthe generally accepted camerastandards allow for quite largemargins of error, especially on
the faster shutter speeds. Anerror of something like +50percent is clearly not good, but itdoes not indicate a faultyshutter, just one havingmediocre accuracy.
With older cameras do not
be surprised if checking thesame shutter setting produces afairly wide range of times.Unless the variations are verywide this does not indicate thatthe shutter is faulty, but it doessuggest that it is long overduefor cleaning and recalibration.
