Optical Circuit

This invention relates to an optical circuit and speaking more in detail, to a so-called photosensor including a light emitting element and a photosensitive element in order to detect an object in an optical manner.

Heretofore so-called photosensors are used for the purpose of detecting optically cards or sheets, or obtaining pulses by means of a disk having radial slits and secured to the rotating shaft of a motor.

A kind of photosensors hitherto widely utilized is described e.g. in Application Manual [Photointerrupter] SHARP (January, 1982) p. 26, FIG. 4-7(d). In this circuit a current is made flow through a light emitting diode from a power source +V.sub.cc through a resistor R and a voltage is applied to a phototransistor from the power source +V.sub.cc through another resistor. The light emitting diode emits light, when a current flows therethrough and the phototransistor is switched-on, when the light reaches it so that a current flows through this phototransistor from the power source +V.sub.cc. Since the current amplification factor of the phototransistor is small, its output is amplified by a transistor in the succeeding stage and a waveform having a large amplitude is taken out. However, since the waveform includes distortions, the amplified output is further shaped in waveform by means of a Schmitt trigger element.

In addition, in this circuit structure, since the circuit comprising the light emitting element and that comprising the photosensitive diode are separated from each other with respect to the power supply, the interface for taking out the output should consist of 3 lines; i.e. power source line, collector line and earth line for the phototransistor.

Recent electronic apparatuses have a tendency to become more and more highly functional and many photosensors are incorporated here and there in the apparatuses. Consequently, since the number of interface signal lines increases proportionally thereto, assembling work for the signal lines in the apparatuses becomes tremendous.

An object of this invention is to provide an optical circuit permitting to reduction the number of interface signal lines.

Another object of this invention is to provide an optical circuit having an amplifier permitting amplification of electric current applied to the light emitting element in dependence upon the output of the photosensitive element.

Still another object of this invention is to provide a photosensor, in which the current through the light emitting element is increased by feeding back the current flowing through the photosensitive element to the light emitting element and thus the amount of emitted light is increased.

This invention is realized in an optical circuit provided with a light emitting element a photosensitive element, which receives light emitted by this light emitting element. Usually the light emitting element is a photodiode and the photosensitive element is a phototransistor. A current control circuit is connected in series with this light emitting element, which circuit controls current flowing through the light emitting element, depending on the current flowing through the photosensitive element. This current control circuit consists preferrably of a bias element and a transistor serving as current amplifier means connected in parallel thereto.

Owing to such a construction as described above, the optical circuit is realized by 2 lines, i.e. one for feeding the light emitting element with electric power and the other for receiving signals from the photosensitive element.

Further, the path for the current flowing through the light emitting element and that for the current flowing through the photosensitive element constitute a feedback loop through the current control circuit. The output current of the photosensitive element is fed back to the light emitting element side. In this way, the current flowing through the light emitting element is amplified by the transistor and thus the amount of emitted light is increased so that the current intensity appearing at the output stage of the photosensitive element is considerably amplified. Consequently no amplifier circuit need be added to the output stage.

FIG. 1 is a perspective view of a mechanism for detecting the rotation of a motor, which is an example of applications of a photosensor;

FIG. 2 is a circuit diagram for the optical detection circuit according to an embodiment of this invention;

FIG. 3 is a time chart for explaining the working mode of the optical detection circuit indicated in FIG. 2; and

FIG. 4 is a circuit diagram for the optical detection circuit according to another embodiment of this invention.

FIG. 1 is a diagram showing a mechanism for detecting the number of rotations of a motor, which is an example of applications of a photosensor. A disk 12 having a hole 121 for detecting the phase of the rotating shaft 111 of a motor is mounted on the shaft. An optical object detection unit 13 for detecting optically the position of the hole formed in the disk 12 is so disposed that the disk 12 is inserted therein. Two signal lines L.sub.1 and L.sub.2 are taken out from this optical object detection unit 13.

In this unit 13 a light emitting element and a photosensitive element are disposed opposing to each other so that the disk 12 is held therebetween. In this way, when the hole 121 reaches the position therebetween in the course of the rotation of the motor 11, light emitted by the light emitting element is detected by the photosensitive element and a current corresponding to the amount of received light flows through the photosensitive element. In this way, the object, i.e. presence or absence of the hole formed in the disk 12 is detected.

FIG. 2 shows a specific optical detection circuit.

In FIG. 2, an optical object detection part 3 and a detection circuit 10 are indicated by broken lines, respectively. The detection circuit 10 is connected with the optical object detection part 3 by means of 2 signal lines; i.e. one L.sub.1 coming from a power source V.sub.cc and the other L.sub.2, which is the output line. The signal line L.sub.1 is connected with the emitter of a transistor 6 and a terminal a of a bias element 7 in the optical object detection part 3. Further, the collector of the transistor 6 and the other terminal b of the bias element 7 are connected with each other and they are connected to the anode of a photodiode 4, which is the light emitting element. The base of the transistor 6 is connected with the collector of a phototransistor 5, which is the photosensitive element. In addition, the cathode of the light emitting element 4 and the emitter of the photosensitive element 5 are connected with each other and they are connected with the detection circuit 10 through the signal line L.sub.2.

The signal line L.sub.2 is connected with a terminal a of a load resistor 8 and the input of a Schmitt trigger element 9 in the detection circuit 10. In addition, the other terminal b of the load resistor 8 is connected to a reference potential such as "earth". In this way, in the optical object detection part 3, the path of the current flowing through the light emitting element 4 and that of the current flowing through the photosensitive element 5 constitute a feedback circuit. When the intensity of the light emitted by the light emitting element 4 increases, the amount of received light increases also and thus the intensity of the current flowing through the photosensitive element 5 increases. Consequently, this is fed back to the path of the current flowing through the light emitting element 4 through the transistor 6, which increases further the intensity of the light emitted by the light emitting element 4. In this way, since increase of the current is great owing to the feedback, it is not necessary to dispose an amplifier means for the output stage of the light receiving side, which was inevitable in the prior art techniques. In addition, since the interface signal line L.sub.2 starts from the cathode of the photosensitive element 5, the number of signal lines is two, smaller than that needed according to the prior art techniques. By this fact reduction in size of the apparatus and in electric power consumption is realized.

Now the working mode of the circuit will be explained below.

Here it is supposed that among the resistance of the bias element 7, the resistance of the load resistor 8, the lower limit V.sub.iL and the upper limit V.sub.iH for the input voltage level the following relationship is valid; ##EQU1## where 1.2 is an approximate value in Volt of the voltage drop in the light emitting element 4, in the case where an infra-red light emitting diode is used as the light emitting element 4. Furthermore it is supposed that the voltage drop in the transistor 6 is negligibly small.

Hereinbelow the working mode of the circuit indicated in FIG. 2 will be explained for a case in which it is applied to a mechanism for detecting the rotation of a motor 11.

When the part of the disk 12, where there is no hole, is located at the optical object detection part 3, no output current I.sub.p flows, because no light emitted by the light emitting element 4 reaches the photosensitive element 5. Therefore, the base current of the transistor 6 is "0" and the transistor 6 is cut off. On the other hand, the current I.sub.D flowing through the light emitting element 4 is equal to the current I.sub.R flowing through the bias element 7 and therefore the amount of emitted light is small. Consequently, the input voltage V.sub.i of the Schmitt trigger element 9 is at the "LOW" level according to the above-mentioned formula (1) and the output voltage V.sub.o of the Schmitt trigger element 9 is at the "HIGH" level.

Then, when the hole 121 in the disk 2 reaches the optical object detection part 3, light emitted by the light emitting element 4 arrives at the photosensitive element 5 and the current I.sub.p begins to flow. Since the photosensitive element 5 is connected with the base of the transistor 6, the collector current I.sub.C of the transistor 6 begins to flow. By this fact, the current I.sub.D flowing through the light emitting element 4 becomes I.sub.R +I.sub.C. Therefore, the amount of emitted light increases and thus I.sub.p augments further. This amount continues to augment, until the moment when the transistor 6 is saturated. At the moment when the transistor 6 is saturated, the input voltage V.sub.i of the Schmitt trigger element 9 becomes V.sub.i =(V.sub.cc -1.2) [V] and according to the above-mentioned formula (2) the output voltage V.sub.o of the Schmitt trigger element 9 becomes "LOW" level.

FIG. 3 is a time chart indicating this relation. In FIG. 3, a, a' and a" on the abscissa are periods of time, during which the hole 121 in the disk is in the optical object detection part 3, and other parts are periods of time, during which the part where there is no hole is in the optical object detection part 3. The current I.sub.D flowing through the light emitting element 4 has a waveform distorted by the bias element 7 and the transistor 6, but pulses having no distortions and well shaped appear at the output V.sub.o after they have been inputted in the Schmitt trigger element 9. Further, at first only the current I.sub.D due to the bias element 7 flows through the light emitting element 4. When the photosensitive element 5 receives light emitted by the light emitted element 4, the current I.sub.p flowing through the photosensitive element 5 is fed back through the base of the transistor 6 and therefore the intensity of I.sub.D and I.sub.p increases further. This constitutes the input voltage V.sub.i for the Schmitt trigger element 9 in the detection circuit 10, and when this signal is outputted by the Schmitt trigger element 9, it is represented by a well shaped pulse voltage V.sub.o. As it is clear from FIG. 2, when an object is inserted between the light emitting element 4 and the photosensitive element 5, electric power consumption is extremely low and thus the invention is efficacious for reduction of electric power consumption of the circuit. In addition, in FIG. 3, when the period of the output voltage V.sub.o of the Schmitt trigger element 9 (time from a to a') is measured, since the number of rotation f is equal to 1/T, the number of rotation of the motor 11 can be detected by using the inverse of the measured value of T.

FIG. 4 is a circuit diagram of the optical object detection circuit according to another embodiment of this invention.

In FIG. 4, the light emitting element 4 is arranged at the side of the power source (V.sub.cc). That is, the signal line L.sub.1 is connected with the anode of the light emitting element 4 and the cathode of the light emitting element 4 is connected with the emitter of the transistor 6 and the terminal a of the bias element 7. Further, the collector of the transistor 6, the terminal b of the bias element 7 and the emitter of the photosensitive element 5 is connected with the common signal line L.sub.2. In addition, the collector of the photosensitive element 5 is connected with the base of the transistor, gist as for the circuit indicated in FIG. 2.

As explained above, according to the embodiments described above, a photosensor having only two interface signal lines is obtained. Furthermore, since the amplifier circuit, which was necessary in the output stage according to the prior art techniques, is no more needed, the circuit is simplified.

This invention is not limited to the embodiments described above, but this invention can be realized with many variations. One of the most usual variations is a circuit, in which the connection direction is inversed, as usually done in electronic circuits where transistors are used. That is, a connection at the collector side is changed to the emitter side, or an emitter-earthed circuit is changed to a collector-earthed one.

As other examples of modifications in the examples indicated in FIGS. 2 and 4, the bias element 7 is not always necessary, because leakage current flows usually through the transistor 6 and this current flows also through the light emitting element.

Furthermore, as another example of variations, in the circuit indicated in FIG. 2, the Schmitt trigger element 9 can be replaced by a comparator circuit while taking the relation in voltage level into account. Further, without making flow starting current by means of the bias element 7, it is possible to form the starting current by using external light or leakage current through the transistor 6 or the photosensitive element 5.

Still further, as an example of variations of the circuit indicated in FIG. 4, it is possible to compare the voltage coming from the signal line L.sub.1 with an earthed voltage by connecting the signal line L.sub.2 with frame earth and by replacing the Schmitt trigger element 9 in the detection circuit 10 by a comparator. In this way, only one signal line L.sub.1 is necessary between the detection part 3 and the detection circuit 10.