WO2001016967A1

WO2001016967A1 - Changeable resistance

Info

Publication number: WO2001016967A1
Application number: PCT/DE2000/002072
Authority: WO
Inventors: Johann Wolf
Original assignee: Siemens Aktiengesellschaft
Priority date: 1999-08-30
Filing date: 2000-06-26
Publication date: 2001-03-08

Abstract

The invention relates to a resistance (1) that is designed as a bipole and consists of a series connection made of a radiation emitter (2) and a radiation receiver (3). The resistance (R) which is determined by the quotient of a voltage (Um) that can be picked off at the connection poles (S1,2) pertaining to the series connection and a current (Im) that flows over said voltage depends upon the degree of external influence on the beam path (5) between the radiation emitter (2) and the radiation receiver (3).

Description

description

Changeable resistance

The invention relates to a variable resistor with two connection poles, on which a voltage can be tapped.

A variable resistance is usually understood to mean a so-called wire rotation or film rotation resistance, which is also referred to as a potentiometer. In this case, a path change caused by a grinder or slide is transferred to a change in resistance, which can be tapped in the form of a corresponding voltage change at a tap of the resistor. Such a variable resistor, which is often used in the field of electrical engineering as a controller - e.g. used as a volume or tone controller, thus enables the most general form of electrical detection of a physical variable in the form of a change in path or angle. However, such a mechanically changeable resistor does not permit a force-free, contactless, and in particular a potential-separated change in resistance.

A non-contact and also potential-isolated resistance control is possible in principle by means of a hot or cold conductor for temperature detection or by means of a photo resistor. Its resistance value depends on the light intensity with which the photo resistor is illuminated. According to this principle of converting an optical signal into an electrical signal, an optocoupler also works, in which an electrical signal on the primary side is converted into an optical signal by means of a light transmitter in the form of a light-emitting diode. The optical signal is received by a light receiver in the form of a photo transistor, with which this can be converted indirectly into an electrical signal. The optocoupler is usually used for the potential separation of two circuits by decoupling them and thus galvanically separating them from each other. For this purpose, the primary-side light transmitter and the secondary-side light receiver must be supplied by means of separate power sources, so that the optocoupler and the photoresistor always represent a four-pole connection with the light source required for lighting.

The invention is based on the object of specifying a two-pole variable resistor in which a change in or influencing of the resistance that is as forceless and contactless as possible and in particular potential-separated is possible.

This object is achieved according to the invention by the features of claim 1. For this purpose, a radiation transmitter and a radiation receiver are electrically connected in series, so that practically a two-pole connection is formed. The change in resistance of this passive bipolar takes place by influencing the

Beam path between the light transmitter and the light receiver. The resistance determined by the quotient of the voltage that can be tapped between the two connection poles and a current flowing through the series connection is dependent on the degree of influencing of the beam path caused by an external means.

The invention is based on the consideration that on the one hand the beam path between a radiation transmitter and a radiation receiver, in the simplest case the light path between a light transmitter and a light receiver of an optocoupler-like arrangement, can be influenced. On the other hand, a series connection of the polar side and the secondary side of such a radiation coupler can be used to implement a two-pole connection. The influencing of the beam path is then reflected independently of the supply voltage of this two-terminal pole in the event of a change in the series or series Circuit flowing current again. The effective resistance of this two-pole system then results from the quotient of the voltage which can be tapped at the two connection poles and the current flowing through the series circuit.

The current flowing through the series connection depends on the degree of influence on the beam path. In order to enable a minimum radiation level or minimum luminance of the primary-side radiation source, the secondary-side radiation receiver is expediently shunted with a current path. In the simplest case, an ohmic resistor is connected in parallel to the radiation receiver. The minimum current thus flowing as a result of the series connection via the radiation transmitter is then essentially dependent on the maximum isolation of the beam path, ie. H. from the maximum coverage of the radiation receiver. By successively releasing the beam path, the radiation receiver becomes increasingly conductive. As a result, the current flowing through the radiation source or the radiation transmitter increases accordingly, so that the radiation intensity or intensity is also increased. The effective resistance value is then determined from the voltage drop that can be tapped at the connection poles and the current flowing through the series connection.

Various external means can be used to influence the beam path between the radiation source and the radiation receiver, the type of which depends, inter alia, on the type of the radiation transmitter and the radiation receiver. For example, the radiation between the transmitter and the receiver can be wholly or partially deflected, reflected or also filtered. In a preferred use of a light transmitter, for example in the form of a light-emitting diode, as a radiation transmitter or source and a light receiver, for example in the form of a photo semiconductor, the beam path can be carried out by partial covering, different levels of reflection or by polarization filtering. Therefore a cover element is expediently introduced into the beam path as an external means.

The cover element can be designed as a reflector, filter or as an opaque plate or flag. Depending on the position or location of the cover element within the beam path, this is influenced to a greater or lesser extent, and thus its degree of influence is determined. The respective position of the cover element within the beam path lies between a first position which releases the beam path and a second position which interrupts the beam path. By successively changing the position or position of the cover element within the beam path between these two positions, the current flowing through the series circuit from the radiation transmitter and the radiation receiver changes. This also changes the resistance of the series connection between two distinct final states of the variable resistor.

In this way, a variety of physical quantities, i.e. H. their changes in time or space are translated into a corresponding change in resistance. The variable resistor, which is designed as a two-pole radiation coupler, is therefore particularly suitable for determining the position of an object, for determining the angle, for determining a temperature-dependent expansion or for level detection and regulation or control. The variable resistance is also suitable for determining the reflectance or transmittance of an object or body, for determining the intensity of polarized light and much more.

The radiation or light transmitter and the radiation or light receiver can be arranged opposite one another in the manner of a fork light barrier or next to or one above the other in the manner of a reflection light barrier. In the latter arrangement, the external means or the cover element is as Designed reflector, which preferably has at least two reflection areas with different degrees of reflection. As a result, on the one hand the reflection intensity and on the other hand the degree of deflection of the beam path past the radiation receiver can be influenced particularly finely.

In a particularly preferred embodiment of the variable resistor with a light-emitting diode as a radiation transmitter and a photo semiconductor or transistor as a radiation receiver, the latter is implemented in a Darlington circuit. Such a Darlington circuit is described, for example, in “semi-conductor circuit technology ^ι , U. Tietze, Ch. Schenk, 6th edition, 1983, pages 64 to 66 and pages 496 to 498. This circuit additionally ensures a minimum - Current via the light transmitter and thus a minimum luminosity also achieved additional amplification. This results in a comparatively fast switching of the phototransistor even with small changes in the light intensity.

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. In it show:

1 shows a block diagram of a two-pole variable resistor with a series connection of one

Radiation transmitter and a radiation receiver, FIG. 2 shows in a voltage / path diagram the resistance curve of the variable resistor which is dependent on the degree of influencing of a beam path, FIG. 3 shows the variable resistor with a series circuit comprising a light-emitting diode and a shunted photo transistor in a block diagram according to FIG 4 shows the phototransistor in a Darlington circuit, and FIG. 5 shows the variable resistor with a transmitter-receiver arrangement based on the reflection principle. Corresponding parts are provided with the same reference symbols in all figures.

1 shows a variable resistor 1 designed as a two-pole system in the form of a radiation coupler with a series circuit comprising a radiation transmitter or a radiation source 2 and a radiation receiver 3. The radiation transmitter 2 is connected on the input side to a first connection pole Si and on the output side to the radiation receiver, which in turn is connected is connected on the output side to the second connection pole S ₂ via an electrical network 4. A supply voltage U _v (±) is applied to these connection poles S 1, S ₂ via an ohmic resistor R 1. A current I _m flowing through the series connection of the radiation transmitter 2 and the radiation transmitter 3 thus causes a tapped voltage U _m at the connection poles S 1, S ₂ , which is referred to below as the measurement voltage. Analogously, the current I _m flowing via the radiation transmitter 2 and via the radiation receiver 3 which is electrically connected in series therewith and also via the electrical network 4 likewise located in this series connection is referred to below as the measurement current. The current arrow indicates the technical current direction of the measuring current I _m .

When the supply voltage U _V ( _± ) is present at the connection poles S1, S2, the measurement current I _{ra flows} via the radiation transmitter 2 to the connection pole S ₂ . The radiation thus emitted by the radiation transmitter 2, the beam path of which is illustrated by the arrows 5, causes a flow of the measuring current I _m through the radiation receiver 3 and in parallel therewith via the electrical network 4, which is designed as a complex electronic circuit, in particular also as an amplifier circuit can. The magnitude of the measuring current I _{m is} directly dependent on the degree of the influence of the beam path 5 symbolized by the arrow 6. The quotient of the measuring voltage U _m which can be tapped at the two connection poles S 1, S ₂ and that via the series connection of the radiation transmitter 2 and The measuring current I _m flowing to the radiation receiver 3 thus determines the resistance according to the relationship R = U _m / I _m . The change in the value of the resistor 1 is thus also directly dependent on the degree of influence on the beam path 5 with the external means indicated by the arrow 6.

2 shows in a voltage / path diagram the resistance curve R of the variable resistor 1 which results as a function of the degree of influence of the beam path 5. The degree of influence x of the beam path 5 is plotted on the abscissa of this diagram, which represents a large number of physical variables, e.g. , B. represents a path or an angle. The measuring voltage U _m is plotted on the ordinate. The window indicated by the arrow 7 shows the usable area of the variable resistor 1.

3 shows the variable resistor 1 with a movable cover element 9 in the form of a flag or a plate on the free end of an actuating element or an actuating axis 8. The cover element 9 extends transversely to the actuation axis 8 and projects into the beam path 5 between a light-emitting diode as the radiation transmitter 2 and a phototransistor connected downstream in series as the radiation receiver 3. The optical path 5 corresponds to the optical path between the light emitting diode 2 and this oppositely arrange the manner of a forked light barrier ^¬ th phototransistor 3. The series circuit of the light emitting diode 2 and the phototransistor 3 is realized, for example on a circuit board or a printed circuit board 10 degrees.

In the exemplary embodiment, the cover element 9 detects a changing state in the form of a rotary movement via the actuating element 8, which is reflected in a corresponding change in position of the cover element 9 within the beam path 5. The change in state to be detected is thus produced by means of a cover 9. called mechanical influence on the beam path 5 transmitted. This influencing of the transmission in turn results in a corresponding change in the measuring current I _m and thus in a corresponding change in the measuring voltage U _m , which drops across the series connection of the light-emitting diode 2 and the photoresistor 10.

For this purpose, the light-emitting diode 2 serving as a radiation source or light transmitter is led on the anode side to the voltage connection or connection pole Si, to which the positive pole U _v (+) of the supply voltage is to be connected via the series resistor R1. On the cathode side, the light-emitting diode 2 is guided, on the one hand, to the collector of the phototransistor 3 serving as a radiation or light receiver and, on the other hand, to a resistor R2 connected in parallel to this on the collector-emitter side.

This parallel resistor R2 serving as a shunt forms an embodiment of the electrical network 4 according to FIG. 1. The resistor R2 connected to the emitter of the phototransistor 3 is connected to the second voltage connection or connection pole S ₂ to which the negative pole U _v (-) of the front - the supply voltage must be connected.

In the similarly constructed circuit of the variable resistor 1 according to FIG. 4, the phototransistor 3 is implemented in a Darling tone circuit. The resistor R2 connected in parallel with the phototransistor 3 according to the embodiment according to FIG. 3 is substituted by the additional transistor 11. As in the embodiment according to FIG. 3, the measurement voltage I _{m is} again between the connection poles S 1 in this embodiment according to FIG. S ₂ tapped.

Depending on the position of the cover element 9 with respect to the arrangement of the light-emitting diode 2 and the photo transistor 3, which, analogously to the embodiment according to FIG. 3, are positioned opposite one another in the manner of a fork light barrier, the beam path 5 between the light-emitting diode 2 and the photo transistor 3 is entirely or partially covered and therefore more or less interrupted. In this case, the cover element 9 can be pivoted or displaced between a first position that completely releases the beam path and a second position that completely covers the beam path 5. The corresponding position of the cover element 9 within the beam path 5 determines its degree of influence.

If the state of the variable to be recorded changes, this change in state is recorded by the actuating element 8 and transmitted to the variable resistor 1 via the cover element 9 which is moved with it. As a result of the coupling of the actuating element 8 with the covering element 9, a rotary movement of the actuating element 8 results in a pivoting movement of the covering element 9. As a result, the covering element 9, which is designed as a movable flag or plate, is influenced by the external influence due to the change in state of the size to be detected Beam path 5 between the light emitting diode 2 and the phototransistor 6 more or less pivoted in or pivoted out of this more or less. This in turn leads to a change in the measurement voltage U _m as a result of a corresponding change in the measurement current I _m flowing via this series connection from the light-emitting diode 2 and the phototransistor 6. The measurement voltage U _m thus represents the change in the resistance 1, which in turn represents the state to be detected or its change as a function of the position of the cover element 9 within the beam path 5 and thus as a function of the degree of influence.

In this fork-barrier-like design of the variable resistor 1, the phototransistor 3 is blocked when the beam path 5 is covered or covered, while the beam path 5 is completely controlled when the beam path 5 is released. These are the two distinct final states of the changeable resistor 1. Between these two final states, the photo current changes through the photo transistor 3 and thus > co r N>P> P ¹

Cπ o cπ O Cπ o Cπ tu PJ 3 b o. Hi φ α

Φ •: H φ Φ g rsi er P- Φ 3 er CΛ σ PJ: uq uq rt CΛ σ d: CL 3 rt öd uq CL φ d φ S, φ 3 φ P- P- Φ rt φ 3 ι-i - ¹ PJ O rt P- er φ PJ φ ι-i Φ Φ Φ rt 3 P- tf Hi CΛ li CΛ P- ι-i ι- <3 rt s: PJ li 0- φ Φ H ι-i φ Φ ι-i he CΛ PJ P- PH pj; uq π- 3 ^J li PJ d Ω CΛ Φ u φ P- 3 Φ 3 P- PJ li Φ CΛ 3 PJ: rt- φ α • d CΛ tr ¹ 3 lr Ω P- d Φ ι-i α er HN Ω co t K ^ P- CΛ CΛ a & £

P- Hi 00 Φ 3 CΛ P- uq 3 ^J Ω li φ <P-; φ Φ er Φ 3 ^J P- φ too rt P- φ Φ uq d: 1- Φ Ω d Φ 3 ^J Ω P- φ 3 rt CΛ P- PJ 3 CΛ P- φ CΛ rt t ι-i CΛ li ^ CΛ d 3 "Hi d CΛ 3 tr CΛ 3 3 ^J Ω li li N rt P 3 ω O 0 o rt Pl CΛ

3 ri H er o. Rt Φ O uq 3 ^{^} uq CΛ φ V α Φ er d Hi uq CΛ rt Ω O CD CΛ uq rt Φ d Ω Φ Φ CΛ CΛ Ό O PJ T tr ¹ 3 ^J li

P- CΛ α Φ Φ CΛ h- ¹ Ω P- Φ 3 P- 3 ^* P- 3 rt O Φ CΛ CΛ, Φ P ^J 3 ι-i r- ¹ Φ r- ¹ 3 J φ PJ * _* c rt ω φ CΛ) ii φ Φ Hi CΛ ι-i <l- ¹ 3 Ω Φ P- 3 φ Φ 21 o ι- (uq uq 3 d 3 3 CO Hl O h- ¹ 3: CL T Ό O Φ Φ < ! ^{^} P- Ω Φ P-> P- Φ S "φ 3 Ω ω

& z d: l-i 3 φ α P Φ P- tr 3 l-i 3 S φ J CΛ t <3 CL 3 3 f O d 3 'P- φ CL t φ

3 CΛ H φ • <ω pj: φ Φ CΛ φ u CΛ φ CΛ 3 α α 3 rt ω 3 d P- l-i CΛ PH

Φ φ Φ d Φ 'CΛ P C CΛ pj: rt Z Φ l-i φ Hi l-i α Ω 3 φ φ uq rt 3 l-i φ d d 3

3 Ω P- 3 3 P- o uq α α CΛ 3 Φ ^ Q Φ li yQ CΛ £: CΛ PJ - Φ CΛ ι-i P- O Ω uq 3 rt tr - t CΛ rt Φ P- Φ φ t rt 3 Φ 3

Φ P- CΛ 21 α o ι-i σ "φ uq rt d P> Φ H CΛ Ό Φ Φ li CΛ CΛ Φ rt li PJ 3 3 Ξ ^ t ^{" 1} 3 Φ 3 P- s; uq Hi P- ι- (1- 'li σ PJ li 3 P- Φ P- N d 3 P- O P- Φ co P- CL CL uq Φ ol \) φ O 3 Ω P- JP ⁾ 3 er 3 α Ω rt 3 α α 21 α rt Ω P- P- Φ φ li ι-i

3 z ι-f? - CΛ Λ H 5 ^ 3 J IM iQ uq d 3 'Φ uq Φ P- P- Φ O 3 ^J CΛ t uq <! P p rt PJ

Φ Φ P- 3 Φ Ω rt Φ d li d Φ φ li CΛ CΛ Λ φ H CΛ rt rt φ Φ 2! p:. d li 3 Φ 3 'ι-i P 3 Φ CΛ 3 ι-i Ω £ ι-a Hl Φ CΛ rt s: CL CΛ tr ¹ H- &> CΛ

CΛ ri- α 1 P- φ PJ Φ iQ 3 rt P- 3 ^J Φ ι-io cυ (D ι-i rt H <! D TJ CL Φ rt Φ Φ Φ 3 P "-3 p- CΛ ⁾ Φ 3 CΛ P- J li • CΛ PJ O uq φ li φ φ d Φ ^ Z li «3 ts 3 o r-> H- H) ιq S 3 3 υq Ω CΛ 3 3 Φ rt 3 3 li Ω ι-i Ω Ω li P-

Φ • d pj: CL fτ CD Φ P- tr P- α TJ φ tr Φ CΛ 3 PJ CΛ cπ uq ι-i "CΛ (D φ

Ω 3 E Φ O l-i P Φ φ • Hi PJ P- uq N 3 rt rt rt rt CL

K σ li CΛ O: uq fi 3 Φ pj: S ¹ uq CΛ φ α α Hi Φ L Φ CL J CO Φ P "

Φ PJ CL ^ P- o PJ CΛ N l-i> 3 φ rt rt 3 φ d l-i P- Φ P- P P- 3 j O

3 Cfi PJ H CΛ rt Φ 13 rt 3 Φ ω d ιq CΛ Φ OOJ: li ^ CΛ H Φ Ω 3 Λ O CL er d α Λ CD Ω P- uq PJ 3 rt Hi φ CΛ 3 S> li ÖJ rt N Ω P- 3 "rt CΛ Φ 9 φ o tr 0 J Φ a PJ ι-i CΛ φ Φ ^ 3 ^* uq 21 -» rt Φ O

P- t Cπ £ 3 er CΛ P- φ P- 3 D, CΛ TP ¹ ^ 0 uq φ Z. P- z ι-i α P- CL g Ω P- Φ P ^J Φ)

CΛ CΛ P- φ PJ Φ CD P ¹ or Φ uq φ C CL PJ φ ii Φ

1-r Φ rt P CL P ⁾ : Cπ CΛ CΛ rt φ φ P- 3 CΛ φ CD 0, φ O CΛ Φ P- φ PJ 3 ^J • li f li α li Φ 3 CΛ Λ ω rt 3 CΛ P- Φ P- rt Ω er CΛ li rt

Φ P- pj: ω CΛ uq d l- ¹ O- φ P- d ^ J 3 Ji. ι-i 3 t Φ Φ CΛ α φ ü CΛ 21

Ω 3 PJ uq rt P- 3 P ⁾ P- P ¹ υ 3 o 3 • J PJ 3 rt PJ 3 P- 3 ^J CL P- w rt Φ 3

S uq α 3 φ Sl φ uq rt et σ PJ M 3 P ¹ PJ 3 uq φ d Φ P-

1 t ^"1 CΛ. O 3 φ 3 O <! PJ 3 3 ö CΛ rt d P- 3 3 PJ 3 φ pj: CL CL CL 3 d P- P ⁾ P- α rt Φ ι-i cn α P- P- Φ 3 φ CL 3 S rt ι- (

0 3 J PJ d Φ α J 3 3 ι- (CΛ O 3 ι-i l-i rt N φ CΛ rt CL. P- CΛ

CL uq φ T

P- yQ Φ CΛ 3 Ω P- p P- P- Φ φ PJ CΛ P- Φ d cn rt ι-i! Λ Φ CΛ V PJ rt

Φ CΛ Hi uq; φ P- Φ ^ P- CΛ 3 rt 3 ω Φ O P- CL P-) i O CΛ fxj li li H ω Φ CΛ rt li O φ K d CΛ p ^j : uq ¹ rt li CΛ φ uq CL CΛ J 3

P- Φ Φ er Φ li ι- (PJ 3 Hi P- li rt Φ p: S CΛ rt li ^"* er S Φ φ L

Ω rt φ Φ <tö P rt 3 rt ^ Q CΛ? O 3 Φ • Φ Φ ι-i PJ P-

Φ tr P- fu; CΛ 3 P- O φ υq 3 ≤ l-i rt d 3 CΛ O ^ ≤ CΛ CΛ 3 3 φ 0 i rt O rt rt Φ CΛ 3 φ φ CL- Z, P- d α

HO 3 1 φ Φ CΛ M o φ rt CΛ tr ¹ 3 φ - Φ d 3 P- P- 3 rt H- υ Φ Φ a 3 ι-iuo CΛ CΛ P- P rt 3 P- CΛ P- d 3 H

Φ 3 CΛ yQ g rt P Φ ι-i li Φ Φ CΛ Ω CL TJ PO 3 9 rt Ω 3 N uq Φ dp CΛ P- Φ Hi σ li d 3 "21 Φ PJ o .. P- rt 3 H 3 ^J uq co 3 PJ P- CΛ 3 <P- ι-i CL S d

P- - '3 rt P li l- ¹ Φ PJ Φ CΛ rt O rt P- gdo P> Φ uq • θ d 3 3 3 3

Φ. φ s rt Φ α α 3 Φ φ PJ Φ P Z 3 Hl

3 IV> 3 Λ N CΛ PJ CΛ CΛ rt Φ <! d l-i CΛ 3 ι-i z Φ CL ^

CΛ φ φ d d CΛ α CΛ 3 o Λ d l-i φ 3 1 Φ CΛ P- uq p- φ O V

1 P CΛ d Φ CΛ £ 0 3 1 l-i uq 3 P- 3

CΛ CΛ rt

Φ rt φ 1 P 3 3 1 Φ P- 1 uq 1 CΛ CΛ a Cπ rt rt O

CΛ 1 uq 1 CΛ φ 1 1 • 1 1

element 9 deflects light emitted from the light source, again embodied as a light emitting diode 2, depending on the position with respect to the light receiver 3, again embodied as a phototransistor, along the indicated beam path 5 in the direction of the phototransistor 3 by or more or less past it. The circuit comprising the light-emitting diode 2 and the phototransistor 3, again in the form of a series circuit, is likewise implemented on a circuit board or a printed circuit board 10. The state detected by the actuating element 8 and via this by the covering or reflecting element 9 or a corresponding change in state is in turn mechanically transmitted to the variable resistor 1.

Depending on the starting position of the covering or reflection element 9 with respect to the light transmitter 2 and with respect to the light receiver 3, which are positioned next to one another in the manner of a reflection barrier, the beam path 5 emanating from the light-emitting diode 2 is more or less reflected in the direction of the phototransistor 3. For this purpose, the covering or reflection element 9 expediently has at least two reflection regions 9a and 9b with different degrees of reflection.

A shift or pivoting of the covering or reflection element 9 in turn leads to a change in the measuring current I _m via the series circuit formed by the light-emitting diode 2 and the phototransistor 3 with a corresponding change in the measuring voltage U _m and S _{2 which can be} tapped at the connection poles S 1, S ₂ a corresponding change in resistance.

The effective resistance of the variable resistor 1 is thus influenced without force or contact. The one caused by the cover or reflection element 9 or by another external means 6, for example by a filter

Influencing takes place in a potential-separated manner. In this case, in the case of the variable resistor designed as a two-pole connection, 1 for the LED 2 and therefore no additional auxiliary voltage is required for the light or radiation transmitter.

Claims

claims

1. Variable resistor with two connection poles (Sι, S ₂ ), at which a voltage (U _m ) can be tapped, with a series connection between the connection poles (Sι, S ₂ ) consisting of a radiation transmitter (2) and a radiation receiver ( 3), the resistance (R) determined by the quotient of the tapped voltage (U _m ) and a current (I _m ) flowing through the series circuit being dependent on the degree to which the beam path (5) is influenced by an external means ( 6.9).

2. Changeable resistor according to claim 1, characterized in that as an external means in the beam path (5) insertable cover element (9) is provided, the position of which is determined by the position between the radiation transmitter (2) and the radiation receiver (3) ,

3. Variable resistor according to claim 1 or 2, characterized in that when a supply voltage (U _v ) is applied, this is led via an ohmic resistor (Rl) to the connection poles (SιS ₂ ).

4. Variable resistor according to one of Claims 1 to 3, characterized in that the radiation transmitter (2) and the radiation receiver (3) are arranged opposite one another, the external means (9) between a first position which releases the beam path (5). tion and a second position interrupting the beam path (5) is pivotable or displaceable.

5. Changeable resistance according to one of claims 1 to 3, that the radiation receiver (3) reflects from the external means (9), so that the radiation receiver (3) reflects

Receives radiation, the means (9) between the beam path (5) in the direction of the radiation receiver (3) deflecting first position and a second position deflecting the beam path (5) past the radiation receiver (3).

6. Variable resistor according to claim 5, so that the external means (9) has at least two reflection areas (9a, 9b) with different reflectance.

7. Variable resistor according to one of claims 1 to 6, so that the radiation transmitter is a light-emitting diode (2) and that the radiation receiver (3) is a photo semiconductor connected downstream of this.

8. Variable resistor according to claim 7, which also means that means for generating a minimum current flowing parallel to the radiation receiver (3)

(I _m ) are provided.

9. Variable resistor according to one of claims 1 to 6, d a d u r c h g e k e n n z e i c h n e t that the radiation receiver (3), an ohmic resistor (R2) is connected in parallel.

10. Variable resistor according to one of claims 1 to 9, d a d u r c h g e k e n n z e i c h n e t that the radiation receiver (3) is designed in Darlington circuit.