DEVICE FOR COMMUNICATION BY LIGHT

Abstract: Device for communication by light, including means for transforming electrical signals to light signals, a transmitter for emitting said light signals, a combined light sensitive means and transforming means (10) for transforming said light signals to elecrical signals, a diode (9) being provided as a light sensitive means, and means being provided for supplying a backward biasing current to said diode. Said diode (9) is arranged in series with a first transistor (T1) in a current mirror coupling for providing a mirror of the current through said diode (9) in a second transistor (T2) of said current mirror coupling, an output of said electrical signals being provided across said second transistor (T2).

Description: DEVICE FOR COMMUNICATION BY LIGHT Systems using infrared transmission are at present frequently utilized for remote control units for TV sets and the like. Recently such systems have been developed also for simple and low speed communication purposes, such as wireless mouse units for computers. The construction of most transmitters and receivers for light, e.g. infrared light, is based on common radio communication technique using a carrier and some kind of modulation, e.g. AM or FM. The carrier frequency is normally around 1 kHz-100 kHz and the bit rate used around 10-100 bits per second. This bit rate is sufficient when for instance switching channel on a television set. No concern is taken on the power consumption on the receiving side as the apparatus comprising the IR receiver normally is connected to mains. A major problem when using IR transmission in a wireless link indoors is the interference caused by strong IR light sources such as light tubes. Outdoors the sun causes the same type of problems. Also, the IR signal it self has a very high dynamic range, which may be diffucult to handle. The same type of problems exists also when other frequences of light is used. .An object of the present invention is to overcome the problems mentioned above and to provide a high speed infrared communication system with low power consumption. A further object of the invention is to provide a more reliable transfer of information through said system. These objects are achieved by the features appearing from the characterizing parts of claim 1 and 4, respectively. Other objects and features of the invention appear from.the subclaims. The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which FIG.l is a block diagram of a reciver according to the invention, FIG.2 is a circuit diagram of a resistor feeded diode detector, FIG.3 is a circuit diagram of a coil feeded diode detector, FIG.4 is a circuit diagram of a current mirror feeded diode detector, FIG.5 shows schematically an optimal IR detector, FIG.6a and FIG.6b are two circuit diagrams of realisations of the receiver with two diffrent types of filter functions, FIG.7a and FIG.7b are two circuit diagrams of realisations of the circuit of FIG.6b with different kinds of transistors, FIG.8 a circuit diagram of a Signal Compression block of FIG.l FIG.9 is a circuit diagram of a Differential Amplifier block of FIG.l, FIG.10 is a circuit diagram of a Shmitt Trigger block of FIG.l FIG.11 is a block diagram of a system using the receiver of FIG.l, FIG.12 a block diagram of another system using the receiver of FIG.l, FIG.13 is a circuit diagram of a simple IR reciver according to the invention, FIG.14 is an alternative embodiment of the receiver of FIG.13,and FIG.15 is a circuit diagram of still another embodiment of the receiver according to the invention. With reference to FIG.l an IR (infra red) receiver according to the invention comprises a detector unit 10 of IR light. Said detector separates an incoming signal from background noise. A signal compression unit 11 is operatively connected to an output of said detector unit 10, and an output of said signal compression unit is connected to an input of an amplifier 12. In the shown embodiment said amplifier is an operational amplifier, which is connected in a conventional manner and is provided with a feed back network. The output of said amplifier is connected to a pulse shaper 13, which in the shown embodiment is a Schmitt trigger. With reference to FIG.2-4 different embodiments of detectors 10 are shown. If a diode of IR type is used as the light sensor the current changes due to incoming IR radiation variations are detected. Also diodes responsive to other types of radiation, such as ultra-violet light and visiable light, can be used. FIG.2 shows an embodiment wherein the current is detected through a resistor R connected in series with the IR diode, which is biased in backward direction. The output signal is taken from the junction between said resistor and said diode. In FIG.3 the resistor is replaced by a coil L and thereby several advantages are acchieved. First of all the backward biasing stability is improved providing maximal voltage over the diode at DC level. Secondly the coil it self is a low pass current filter and it will provide approximately dx/dt at the output between D and L. X is the received light signal. This is an estimation because of the unlinear behavoir of the diode it self. This is not a preferred embodiment due to the physical size and the price of a coil. FIG 4 show how to overcome the unlinear behavior of the diode at the cost of increased current losses. A current mirror comprising a first transistor Tl connected in series with the IR diode and a second transistor T2 connected in series with a coil is utilized in this embodiment. The output is taken from the junction between said second transistor and said coil. In FIG.5 the coil of FIG.4 is replaced by a transistor Til connected in series with a transistor T2a forming a first functional part of the second transistor T2 of said current mirror, a filter 14 and a further transistor T12. Said latter transistor T12 is connected in series with a transistor T2b forming a second functional part of the second transistor T2 of said current mirror. By using two mirror images of the current through the IR diod (T2a and T2b) it is possible to use a more common filter of low pass type, which is used in a current feed forward control circuit comprising also said transistor Vll. Instead of the low pass filter used in the current control circuit a high pass filter in the signal path could be used. Two implementations of said filter 14 are shown in FIG.6a and FIG.6b, respectively. FIG 6a shows a construction using a low pass Sallen-Key filter and FIG 6b shows a common low pass RC-filter. The function of the implementations shown in FIG.5 and FIG.6a,b is very similar to the function of an ideal coil. .Another feature of the present invention is shown at the transistors Tla and Tib. Said transistors are connected in parallel and replace together transistor Tl. The two transistor coupling will reduce the current losses through T2a and T2b. .Another way of reducing the current is to use a receiver diode with smaller (receiveing) area. Any kind of transistors can be used in the receiver. FIG 7a and 7b show two different solutions using only MOS transistors or only bipolar transistors. Signals interfering with the IR signal normally have a very low frequency component and can be filtered out by capacitor (see FIG.13, Cl) . Also high frequency light signals, such as lightnings, and light other than IR light interfere when an IR transmission link is used. These interferences are mostly filtered out by a IR transparent coating of the receiver diode. Normally a signal compression unit 11 is required to overcome some of the drawbacks of a simple detector used in some embodiments. In FIG. 8 the circuit of one suitable embodiment of a signal compression unit is shown. The circuit is conventional and is not further described. Also other signal compression circuits can be used. An output signal from the compression unit 11 is transferred to an amplifier 12, preferably a differential amplifier. The circuit of an embodiment of a differential amplifier is shown in FIG. 9. Also this circuit is conventional and it is further described. An output signal from said differential amplifier is normally irregular and therefore c pulse shaper is normally conngcted to said amplifier so as to provide a signal suitable for further processing. With reference to FIG. 10 a circuit of one embodiment of a pulse shaper is shown. The circuit shown is a so called Schmitt trigger and it is constructed in a conventionl manner and is further described. FIG 11 shows a communication system. A system computer 32, for instance a PC, is operatively connected to a control processor 33. Arrow 13 represents a one way communication on an AT or Micro channel bus or similar bus from said system computer 32 to said control processor 33 controlling a central IR transmitter 34 through a control line 24. The transmitter 34 transmitts an IR signal formed as pulse trains. Arrow 25 represents IR radiation from said transmitter 34 to said IR reciver, which is operatively connected to a demodulator 36. Said demodulator 36 transmitts a demodulated signal to a local control unit 37, and said two latter units carry out the demodulation and verification of a signal/message received from said transmitter. Two methods of verification form part of the present invention. A first verification method includes use of a signal path for a verification signal from said local CPU 37 to a local transmitter 39 through a connection 29. Said transmitter 39 can be simplier than the central transmitter 34 because only an acknowledgement signal for each block\message correctly received from the receiver 35 has vo be transmitted. A central receiver 40 receives the acknowledgement from the transmitter 39 by IR transmission 30 and passes it on to the control processor 33 through a connection 31. If said control processor 33 does not recive an acknowledgement from said central receiver after transmission of a signal/message a retransmission via said central Ir transmitter 34 is initiated. A second verification method is also shown in FIG. 11. In the local CPU 37 there is provided a timer, which preferably is programable, and a display 38 is operatively connected to said local CPU 37. Each time a correct message is received said timer is cleared. If no correct message is received within a prescribed time period a signal is generated, sent through a connection 28, and displayed on said display 38. Any operator, staff and customer may now read and observe (optical read out 43) that the communication unit comprising units 35-38 has not recived any message for a long time and that the data present therein may be invalid or unsafe. FIG.13-15 show different practical emdodiments of a receiver according to the invention. In the embodiment of FIG. 13 bipolar transistors are used, and in the embodiment of FIG. 14 MOS FET transistors are used. FIG. 15 shows a preferred embodiment. Even if IR light is mentioned in the first place above the inventive concept of the present application is applicable also in other frequency intervalls. Also other mediums, such as pressure, can be used to transmitt information signals to a detector according to the present invention.

Claims: CLAIMS 1. Device for communication by light, including means for transforming electrical signals to light signals, a transmitter for emitting said light signals, a combined light sensitive means and transforming means (10) for transforming said light signals to electrical signals, a diode (9) being provided as a light sensitive means, and means being provided for supplying a backward biasing current to said diode, c h a r a c t e r i s e d in that said diode (9) is arranged in series with a first transistor (Tl) in a current mirror coupling for providing a mirror of the current through said diode (9) in a second transistor (T2) of said current mirror coupling, an output of said electrical signals being provided across said second transistor (T2). 2. Device according to claim 1, c h a r a c t e r i s e d by means (15) for low pass filtering said biasing current to said diode (9). 3. Device according to claim 1, c h a r a c t e r i s e d in that said means (15) for low pass filtering comprises a coil. 4. Device according to claim 1, c h a r a c t e r i s e d in that said second transistor (T2) is divided into a third transistor (T2a) operatively replacing said second transistor (T2), and a fourth transistor (T2b) , the base of said fourth transistor (T2b) being connected to the base of said third transistor (T2a), and the emitter of said fourth transistor (T2b) being connected to the emitter of said third transistor (T2a) , and that a low pass voltage filter (14) is provided between the collector of said third transistor (T2a) and the collector of said fourth transistor (T2b), , an output of said electrical signals being provided across said fourth transistor (T2b) . 5. Device according to claim 1 , c h a r a c t e r i s e d in that compressing means (11) is operatively connected to said transforming means (10) for compressing said electrical signals, that amplifying means (12) is operatively connected to said compressing means (11), and that signal shaping means (13) is operatively connected to said amplifying means (12) . 6. System for verification in a communication system according to any of the preceeding claim, data being transferred between a transmitter (34) and a selected receiving unit, comprising a receiver (35), a demodulator (36), a local CPU unit (27) and a display (38) to be displayed on said display (38), c h a r a c t e r i s e d by measuring in said receiving unit a time interval between consecutive data transfers, comparing said time interval with a predetermined time interval, and displaying on said display (38) a signal indicative of the time interval passed when the measured time interval exceeds said predetermined time interval.

Inventors: AHLM ROGER, AHLM, ROGER
Applicants: UNIGRAFIC AG
Application year: 1992
Main classification: H04B10
IPC codes: H03F3, H04B10