GB2123144A

GB2123144A - Bistable photoelectric sensing apparatus

Info

Publication number: GB2123144A
Application number: GB08317343A
Authority: GB
Inventors: Egon Krogh
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1982-07-01
Filing date: 1983-06-27
Publication date: 1984-01-25
Also published as: GB8317343D0; SE8303485D0; DK291783A; DE3224531A1; SE8303485L; JPS5920839A; DK291783D0; FR2529685A1

Abstract

A bistable photoelectric sensing apparatus comprises a lamp (1) connected in series with a limiting resistor (2) to an operating voltage source (3) and a photoelectronic semiconductor (4) on which light impinges during sensing of an article. As a result of illumination by the lamp (1), the conductivity of the semi-conductor (4) increases and brings about an increase in the lamp current. The lamp (1) is arranged to be ON continuously and the current of the semi-conductor (4) brings about an at least partial short-circuiting of the limiting resistor (2). <IMAGE>

Description

SPECIFICATION Bistable photoelectric sensing apparatus The invention relates to bistable photoelectric sensing or scanning apparatus comprising a lamp applied in series with a limiting resistorto an operating voltage source and a photoelectronic semiconductor which is impinged in response to scanning of an article and, through illumination by the lamp, receives a higher conductivity and brings about an increase in the lamp current.

In a known scanning apparatus ofthe aforementioned kind, the limiting resistor, the lamp and the photoelectronic semi-conductor are applied in series to the operating voltage source. A light pulse must be given to the photoelectronic semi-conductor by an external lightsourceto initiate a current in this series circuit because it is only when the photoelectronic semi-conductor is conductive that the lamp in series with itwill produce lightand hold the photoelectronic semi-conductor in the conductive state. If the light rays ofthe lamp striking the photoelectronic semiconductor are interrupted, the current is interrupted and the lamp is switched off again so that the current remains interrupted.It is only with renewed illumination of the photoelectronic semi-conductor by means oftheexternal light source thatthe current in the series circuit can be switched on again. The additional light source and operation ofthis light source is expensive if this scanning apparatus is to be used for scanning an article such as a rotating programme disc of a programme time switch, the article allowing the light rays of the lamp to pass or reflecting same to initiate a switching step depending on its transparencyorsurface properties because, after every interruption of the light between the lamp and semi-conductor, the external lightsourcewould again have to be switched on to deliver a light pulse.

The invention is based on the problem of providing a scanning apparatus of the aforementioned kindthat can be used to scan articles without an additional light source and is of simple construction.

The present invention provides: A bistable photoelectric sensing apparatus comprising a lamp arranged in series with a limiting resistor for connection to an operating voltage source and a photoelectronicsemi-conductorso arranged that, in use, light impinges on the semi-conductor in response to sensing of an article and, th rough illumination by the lamp, the conductivity ofthe semi-conductor is increased and brings about an increase in the lamp current, wherein, in use, the lamp is arranged to be ON continuously and the current of the semi-conductor brings about an at least partial short-circuiting of the limiting resistor.

According to the invention, this problem is solved in thatthe lamp is constantly on and the current of the semi-conductor brings about at least partial shortcircuiting ofthe limiting resistor.

In this construction, a single lamp suffices for controlling the photoelectronicsemi-conductor.As soon as the light between it and semi-conductor is freed by the article to be scanned, the light current of the lamp increases the conductivity of the photoelectronic semi-conductor and thus reduces the overall resistance in the lamp circuit so that the light current of the lamp and thus the semi-conductor current are amplified in the sense of positive feedback. This leads to a trigger behaviour of the scanning apparatus.

Consequently, during scanning ofthe article, which leads to a marked change in the light current ofthe lamp, sudden changes in the semi-conductor current are produced which can beevaiuatedasaclear indication ofthe presence of a recess, hole or other change in the property of the material of the article to be scanned.

The semi-conductor may be in parallel with the limiting resistor to short-circuitthe limiting resistor.

This results in particularly simple circuitry.

However, it is also possibleforthe base-emitter current of a transistor in shunt with the limiting resistorto be controllable by the semi-conductor current. This leads to amplification of the feedback currentthrough thetransistor and thusto particularly steep pulse flanks in the semi-conductor current in dependence on the scanning of an article.

A pre-resistor may be connected in series with the transistor in the shunt circuitforthe purpose of limiting the transistor current and th us the lamp current.

The current of a transistor in an output circuit may be controllable bythe semi-conductor current. Instead ofthe semi-conductor current, the current ofthe transistor in the output circuit may in this case be evaluated without affecting the semi-conductor current, amplification being produced at the same time.

Further,the connector of the transistor may be connected by way of a pre-resistor directly to the voltage source and by way of a diode which is poled in the same sense as the pass direction ofthetransistor to the junction between the semi-conductor and limiting resistor. In this way, a feedback effect by the output circuit on the lamp currentwhen the transistor is blocked is prevented by interruption ofthe rays between the lamp and semi-conductor. Nevertheless, a single transistor will suffice for amplifying the feedback current and output current.

Ifthe diode is at the sametime a luminous diode, it cannotonlyuncoupletheoutputcircuitand lamp circuit when the rays are interrupted but, by lighting up when the transistor is conductive indicate the passage of lightthrough the article being scanned.

The semi-conductor circuit may be operatively connected to the lamp circuit only by way of a photoelectric coupler. This coupler ensures galvanic separation between the semi-conductor and lamp circuits.

The photoelectric coupler can be connected on the input side to the output of a transistor which amplifies the semi-conductor current and on the output side to the base of a transistor in parallel with the limiting resistor. In this way the output current as well as the feed-back current can be amplified independently of each other.

The lamp may be a luminous diode. With a relatively smail size, it has a long life in comparison with, say, an incandescent lamp.

A photo-transistor may be used as photoelectric semi-conductorwhich, as such, already has an amplifying effect with a viewto achieving steep pulse flanks.

Bistable photoelectric sensing apparatus con structed in accordance with the invention will now be described by way of example only with reference to the accompanying drawing, in which: Figs. 1 to 5 are each a circuit diagram of a different example of sensing apparatus according to the invention.

The example of a bistable photoelectric sensing apparatus shown in Fig. 1 comprises a series circuit of a lamp 1 in the form of a luminous diode and an ohmic limiting resistor 2 connected to an operating voltage source 3. In parallel with the limiting resistor 2 there is a photoelectric semi-conductor 4 in the form of a photo-transistor of which the conductivity increases upon illumination. The lamp 1 and semi-conductor 4 are each arranged in a respective limb of a fork-like holderso thatthe light of lamp 1 is directed onto the semi-conductor 4 but substantially screens off the surrounding light.By means of the programme disc5 of a programme time switch, which disc is provided with cut-outs atthe peripherythereof and rotates between the limbs ofthefork-like holder, the light of lamp 1 is alternately allowed to pass to the semiconductor4 and interrupted. Only that part of the programme discS which is disposed above the rotary axis shown in chain-dotted lines is illustrated.The limiting resistor 2 is dimensioned so thatthe light of lamp 1 is comparativelyweakwhen the semi- conductor4is not illuminated and is just sufficient to initiate the photo-current of the semi-conductor when illumination ofthe semi-conductor4 is permitted through the programme discS. The semi-conductor 4 is able to bring about at least partial short-circuiting of the limiting resistor2 so that the lamp 1 lights up more intensively. The now stronger illumination of semiconductor4 allows its current and thus the light of lamp 1 to become even stronger until the semiconductor 4 is finally completely conductive. By reason ofthis positive feedback, the voltage at the output6formed bythecollectorofthe phototransistor4 rises suddenly.Conversely, the output voltage decreases suddenly when the lightfrom lamp 1 is interrupted by the programme discS. In this way, sudden changes in the voltage at the output 6 are produced when scanning the edges of programme disc cut-outs, so that the output voltage is a square voltage with steep flanks even if the programme disc 5 rotates only comparatively slowly.

In the Fig. 2 example, the currentflowing through thesemi-conductor4is amplified by a transistor 7 which is disposed in the output circuit in a branch parallel to the limiting resistor resistor 2, a further ohmicresistor8being provided in serieswith the transistor7 to limit the current in the parallel branch.

The current of the semi-conductor4flows through a voltage divider which is formed by ohmic resistors 9 and 10 and the tapping ofwhich is connected to the base of transistor 7. In this way, the semi-conductor current controls the collector-emitter current oftran- sistor7 and thus controls that part ofthe current flowing through lamp 1 which is superimposed over the basic currentflowing through the limiting resistor 2 and determining the basic brightness of lamp 1.

In the Fig. 3 example, compared with the Figs. 2 example an additional resistor 11 is provided and connected between the collector oftransistor7 and the positive pole of operating voltage source 3.

Further, a diode 12 is disposed between the collector oftransistor7 and the junction of lamp 1 and limiting resistor 2 in the parallel branch. This results in a defined output voltage at the output 6 even if the semi-conductor4 is not illuminated, that is, is de-energised, so that the transistor 7 is blocked. In this case, the diode 12 decouples the lamp current from the output circuit.

Further, the diode 12 is a luminous diode so that it lights up upon illumination of semi-conductor4and and thus gives a readily perceivable optical indication of the presence of a recess in the programme discS.

Whereas in the Fig. 3 example the transistor 7 amplifies the feedback current as well as the output current, these functions are divided between two transistors 7a and 7b in the example of Fig. 4so that the feedback current is independent of the output current, that is the load. The transistor7a lies only in the branch parallel to the limiting resistor 2 and the transistor 7b only in the output circuit. Both are controlled in parallel by the semi-conductor4 or rather the semi-conductor current by way of a respective voltage divider 9a, 1 Oa or9b, 10b preceding the bases oftransistors 7a and 7b.

In the example of Fig. 5, the lamp circuit and semi-conductor circuit are galvanically separated, each being fed by its own voltage source 3. As in the caseofthe Fig. 4example,the current of semi- conductor4determinesthe base current of the amplifying transistor 7b in the output circuit. Its current is, however, fed to the base of the feedback transistor7a byway of the lamp 13, in this case a luminous diode, of an electro-optical coupler 14that is disposed in the collector circuit oftransistor7b. For this purpose, the coupler 14 also comprises a photoelectronic component 15 in the form of a transistor.

This component 15 is illuminated by lamp 13 and transforms the light of lamp 13 into a substantially proportional current. Consequently, in this case a current substantially proportional to the current of semi-conductor4 is again positively fed back (in the sense of a current increase) to the circuit containing lamp 1. In this case, the output circuit and feedback circuit are operatively interconnected by way of the coupler 1 4which merely separates them galvanically, so thatthefeedback circuit does not produce an additional load on the output circuit, and vice versa.

Further modifications of the illustrated examples are within the scope ofthe invention. Thus, the lamps 1 and 13 may be incandescant lamps instead of luminous diodes. Similarly, the photoelectronic components 4 and 15may comprise photo-resistors or photo-diodes instead ofthe photo-transistors. The lamp 1 and semi-conductor4 may be so arranged that the light from lamp 1 is reflected from the article to be scanned onto the semi-conductor 4.

Claims

1. Abistable photoelectric sensing apparatus comprising a lamp arranged in series with a limiting resistorforconnection to an operating voltage source and a photoelectronic semi-conductor so arranged that, in use, light impinges on the semi-conductor in response to sensing of an article and, through illumination by the lamp, the conductivity ofthe semi-conductor is increased and brings about an increase in the lamp current, wherein, in use, the lamp is arranged to be ON continuously and the current of the semi-conductor brings about an at least partial short-circuiting ofthe limiting resistor.

2. Apparatus according to claim 1, wherein the semi-conductor is in parallel with the limiting resistor.

3. Apparatus according to claim 1, wherein a transistor is connected in shunt with the limiting resistor and the base-emitter current ofthat transistor is arranged to be controlled by the semi-conductor current.

4. Apparatus according to claim 3, wherein a resistor is connected in series with the transistor in the shunt circuit.

5. Apparatus according to one of claims 1 to 4, including a transistor connected in an output circuit and arranged to have its current controlled by the semi-conductor current.

6. Apparatus according to one of claims 3 to 5, wherein the collector of the shunt-connected transistor is connected by way of a resistor directly, in use, to the voltage source and is connected byway of a diode, poled in the same sense as the pass direction of the shunt-connected transistor, to the junction between the semi-conductor and the limiting resistor.

7. Apparatus according to claim 6, wherein the diode is a luminous diode.

8. Apparatus according to one of claims 1 to 7, wherein the semi-conductor circuit is operatively connected to the lamp circuit byway of a photoelectric coupler.

9. Apparatus according to claim 8, wherein the photoelectric coupler is connected on the input side to the output ofatransistorarranged to amplifythe semi-conductor current and on the output side to the base of a transistor in parallel with the limiting resistor.

10. Apparatus according to one of claims 1 to 9, wherein the lamp is a luminous diode.

11. Apparatus according to one of claims 1 to 10, wherein the semi-conductor is a phototransistor.

12. A bistable photoelectric sensing apparatus substantially as herein described with reference to, and as illustrated by, any one of Figures 1 to 5 ofthe accompanying drawing.