datasheet tcrt 5000

TELEFUNKEN Semiconductors ANT014

Issue: 09. 96

Application of Optoelectronic Reflex Sensors

TCRT1000, TCRT5000, TCRT9000, CNY70

ANT014 TELEFUNKEN Semiconductors

Issue: 09. 96

Table of contents

Drawings of the sensors....................................................................................................................... 2

1. Optoelectronic sensors..................................................................................................................... 3

1.1 General principles ...................................................................................................................... 3

2. Parameters and practical use of the reflex sensors .......................................................................... 5

2.1 Coupling factor, k ...................................................................................................................... 52.2 Working diagram ....................................................................................................................... 72.3 Resolution, trip point ................................................................................................................. 92.4 Sensitivity, dark current and crosstalk..................................................................................... 122.5 Ambient light ........................................................................................................................... 12

3. Application examples, circuits ...................................................................................................... 14

3.1 Application example with dimensioning ................................................................................. 143.2 Circuits with reflex sensors...................................................................................................... 17


Issue: 09. 96 1

TEMIC optoelectronic sensors contain infrared emitting diodes as a radiation source andphototransistors as detectors.

Typical applications include:

• Copying machines

• Video recorders

• Proximity switch

• Vending machines

• Printers

• Object counters

• Industrial control

• Facsimile

Special features:

• Compact design

• Operation range 0 mm to 20 mm

• High sensitivity

• Low dark current

• Minimized crosstalk

• Ambient light protected

• Cut-off frequency up to 40 kHz

• High quality level, ISO 9000

• Automated high-volume production

These sensors present the quality of perfected products. The components are based on TEMIC’smany years experience as one of Europe's largest producers of optoelectronic components.

TELEFUNKEN SemiconductorsANT014

Issue: 12. 942

Drawings of the sensors

94 9318

TCRT1000

94 9442

TCRT5000

94 9346

TCRT9000

94 9320

CNY70


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1. Optoelectronic sensorsIn many applications, optoelectronic transmit-ters and receivers are used in pairs and linkedtogether optically. Manufacturers fabricatethem in suitable forms. They are available fora wide range of applications as ready-to-usecomponents known as couplers, transmissivesensors (or interrupters), reflex couplers andreflex sensors. Increased automation in indus-try in particular has heightened the demand forthese components and stimulated the develop-ment of new types.

1.1 General principlesThe operating principles of reflex sensors aresimilar to these of transmissive sensors. Basi-cally, the light emitted by the transmitter is in-fluenced by an object or a medium on its wayto the detector. The change in the light signalcaused by the interaction with the object thenproduces a change in the electrical signal inthe optoelectronic receiver.The main difference between reflex couplersand transmissive sensors is in the relative posi-tion of the transmitter and detector with re-spect to each other. In the case of the trans-missive sensor, the receiver is opposite thetransmitter in the same optical axis, giving adirect light coupling between the two. In thecase of the reflex sensor, the detector is posi-tioned next to the transmitter, avoiding a directlight coupling.The transmissive sensor is used in most appli-cations for small distances and narrow objects.The reflex sensor, however, is used for a widerange of distances as well as for materials andobjects of different shapes. It sizes by virtue ofits open design.In this article, we will deal with reflex sensors– placing particular emphasis on their practicaluse. The components TCRT1000, TCRT5000,TCRT9000 and CNY70 are used as examples.However, references made to these compo-

nents and their use apply to all sensors of asimilar design.The reflex sensors TCRT1000, TCRT5000,TCRT9000 and CNY70 contain IR-emittingdiodes as transmitters and phototransistors asreceivers. The transmitters emit radiation of awavelength of 950 nm. The spectral sensitivityof the phototransistors are optimized for thiswavelength.There are no focusing elements in the sensorsdescribed, though inside the TCRT5000 inboth active parts (emitter and detector) lensesare incorporated. The angular characteristicsof both are divergent. This is necessary to real-ize a position-independent function for easypractical use with different reflecting objects.In the case of TCRT5000, the concentration ofthe beam pattern to an angle of 16° for theemitter and 30° for the detector, respectively,results in operation on an increased range withoptimized resolution. The emitting and accep-tance angles in the other reflex sensors areabout 45°. This is an advantage in short dis-tance operation. The best local resolution iswith the reflex sensor TCRT9000.The main difference between the sensor typesis the mechanical outline (as shown in thephotos, see page before), resulting in differentelectrical parameters and optical properties. Aspecialization for certain applications is neces-sary. Measurements and statements on the dataof the reflex sensors are made relative to areference surface with defined properties andprecisely known reflecting properties. Thisreference medium is the diffusely reflectingKodak neutral card, also known as grey card(KODAK neutral test card; KODAK publica-tion No. Q-13, CAT 1527654). It is also usedhere as the reference medium for all details.The reflection factor of the white side of thecard is 90% and that of the grey side is 18%.Table 1 shows the measured reflection of anumber of materials which are important for


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the practical use of sensors. The values of thecollector current given are relative and corre-spond to the reflection of the various surfaceswith regard to the sensor's receiver. They weremeasured at a transmitter current of IF =20 mA and at a distance of the maximum lightcoupling. These values apply exactly to theTCRT9000, but are also valid for the otherreflex sensors. The ‘black on white paper’-

section stands out in table 1. Although all sur-faces appear black to the ‘naked eye’, theblack surfaces emit quite different reflectionsat a wavelength of 950 nm. It is particularlyimportant to account for this fact when usingreflex sensors. The reflection of the variousbody surfaces in the infrared range can deviatesignificantly from that in the visible range.

Table 1 Relative collector current (or coupling factor) of the reflex sensor TCRT9000 for reflection on various materials. Reference is the white side of the Kodak neutral card. The sensor is positioned perpendicular with respect to the surface. The wavelength is 950 nm.

Kodak neutral card

White side (reference medium) 100%

Gray side 20%

Paper

Typewriting paper 94%

Drawing card, white (Schoeller Durex) 100%

Card, light gray 67%

Envelope (beige) 100%

Packing card (light brown) 84%

Newspaper paper 97%

Pergament paper 30-42%

Black on white typewriting paper

Drawing ink (Higgins, Pelikan, Rotring) 4-6%

Foil ink (Rotring) 50%

Fiber-tip pen (Edding 400) 10%

Fiber-tip pen, black (Stabilo) 76%

Photocopy 7%

Plotter pen

HP fiber-tip pen (0.3 mm) 84%

Black 24 needle printer (EPSON LQ-500) 28%

Ink (Pelikan) 100%

Pencil, HB 26%

Plastics, glass

White PVC 90%

Gray PVC 11%

Blue, green, yellow, red PVC 40-80%

White polyethylene 90%

White polystyrene 120%

Gray partinax 9%

Fiber glass board material

Without copper coating 12-19%

With copper coating on the reverse side 30%

Glass, 1 mm thick 9%

Plexiglass, 1 mm thick 10%

Metals

Aluminum, bright 110%

Aluminum, black anodized 60%

Cast aluminum, matt 45%

Copper, matt (not oxidized) 110%

Brass, bright 160%

Gold plating, matt 150%

Textiles

White cotton 110%

Black velvet 1.5%


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2. Parameters and practical use of the reflex sensorsA reflex sensor is used in order to receive a re-flected signal from an object. This signal givesinformation on the position, movement, size orcondition (e.g. coding) of the object in que-stion. The parameter that describes the func-tion of the optical coupling precisely is the so-called optical transfer function (OT) of thesensor. It is the ratio of the received to theemitted radiant power.

OT = r

e

ΦΦ

Additional parameters of the sensor, such asoperating range, the resolution of optical dis-tance of the object, the sensitivity and theswitching point in the case of local changes inthe reflection, are directly related to this opti-cal transfer function.In the case of reflex sensors with phototransis-tors as receivers, the ratio Ic/IF (the ratio ofcollector current Ic to the forward current IF)of the diode emitter is preferred to the opticaltransfer function. As with optocouplers, Ic/IF isgenerally known as the coupling factor, k. Thefollowing approximate relationship existsbetween k and OT:

k = Ic/ IF = [(S × B)/h] × Φr/Φe

where B is the current amplification, S = Ib/Φr(phototransistor's spectral sensitivity), andh = IF/Φe (proportionality factor between IFand Φe of the transmitter).In figure 1 and figure 2, the curves of the radi-ant intensity, Ie, of the transmitter to the for-ward current, IF, and the sensitivity of thedetector to the irradiance, Ee, are shown re-spectively. The gradients of both are equal tounity slope. This represents a measure of the deviation ofthe curves from the ideal linearity of the para-meters. There is a good proportionality be-tween Ie and IF and between Ic and Ee where

Figure 1 Radiant intensity, Ie = f (IF), of the IRtransmitter

the curves are parallel to the unity gradient.Greater proportionality improves the relation-ship between the coupling factor, k, and theoptical transfer function.

2.1 Coupling factor, kIn the case of reflex couplers, the specificationof the coupling factor is only useful by a de-fined reflection and distance. Its value is givenas a percentage and refers here to the diffusereflection (90%) of the white side of Kodakneutral card at the distance of the maximumlight coupling. Apart from the transmitter cur-rent, IF, and the temperature, the couplingfactor also depends on the distance from thereflecting surface and the frequency – that is,the speed of reflection change.For all reflex sensors, the curve of the cou-pling factor as a function of the transmitter


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Figure 2 Sensitivity of the reflex sensors' detector

current, IF, has a flat maximum at approxi-mately 30 mA (figure 3). As shown in thefigure, the curve of the coupling factor followsthat of the current amplification, B, of the pho-totransistor. The influence of temperature onthe coupling factor is relatively small andchanges approximately -10% in the range of-10°C to +70°C (figure 4). This fairly favor-able temperature compensation is attributableto the opposing temperature coefficient of theIR diode and the phototransistor.The maximum speed of a reflection changethat is detectable by the sensor as a signal isdependent either on the switching times or thethreshold frequency, fc, of the component. Thethreshold frequency and the switching times ofthe reflex sensors TCRT1000, TCRT5000,TCRT9000 and CNY70 are determined by theslowest component in the system – in this casethe phototransistor. As usual, the threshold fre-quency, fc, is defined as the frequency atwhich the value of the coupling factor hasfallen by 3 dB (approximately 30%) of itsinitial value. As the frequency increases, f > fc,the coupling factor decreases.

Figure 3 Coupling factor k = f (IF) of the reflex sensors

Figure 4 Change of the coupling factor, k, withtemperature, T

As a consequence, the reflection change is nolonger easily identified.Figure 5 illustrates the change of the cut-offfrequency at collector emitter voltages of 5, 10and 20 V and various load resistances. Highervoltages and low load resistances significantlyincrease the cut-off frequency.The cut-off frequencies off all TEMIC reflexsensors are high enough (with 30 kHz to50 kHz) to recognize extremely fast mechani-cal events. In practice, it is not recommended to use alarge load resistance to obtain a large signal,dependent on the speed of the reflectionchange. Instead, the opposite effect takesplace, since the signal amplitude is markedlyreduced by the decreasing in the cut-off fre-


Issue: 09. 96 7

quency. In practice, the better approach is touse the given data of the application (such asthe type of mechanical movement or thenumber of markings on the reflectivemedium). With these given data, the maximumspeed at which the reflection changes can bedetermined, thus allowing the maximum fre-quency occurring to be calculated. The maxi-mum permissible load resistance can then beselected for this frequency from the diagram fcas a function of the load resistance, RL.

2.2 Working diagramThe dependence of the phototransistor collec-tor current on the distance, A, of the reflectingmedium is shown in figure 6 and figure 7 forthe reflex sensors TCRT1000 and TCRT9000respectively.The data were recorded for the Kodak neutralcard with 90% diffuse reflection serving as thereflecting surface, arranged perpendicular tothe sensor. The distance, A, was measuredfrom the surface of the reflex sensor. The emitter current, IF, was held constant dur-ing the measurement. Therefore, this curvealso shows the course of the coupling factorand the optical transfer function over distance.It is called the working diagram of the reflexsensor.

The working diagrams of all sensors (figure 6)show a maximum at a certain distance, Ao.Here the optical coupling is the strongest. Forlarger distances, the collector current falls inaccordance with the square law. When theamplitude, I, has fallen not more than 50% ofits maximum value, the operation range is atits optimum.

Figure 5 Cut-off frequency, fc


8 Issue: 09. 96

a) TCRT5000

b) CNY 70

c) TCRT9000

d) TCRT1000

Figure 6 Working diagram of reflex sensors TCRT5000, CNY70, TCRT9000 and TCRT1000


Issue: 09. 96 9

2.3 Resolution, trip pointThe behavior of the sensors with respect toabrupt changes in the reflection over a dis-placement path is determined by twoparameters: the resolution and the trip point.If a reflex sensor is guided over a reflectingsurface with a reflection surge, the radiationreflected back to the detector changes gradu-ally, not abruptly. This is depicted in figure 7a.The surface, g, seen jointly by the transmitterand detector, determines the radiation receivedby the sensor. During the movement, this sur-face is gradually covered by the dark reflectionrange. In accordance with the curve of theradiation detected, the change in collector cur-rent is not abrupt, but undergoes a wide,gradual transition from the higher to the lowervalue.As illustrated in figure 7b, the collector currentfalls to the value Ic2, which corresponds to thereflection of the dark range, not at the pointXo, but at the points Xo+Xd/2, displaced byXd/2.The displacement of the signal corresponds toan uncertainty when recording the position ofthe reflection change, and it determines theresolution and the trip point of the sensor.The trip point is the position at which the sen-sor has completely recorded the light/darktransition, that is, the range between the pointsXo+Xd/2 and Xo - Xd/2 around Xo. The dis-placement, Xd, therefore, corresponds to thewidth or the tolerance of the trip point. Inpractice, the section lying between 10% and90% of the difference Ic = Ic1 - Ic2 is taken asXd. This corresponds to the rise time of the ge-nerated signal since there are both movementand speed. Analogous to switching time, dis-placement, Xd, is described as a switching dis-tance.The resolution is the sensor's capability to rec-ognize small structures. Figure 8 illustrates the

example of the curve of the reflection and cur-rent signal for a black line measuring d inwidth on a light background (e.g. on a sheet ofpaper). The line has two light/dark transitions– the switching distance Xd/2 is, therefore,effective twice.

g

a)

b)

Figure 7 Abrupt reflection change with associated Iccurve

The line is clearly recognized as long as theline width is d ≥ Xd. If the width is less thanXd, the collector current change, Ic1 - Ic2, thatis the processable signal, becomes increasinglysmall and recognition increasingly uncertain.The switching distance – or better its inverse –can therefore be taken as a resolution of thesensor.The switching distance, Xd, is predominantlydependent on the mechanical/ optical design ofthe sensor and the distance to the reflectingsurface. It is also influenced by the relative po-sition of the transmitter/ detector axis.


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X < line widthd

X > line widthd

Collector current

IC1

IC1

IC2

IC2

Collector current

X d

X d

d X

X

X

R1

R2

ReflectionLine

d = line width

Figure 8 Reflection of a line of width d and corre-sponding curve of the collector current Ic

Figure 9 shows the dependence of the switch-ing distance, Xd, on the distance A with thesensors placed in two different positions withrespect to the separation line of the light/ darktransition.The curves marked position 1 in the diagramscorrespond to the first position. The transmit-ter/ detector axis of the sensor was perpendicu-lar to the separation line of the transition. Inthe second position (curve 2), the transmitter/detector axis was parallel to the transition.In the first position (1) all reflex sensors havea better resolution (smaller switching distan-

ces) than in position 2. The device showing thebest resolution is TCRT9000. It can recognizelines smaller than half a millimeter at a dis-tance below 0.5 mm.It should be remarked that the diagram ofTCRT5000 is scaled up to 10 cm. It showsbest resolution between 2 cm and 10 cm.All sensors show the peculiarity that themaximum resolution is not at the point ofmaximum light coupling, Ao, but at shorterdistances.In many cases, a reflex sensor is used to detectan object that moves at a distance in front of abackground, such as a sheet of paper, a bandor a plate. In contrast to the examples exam-ined above, the distances of the object surfaceand background from the sensor vary.Since the radiation received by the sensor's de-tector depends greatly on the distance, the casemay arise when the difference between the ra-diation reflected by the object on the back-ground is completely equalized by the distancedespite varying reflectance factors. Even if thesensor has sufficient resolution, it will nolonger supply a processable signal due to thelow reflection difference. In such applicationsit is necessary to examine whether there is asufficient contrast. This is performed with thehelp of the working diagram of the sensor andthe reflectance factors of the materials.


Issue: 09. 96 11

a) TCRT5000

b) CNY70

c) TCRT1000

d) TCRT9000

Figure 9 The switching distance as a function of the distance A for the reflex sensors TCRT5000, CNY70, TCRT1000and TCRT9000


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2.4 Sensitivity, dark currentand crosstalk

The lowest photoelectric current that can beprocessed as a useful signal in the sensor's de-tector determines the weakest usable reflectionand defines the sensitivity of the reflex sensor.This is determined by two parameters – thedark current of the phototransistor and thecrosstalk.The phototransistor as receiver exhibits asmall dark current, ICEO, of a few nA at 25°C.However, it is dependent on the applied col-lector-emitter voltage, VCE, and to a muchgreater extent on the temperature, T (see figure10). The crosstalk between the transmitter anddetector of the reflex sensor is given with thecurrent, Icx. Icx is the collector current of thephotoelectric transistor measured at normal IRtransmitter operating conditions without a re-

Figure 10 Temperature-dependence of the collectordark current

flecting medium.It is ensured that no (ambient) light falls ontothe photoelectric transistor. This determineshow far it is possible to guarantee avoiding adirect optical connection between the transmit-ter and detector of the sensor.

At IF = 20mA, the current Icx is approximately50 nA for the TCRT9000 and 15 nA for theCNY70, TCRT1000 and TCRT5000.Icx can also be manifested dynamically. In thiscase, the origin of the crosstalk is electricalrather than optical.For design and optical reasons, the transmitterand detector are mounted very close to eachother.Electrical interference signals can be generatedin the detector when the transmitter is operatedwith a pulsed or modulated signal. The trans-fer capability of the interference increasesstrongly with the frequency. Steep pulse edgesin the transmitter's current are here particularlyeffective since they possess a large portion ofhigh frequencies. For all TEMIC sensors, theac crosstalk, Icxac, does not become effectiveuntil frequencies of 4 MHz upwards with atransmission of approximately 3 dB betweenthe transmitter and detector.The dark current and the dc- and ac crosstalkform the overall collector fault current, Icf. Itmust be observed that the dc-crosstalk current,Icxdc, also contains the dark current, ICEO, ofthe phototransistor.

Icf = Icxdc + Icxac

This current determines the sensitivity of thereflex sensor. The collector current caused bya reflection change should always be at leasttwice as high as the fault current so that a pro-cessable signal can be reliably identified bythe sensor.

2.5 Ambient light

Ambient light is another feature that can im-pair the sensitivity and, in some circumstan-ces, the entire function of the reflex sensor.However, this is not an artifact of the com-ponent, but an application-specific character-istic.The effect of ambient light falling directly onthe detector is always very troublesome. Weak


Issue: 09. 96 13

steady light reduces the sensor's sensitivity.Strong steady light can, depending on the di-mensioning (RL, VC), saturate the photoelec-tric transistor. The sensor is ‘blind’ in thiscondition. It can no longer recognize any re-flection change. Chopped ambient light givesrise to incorrect signals and feigns non-exis-tent reflection changes.Indirect ambient light, that is ambient lightfalling onto the reflecting objects, mainly re-duces the contrast between the object andbackground or the feature and surroundings.The interference caused by ambient light ispredominantly determined by the various re-flection properties of the material which inturn are dependent on the wavelength.If the ambient light has wavelengths for whichthe ratio of the reflection factors of the objectand background is the same or similar, its in-fluence on the sensor's function is small. Itseffect can be ignored for intensities that arenot excessively large. On the other hand, theobject/ background reflection factors can differfrom each other in such a way that, for exam-ple, the background reflects the ambient lightmuch more than the object. In this case, thecontrast disappears and the object cannot bedetected. It is also possible that an uninter-esting object of feature is detected by thesensor because it reflects the ambient lightmuch more than its surroundings. In practice, ambient light stems most fre-quently from filament, fluorescent or energy-saving lamps. Table 2 gives a few approximate

values of the irradiance of these sources. Thevalues apply to a distance of approximately50 cm, the spectral range to a distance of 850nm to 1050 nm. The values of table 2 are onlyintended as guidelines for estimating the ex-pected ambient radiation.In practical applications, it is generally ratherdifficult to determine precisely the ambientlight and its effects. Therefore, an attempt tokeep its influence to a minimum is made fromthe outset by using a suitable mechanical de-sign and optical filters. The detectors of thesensors are equipped with optical filters toblock such visible light. Furthermore, the me-chanical design of these components is suchthat it is not possible for ambient light to falldirectly or sideways onto the detector for ob-ject distances of up to 2 mm.If the ambient light source is known and isrelatively weak, in most cases it is enough toestimate the expected power of this light onthe irradiated area and to consider the resultwhen dimensioning the circuit..AC operation of the reflex sensors offers themost effective protection against ambientlight. Pulsed operation is also helpful in somecases.Compared with dc operation, the advantagesare greater transmitter power and at the sametime significantly greater protection againstfaults. The only disadvantage is the greatercircuit complexity, which is necessary in thiscase. The circuit in figure 14 is an example ofoperation with chopped light.

Table 2 Examples for the irradiance of ambient light sources

Light source (at 50 cm distance) Irradiance Ee (µW/cm2)

850 nm - 1050 nm

Frequency (Hz)

Steady light AC light (peak value)

Filament lamp (60 W) 500

Fluorescent lamp OSRAM (65 W) 25 30 100

Economy lamp OSRAM DULUX (11 W) 14 16 100


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3. Application examples, circuitsThe most important characteristics of theTEMIC reflex sensors are summarized in table3. The task of this table is to give a quickcomparison of data for choosing the right sen-sor for a given application.

3.1 Application example withdimensioning

With a simple application example, the dimen-sioning of the reflex sensor can be shown inthe basic circuit with the aid of the componentdata and considering the boundary conditionsof the application.The reflex sensor TCRT9000 is used for speedcontrol. An aluminum disk with radial strips asmarkings fitted to the motor shaft forms the re-flecting object and is located approximately3 mm in front of the sensor. The sensor signalis sent to a logic gate for further processing.Dimensioning is based on dc operation, due tothe simplified circuitry.The optimum transmitter current. IF, for dc

operation is between 20 mA and 40 mA. IF =20 mA is selected in this case.As shown in figure 3, the coupling factor is atits maximum. In addition, the degradation (i.e.the reduction of the transmitted IR output withaging) is minimum for currents under 40 mA(< 10% for 10000 h) and the self heating islow due to the power loss (approximately50 mW at 40 mA).

+5 V

TCRT 9000

180 15 K

74HCTXX

Q

GND

RE

RS

Figure 11 Reflex sensor – basic circuit

Table 3

Parameter Symbol Reflex sensor type

CNY70 TCRT1000 TCRT5000 TCRT9000

Distance of optimum

coupling

A0 0.3 mm 1 mm 2 mm 1 mm

Distance of best resolution Ar 0.2 mm 0.8 mm 1.5 mm 0.5 mm

Coupling factor k 5% 5% 6% 3%

Switching distance (min.) xd 1.5 mm 0.7 mm 1.9 mm 0.5 mm

Optimum working distance Xor 0.2 mm - 3 mm 0.4 mm - 2.2 mm 0.2 mm - 6.5 mm 0.4 mm - 3 mm

Operating range Aor 9 mm 8 mm > 20 mm 12 mm


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Table 4

Application data

Aluminum disk Diameter 50 mm, distance from the sensor 3 mm, markings printed on the aluminum

Markings 8 radial black stripes and 8 spacings, the width of the stripes and spacings in front of

the sensor is approximately = 4 mm (in a diameter of 20 mm)

Motor speed 1000 rpm to 3000 rpm

Temperature range 10°C to 60°C

Ambient light 60 W fluorescent lamp, approximate distance 2 m

Power supply 5 V ± 5%

Position of the sensor Position 1, sensor/ detector connecting line perpendicular to the strips

Special attention must also be made to thedownstream logic gate. Only components witha low input offset current may be used. In thecase of the TTL gate and the LS-TTL gate, theILH current can be applied to the sensor outputin the low condition. At -1.6 mA or -400 µA,this is above the signal current of the sensor. Atransistor or an operational amplifier should beconnected at the output of the sensor whenTTL or LS-TTL components are used. A gatefrom the 74HCTxx family is used.According to the data sheet, its fault currentILH is approximately 1 µA.The expected collector current for the mini-mum and maximum reflection is now esti-mated.According to the working diagram in figure6c, it follows that when A = 3 mm

Ic = 0.5 × Icmax

Icmax is determined from the coupling factor,k, for IF = 20 mA.

Icmax = k × IF

At IF = 20 mA, the typical value

k = 2.8%

is obtained for k from figure 3.However, this value applies to the Kodak neu-tral card or the reference surface. The couplingfactor has a different value for the surfaces

used (typewriting paper and black-fiber tippen). The valid value for these material sur-faces can be found in table 1:

k1 = 94% × k = 2.63% for typing paper and

k2 = 10% × k = 0.28% for black-tip pen (Edding)

Therefore: Ic1 = 0.5 × k1 × IF = 263 µAIc2 = 0.5 × k2 × IF = 28µA

Temperature and aging reduce the collectorcurrent. They are therefore important to Ic1and are subtracted from it.Figure 4 shows a change in the collector cur-rent of approximately 10% for 70°C. Another10% is deducted from Ic1 for aging

Ic1 = 263 µA - (20% × 263 µA) = 210 µA

The fault current Icf (from crosstalk and col-lector dark current) increases the signal currentand is added to Ic2. Crosstalk with only a fewnA for the TCRT9000 is ignored. However,the dark current can increase up to 1 µA at atemperature of 70°C and should be taken intoaccount.In addition, 1 µA, the fault current of the74HCTxx gate, is also added

Ic2 = 30 µA


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The effect of the indirect incident ambientlight can most easily be seen by comparing theradiant powers produced by the ambient lightand the sensor's transmitter on 1 mm2 of thereflecting surface. The ambient light is thentaken into account as a percentage in accor-dance with the ratio of the powers.

From table 2:

Ee (0.5 m) = 40 µW/ cm2 (dc + ac/ 2)Ee (2 m) = Ee(0.5 m) × (0.5/ 2)2

(Square of the distance law)

Ee (2m) = 2.5 µW/ cm2

Φsf = 0.025 µW

The radiant power (Φsf= 0.025 µW) thereforefalls on 1 mm2.When IF = 20 mA, the sensor's transmitter hasthe radiant intensity:

I 0.5 mW / sree= =Φ

Ω(figure 1)

The solid angle for 1 mm2 surface at a distanceof 3 mm is

( )Ω = =1 mm

3 mmsr

2

2

1

9

It therefore follows for the radiant power that:

Φe = Ie × Ω = 55.5 µW

The power of 0.025 µW produced by the am-bient light is therefore negligibly low com-pared with the corresponding power (approxi-mately 55 µW) of the transmitter.The currents Ic1, Ic2 would result with full re-flecting surfaces, that is, if the sensor's visualfield only measures white or black typing pa-per. However, that is not the case. The reflect-ing surfaces exist in the form of stripes.The signal can be markedly reduced by thelimited resolution of the sensor if the stripesare narrow. The suitable stripe width for agiven distance should therefore be selected

from figure 9. In this case, the minimum per-missible stripe width is approximately 3.8 mmfor 3 mm distance (position 1, figure 9d). Themarkings measuring 4 mm in width were ex-pediently selected in this case. For this width,a signal reduction of about 20% can be permit-ted with relatively great certainty, so that 10%of the difference (Ic1 - Ic2) can be subtractedfrom Ic1 and added to Ic2.

Ic1 = 210 µA - 18 µA = 192 µA

Ic2 = 30 µA + 18 µA = 48 µA

The suitable load resistance, RE, at the emitterof the photo-transistor is then determined fromthe low and high levels 0.8 V and 2.0 V for the74HCTxx gate.

RE < 0.8 V/ Ic2 and RE > 2.0 V/ Ic1, i.e., 10.2 kΩ < RE < 16.7 kΩ

12 kΩ is selected for RE

The corresponding levels for determining REmust be used if a Schmitt trigger of the74HCTxx family is employed.The frequency limit of the reflex sensor is thendetermined with RE = 12 kΩ and comparedwith the maximum operating frequency in or-der to check whether signal damping attribut-able to the frequency that can occur.Figure 5 shows for Vs = 5 V and RE = 12 kΩapproximately, for the TCRT9000, fc =1.5 kHz.Sixteen black/ white stripes appear in front ofthe sensor in each revolution. This produces amaximum signal frequency of approximately400 Hz for the maximum speed of 3000 rpmup to 50 rps. This is significantly less than thefc of the sensor, which means there is no riskof signal damping.In the circuit in figure 11, a resistor, Rc, can beused on the collector of the photoelectric tran-sistor instead of RE. In this case, an invertedsignal and somewhat modified dimensioningresults. The current Ic1 now determines thelow signal level and the current Ic2 the high.The voltages (Vs - 2 V) and (Vs - 0.8 V) and


Issue: 09. 96 17

not the high level and low level 2 V and 0.8 V,are now decisive for determining the resis-tance, Rc.

3.2 Circuits with reflexsensors

The couple factor of the reflex sensors is rela-tively small. Even in the case of good reflect-ing surfaces, it is less than 10%. Therefore, thephotocurrents are in practice only in the regionof a few µA. As this is not enough to processthe signals any further, an additional amplifieris necessary at the sensor output. Figure 12shows two simple circuits with sensors andfollow-up operational amplifiers. The circuit in figure 12b is a transimpedancewhich offers in addition to the amplificationthe advantage of a higher cut-off frequency forthe whole layout. Two similar amplification circuits incorporat-ing transistors are shown in figure 13.The circuit in figure 14 is a simple example foroperating the reflex sensors with choppedlight. It uses a pulse generator constructedwith a timer IC. This pulse generator operateswith the pulse duty factor of approximately 1.The frequency is set to approximately 22 kHz.On the receiver side, a conventional LC reso-nance circuit (fo = 22 kHz) filters the funda-mental wave out of the received pulses anddelievers it to an operational amplifier via thecapacitor, Ck. The LC resonance circuit simul-taneously represents the photo transistor’s loadresistance. For direct current, the photo tran-sistor’s load resistance is very low – in thiscase approximately 0.4 Ω, which means thatthe photo transistor is practically shorted fordc ambient light.At resonance frequencies below 5 kHz, thenecessary coils and capacitors for the oscilla-tor get unwieldy and expensive. Therefore,active filters, made up with operational ampli-fiers or transistors, are more suitable (figures15 and 16). It is not possible to obtain the

quality characteristics of passive filters. Inaddition to that, the load resistance on theemitter of the photo transistor has remarkablyhigher values than the dc resistance of a coil.On the other hand, the construction with activefilters is more compact and cheaper. Thesmaller the resonance frequency becomes, thegreater the advantages of active filters com-pared to LC resonant circuits. In some cases, reflex sensors are used to countsteps or objects, while at the same time rec-ognition of a change in the direction of rota-tion (= movement direction) is necessary. Thecircuit shown in figure 17 is suitable for suchapplications. The circuit is composed of twoindependent channels with reflex sensors. Thesensor signals are formed via the Schmitt-trigger into TTL impulses with step slopes,which are supplied to the pulse inputs of thebinary counter 74LS393. The outputs of the74LS393 are coupled to the reset inputs. Thisis made in such a way that the first output,whose condition changes from ‘low’ to ‘high’,sets the directly connected counter. In thisway, the counter of the other channel is de-leated and blocked. The outputs of the activecounter can be displaced or connected to moreelectronics for evaluation.It should be mentioned that such a circuit isonly suited to evenly distributed objects andconstant movements. If this is not the case, thechannels must be close to each other, so thatthe movement of both sensors are collectedsuccessively. The circuit also works perfectlyif the last mentioned condition is fulfilled.Figure 18 shows a pulse circuit combininganalog with digital components and offeringthe possibility of temporary storage of the sig-nal delivered by the reflex sensor. A timer ICis used as the pulse generator.The negative pulse at the timer's output trig-gers the clock input of the 74HCT74 flip-flopand, at the same time, the reflex sensor'stransmitter via a driver transistor. The flip-flopcan be positively triggered, so that the


18 Issue: 09. 96

condition of the data input at this point can bereceived as the edge of the pulse rises. Thisthen remains stored until the next rising edge.The reflex sensor is therefore only active forthe duration of the negative pulse and can onlydetect reflection changes within this time pe-

riod. During the time of negative impulses,electrical and optical interferences are sup-pressed. A sample and hold circuit can also beemployed instead of the flip-flop. This isswitched on via an analog switch at the sensoroutput as the pulse rises.

+10 Vb)+10 Va)

Reflex sensor

7 2

36

TLC271

Reflex sensor

7

4

2

36

TLC271

220 K

6

R

1 K1 K390

4

1 K390

R1 K

220 K

GNDGND

Output

R RE

I F= 20 mA

S

I F

= 20 mA

OutputRS RE RF

RF

l

l

Figure12 Circuits with operational amplifier

+10 Vb)

+10 Va)

1 KBC178BPNP

220Reflex sensor 1 K

220 KBC108BNPN

C

2.2 µF

10 K

Reflex sensor

390 GND390 1 K

GND

R R

R

R

E

S

= 20 mA

I F

C

L = 20 mA

I F

RS RE

R L

RF

Output

Output

K

Figure 13 Circuits with transistor amplifier


Issue: 09. 96 19

V = + 5 V82

Reflex sensor

1.2 K

2.7 K Q 3

TR 2

GND

1

THR 6CV

5

DIS 7

R

48 555

100 nF

C TLC 271

4

3

26

7

0.86 L

10 K100

62 nFC100 nF10 nF

GND

RF

Output

S

K

mH

Figure 14 AC operation with oscillating circuit to suppress ambient light

+V (10 V)

R

9.1 K

R

220C

1 nF

4

2

36

733 K

R

33 K

R

C

1 µF

Reflex sensor

Q 3

TR 2GND1

THR 6

CV 5

DIS 7

R

48 Timer

555

R

5.1 K

C

100 nF 100 nF

R

510

R1 K

TLC 271(CA3160)

C

22 nF

GNDGND

A

S

E

Output

S

B

F

K

l q

Timer dimensions:

Active filter: C =

V =

Q = 0.5 ×

t (pulse width) = 0.8 RC = 400 µsT (period) = 0.8 (R + R ) × C = 1 msA B

f = 1/(6.28 × C × R)o uo

p

√ C × Cf q √ C

fCq

2 RRE

Q2×

Figure 15 AC operation with active filter made up of an operational amplifier, circuit and dimensions


20 Issue: 09. 96

+V (10 V)

R

220R

1 KC

1.5 nF

NPN

R

51 K

C

1 µF

R

51 KC

1 µF

Reflex sensorR

9.1 K

Q 3

TR 2

GND

1

THR 6

CV 5

DIS 7

R

48 Timer

555R

5.1 K

C

100 nF 100 nF

R

1.8 K

C

33 nF

GNDGND

A

B

E

S

C

Output

V

K

F

K

q

Timer dimensions:

Active filter: C =

V =

Q = 0.5 ×

t (pulse width) = 0.8 RC = 400 µsT (period) = 0.8 (R + R ) × C = 1 msA B

f = 1/(6.28 × C × R)o uo

p

√ C × Cf q √ C

fCq

2 RRE

Q2×

Figure 16 AC operation with transistor amplifier as active filter

CLK

CLR

QAQBQCQD

A

LS393

+5 V

Reflex sensor

A

74HCT14

CLK

CLR

QAQBQCQD

B

LS393

3.3 K

+5 VDQ

CLKQ

D

RD

GND

15 K

Reflex sensor

+5 V DQCLK

Q

SD

RD GND

Reset

CLK

CLR

QAQBQCQD

A

LS393

B

74HCT14

15 K

R

100

GND

CLK

CLR

QAQBQCQD

B

LS393

Left

Right

Display system

or report

Display system

or report

REV

RE

B7474

S

A

Figure 17 Circuit for objects count and recognition of movement direction


Issue: 09. 96 21

(+5 V)V

PNP

3.3 KR

PNP

82 RR

Q 3

TR 2GND

1

THR 6

CV 5

DIS 7 R48

555

R100

Reflex

CD 2 Q 5

CLK 3

Q 6

SD

4

RD 1

74HCT74R

100 nFC

GND

sensor

A CS

B

l

K

2

Figure 18 Pulse circuit with buffer storage

datasheet tcrt 5000

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