DE102005036937B4 - Evaluation circuit for an optical receiver - Google Patents

Abstract

Evaluation circuit for an optical receiver, comprising a voltage divider circuit which is constructed from a measuring resistor (R _mess ) and a first switching element (T1) and to which a supply voltage (Vdd) is applied, the first switching element (T1) corresponding to a reception of electromagnetic radiation is electrically conductively switched by a receiver and an evaluation signal (V _IN ) between the measuring resistor (R _mess ) and the first switching element (T1) is tapped, characterized in that the first switching element (T1) in series with a parallel circuit of the measuring resistor ( R _mess ) and an additional current _source circuit (A _add ) is connected, wherein the additional current _source circuit (A _add ) in the case of an over-radiation of the receiver causes an additional current flow through the first switching element (T1), so that the first switching element (T1) even in the case of an over-radiation the receiver works with a favorable signal-to-noise ratio ,

Description

The present invention relates to an evaluation circuit for an optical receiver according to the preamble of claim 1, and in particular to such an evaluation circuit, as it can be used for example in a touch-sensitive push button or a remote control or communication receiving part.

Touch-sensitive push-buttons, in particular infrared push-buttons, such as those used in domestic appliance controls, and remote control or communication receiving parts typically include an optical sensor having a receiver receiving electromagnetic radiation and an evaluation circuit for evaluating one of the optical receiver Sensors generated measurement signal and generating a corresponding evaluation signal for further processing. The basic structure of such a conventional evaluation circuit is exemplary in 3 shown.

The evaluation circuit of 3 includes a voltage divider circuit, which is composed of a measuring resistor R _mess and a transistor T1 and to which a supply voltage Vdd is applied. The measuring signal V _{E of} the receiver switches the transistor T1, and the evaluation signal V _IN is tapped between the measuring resistor R _mess and the transistor T1. In normal operation, the transistor T1 without actuation z. B. high impedance of the touch switch, so that the evaluation signal V _IN has a high signal level. Upon actuation of the key switch, the transistor T1 is switched low impedance by the measurement signal V _{E of} the receiver, so due to the current flow through the evaluation of the signal level of the evaluation signal V _IN falls accordingly, which can be evaluated by a connected microprocessor as an operation of the push-button.

A well-known problem of such devices is the over-radiation of the receiver, for example, by extraneous light, which can cause such an insufficient signal / noise ratio that no useful signals can be evaluated by the evaluation circuit. In an over-radiation of the receiver, the transistor T1 is so low that the determined by Vdd and R _mess maximum current flow is achieved by the evaluation circuit, so that upon actuation of the key switch no increase in the current flow and thus no stroke of the evaluation signal V _IN is possible.

A possible approach to problem solving is to reduce the measuring resistor R _mess in the event of extraneous light and thus to increase the possible current flow in the evaluation circuit. At the same time, however, the useful signal decreases with such a reduction of the extraneous light sensitivity, so that the signal-to-noise ratio can not be adjusted as favorably as desired.

In this context, for example, from the DE 38 03 034 C3 a photoreceiver with a special control loop that includes a negative feedback network to compensate for slow changes in the photocurrent through the phototransistor while fulfilling a gain function.

The GB-A-1,252,727 discloses an evaluation circuit of a photodetector having a phototransistor and an auxiliary current source connected to the gate terminal of the phototransistor for keeping the photocurrent low in the event of an over-irradiation of the phototransistor.

It is an object of the present invention to reduce the extraneous light sensitivity of an evaluation circuit for an optical receiver with a simple structure and at the same time to maintain a sufficiently high useful signal or signal-to-noise ratio.

This object is achieved by an evaluation circuit for an optical receiver having the features of claim 1. Advantageous embodiments and modifications of the invention are the subject of the dependent claims.

The evaluation circuit for an optical receiver includes a voltage divider circuit, which is constructed from a measuring resistor and a first switching element and to which a supply voltage is applied, wherein the first switching element is electrically connected in accordance with a reception of electromagnetic radiation by a receiver and an evaluation signal between the measuring resistor and the first switching element is tapped. The evaluation circuit is characterized in that the first switching element is connected in series with a parallel circuit of the measuring resistor and an additional current source circuit, wherein the additional current source circuit causes an additional current flow through the first switching element in the event of over-radiation of the receiver, so that the first switching element in the case of Over-radiation of the receiver works with a favorable signal-to-noise ratio.

In normal operation, ie without over-radiation of the receiver, the evaluation circuit basically works like the in 3 illustrated conventional evaluation circuit, since the additional power source circuit essentially no additional Provides electricity. In an over-radiation of the receiver, however, when in a conventional evaluation circuit, even in, for example, a key press no power increase is possible, the additional power source circuit supplies an additional current through the first switching element. Since the measuring resistor can be kept at a high resistance value at the same time, the first switching element is switched by the additional current source circuit into an operating range with a sufficiently good signal-to-noise ratio so that an evaluable useful signal can be output by the evaluation circuit even in the event of over-radiation of the receiver ,

The first switching element is for example a photodiode or a phototransistor, and the measuring resistor is advantageously a measuring resistor with a constant (high) resistance value.

In one embodiment of the invention, the additional current source circuit comprises a series circuit of a first resistor and a second switching element, which is connected in parallel to the measuring resistor; the resistance of the first resistor of the auxiliary power source circuit is (significantly) lower than the resistance of the measuring resistor; and the second switching element is switched to an electrically conductive state in the event of an over-radiation of the receiver. The second switching element serves, so to speak, as an additional power source for the first switching element.

In one embodiment of the invention, the second switching element is a transistor whose gate terminal is permanently at the potential of the evaluation signal via at least one further resistor. Furthermore, the additional current source circuit has a current-smoothing capacitor in order to keep the additional current flow through the first switching element at a substantially constant value, independently of fluctuations in the potential of the evaluation signal. The thus constructed additional power source circuit works relatively slow and is suitable for example for remote control or communication receiving parts.

In an alternative embodiment of the invention, the second switching element is a transistor whose gate terminal is driven via at least one further resistor and a storage capacitor at least to the evaluation times of the evaluation circuit for switching through the second switching element. In this case, for example, a connection of the storage capacitor is connected to the supply voltage, while the other terminal of the storage capacitor connected before the evaluation of the evaluation via a third switching element, preferably a transistor which is driven via a controller of the receiver, with the potential of the evaluation signal. The thus constructed additional power source circuit operates relatively quickly and is suitable, for example, for touch-sensitive push button switch, in which the reception times, d. H. the evaluation times are known in advance.

One possible application of the invention is a touch-sensitive push-button switch with an optical sensor which has a receiver emitting electromagnetic radiation and a receiver receiving electromagnetic radiation, and an evaluation circuit according to the invention for evaluating a measurement signal generated by the receiver of the optical sensor and generating a corresponding evaluation signal.

Another possible application of the invention is a remote control or communication receiving part, comprising an optical sensor having a receiver receiving electromagnetic radiation, and an evaluation circuit according to the invention for evaluating a measurement signal generated by the receiver of the optical sensor and generating a corresponding evaluation signal.

The above and other objects, features and advantages of the invention will become more apparent from the following description of preferred, non-limiting examples, with reference to the accompanying drawings. Show:

1 a schematic block diagram of an evaluation circuit for an optical receiver according to a first embodiment of the present invention;

2 a schematic block diagram of an evaluation circuit for an optical receiver according to a second embodiment of the present invention, and

3 a schematic block diagram of a conventional evaluation circuit for an optical receiver.

The invention will be described below with reference to various embodiments with reference to 1 and 2 explained in more detail.

In the 1 illustrated evaluation circuit is particularly suitable for use for remote control or communication receiving parts. The evaluation circuit of 2 On the other hand, it is preferably suitable for applications in which the reception or evaluation times are known in advance are, for example, with touch-sensitive push-buttons, as they are increasingly used, for example, in operating parts of electronic household appliances.

The evaluation circuit of the first embodiment of 1 like the one above based on 3 described conventional evaluation circuit on a voltage divider circuit, which is composed of a measuring resistor R _mess and a first switching element T1, for example a photodiode or a phototransistor, and to which a supply voltage Vdd, for example 5 V is applied. The evaluation signal V _IN is tapped between the measuring resistor R _mess and the first switching element T1 and fed to a microprocessor (not shown) for further processing. The first switching element T1 is turned on in dependence on a measurement signal V _{E of} the receiver (not shown), so that, if appropriate, a current flow is effected by the evaluation circuit, which results in a corresponding evaluation signal V _IN . The measuring resistor R _mess is advantageously a resistor with a constant high resistance to achieve a favorable signal-to-noise ratio.

In addition, this evaluation circuit has an additional current source circuit A _add , which is provided parallel to the measuring resistor R _mess and should cause an additional current flow through the first switching element T1 in the case of over-radiation of the receiver to this in a work area with a favorable signal / noise ratio to operate.

The additional _{current source} circuit A _add contains a series circuit of a first resistor R1 whose resistance value is chosen to be significantly smaller than that of the measuring resistor R _mess , and a second switching element T2 in the form of a transistor. The gate terminal of the transistor T2 is connected via a second (relatively high) resistor R2 and a capacitor C1 to the potential of the supply voltage Vdd and via the second resistor R2 and a third (relatively high) resistor R3 to the potential of the evaluation signal V _IN connected. The resistance values of the resistors R2 and R3 are chosen so that in the case of an over-radiation of the receiver to the gate terminal of the second switching element T2 such a negative voltage is applied that the second switching element T2 is turned on. Then, via the low-resistance second switching element T2 and the relatively low second resistor R2, an additional current can flow through the first switching element T1.

The functioning of the evaluation circuit of 1 is as follows.

In normal operation of the receiver, ie without overexposure by extraneous light, and without a signal reception, the first switching element T1 is very high impedance. The level of the evaluation signal V _IN is very high (it corresponds almost to the value of the supply voltage Vdd), as judged by the microprocessor as a non-existence of an input signal. Since the evaluation signal V _IN is substantially at the level of the supply voltage Vdd, there is substantially no negative voltage at the gate terminal of the second transistor T2 via the two resistors R2 and R3, so that the second transistor T2 remains blocked. In this case, the additional current source circuit A _add does not supply any additional current, ie the evaluation circuit functions like a conventional evaluation circuit.

Is now (still without extraneous light overexposure) receive a signal from the receiver, the first switching element T1 is turned conductive by the corresponding measurement signal V _E of the recipient at the time of a signal pulse. Through the main circuit of the evaluation circuit, a current flows, which causes a voltage drop across the measuring resistor R _mess , which in turn has a drop in the level of the evaluation signal V _IN result. This stroke in the evaluation signal V _IN is evaluated by the microprocessor as a signal input. The height of the stroke in the evaluation signal V _IN , in other words the signal / noise ratio of the useful signal is determined by the size of the measuring resistor R _mess .

The additional current source circuit A _add is designed so slow that it does not provide additional current through the first switching element T1 during the short signal pulse received by the receiver. The evaluation circuit of the invention thus also works here like a conventional evaluation circuit.

If, on the other hand, external light, such as sunlight or artificial light sources, causes over-radiation of the receiver, the first switching element T1 is switched to be continuously conductive by the corresponding measuring signal V _{E of} the receiver. And this to an extent that the first switching element T1 is so strong low resistance that the current through the first switching element T1 theoretically could be even greater if this would allow the rest of the evaluation circuit, in particular the limitation by the measuring resistor R _mess . The evaluation signal V _IN is thus greatly reduced, much stronger than in the case of signal reception.

Due to the greatly reduced evaluation signal V _IN is now also at the gate terminal of the second switching element T2 is a (relative to the potential of the supply voltage Vdd) strongly negative Voltage, through which the second transistor T2 is turned on. Due to the very low second resistance R2 and the very low-resistance second switching element T2 can flow through these two elements, an additional current through the first switching element T1, which is significantly higher than that which is limited by the measuring resistor R _mess . The first switching element T1 is thus regulated by the additional _{current source circuit} A _add again into an operating range with a favorable signal / noise ratio.

If a signal pulse is received by the receiver in this adjusted operating range of the first switching element T1, the first switching element T1 is switched even lower impedance by the corresponding measurement signal V _E , so that the current flow through the first switching element T1 can be further increased, due to the additional current source circuit A. _add of the invention is possible. Also in this case (signal pulse with simultaneous over-radiation of the receiver) is thus a level drop in the evaluation signal V _IN recognizable, which can be evaluated by the microprocessor.

According to the additional _{current source circuit} A _{add of} the invention is thus ensured that the first switching element T1 is always in an operating range with a good signal / noise ratio. In addition, for this purpose, the measuring resistor R _mess does not have to be reduced, ie it can be kept constantly high, so that a clearly recognizable useful signal is generated as an evaluation signal V _IN in the event of overshoot.

The capacitor C1 in the additional current source circuit A _add is used for smoothing (high-frequency) fluctuations in the level of the evaluation signal V _IN , which are not generated by a signal pulse to provide a constant current flow through the first switching element T1. The capacitor C1 can therefore also be referred to as a "current smoothing capacitor".

As can be seen from the above explanations, the additional current _{source circuit} A _add only supplies an additional current through the first switching element T1 when the receiver is in the region of an over-radiation. In the normal operating state of the receiver no additional current should flow, so that the first switching element T1 is controlled in any case in a working area with a sufficiently good signal-to-noise ratio.

With reference to 2 Hereinafter, a second embodiment of the present invention will be explained. The same components and components are denoted by the same reference numerals as in the first embodiment.

While the evaluation circuit of the above first embodiment has a rather sluggish additional _{current source} circuit A _add , the evaluation circuit of this embodiment of 2 characterized by a fast reaction time. As already mentioned, it is therefore particularly suitable for applications in which the reception or evaluation times are known in advance, such as, for example, touch-sensitive push-buttons, such as are increasingly used in operating parts of electronic household appliances.

The evaluation circuit of the second embodiment differs from that of the first embodiment described above in the structure and operation of the additional _{current source} circuit A _add . As in 2 in this embodiment, in particular, the third resistor R3 in the boost _{current source circuit} A _{add is} replaced by a third switching element T3; the resistance of the second resistor R2 is adjusted accordingly. The remaining components and components are unchanged from the first embodiment.

While the third switching element T3 in 2 As a switch, which may be formed by a reed contact, for example, it is often advantageous in practice to form the third switching element T3 as a transistor which is driven by a controller control, which also includes, for example, the optical sensor the touch-sensitive pushbutton controls.

The third switching element T3 is closed in each case shortly before an expected signal pulse at the receiver, ie for example whenever the transmitter of the optical sensor emits electromagnetic radiation, ie in each case shortly before an evaluation time of the evaluation circuit. When the switch T3 is closed, the potential of the evaluation signal V _IN is present at the gate terminal of the second switching element T2. In the normal operating state of the receiver (ie, no blooming), this potential almost corresponds to the supply voltage Vdd, so that the second switching element T2 remains blocked and the evaluation circuit operates analogously to a conventional evaluation circuit, as in the case of the above embodiment.

However, if the receiver is outshone, then a signal which is clearly negative (compared to the potential of the supply voltage) is present at the gate terminal of the second switching element T2 via the closed switch T3, and the storage capacitor C2 is charged. At the evaluation time, ie when a reception of a signal pulse by the receiver is expected, the third switching element T3 is opened again. The second switching element T2 is now turned on by the storage capacitor C2, so that analogously to the above embodiment, an additional current can flow through the first switching element T1 when a pulse signal is received. The additional _{current source} circuit A _{add of} the second embodiment forms a kind of "sample and hold" circuit.

It is thus also in the evaluation of 2 ensures that the first switching element T1 (at least during the expected evaluation times) is always in a work area with a favorable signal-to-noise ratio. Here, too, the measuring resistor R _{mess is} advantageously a resistor with a constant (high) resistance value in order to be able to evaluate a sufficiently large useful signal even when the receiver is overshadowed.

Claims (11)

Evaluation circuit for an optical receiver, comprising a voltage divider circuit which is constructed from a measuring resistor (R _mess ) and a first switching element (T1) and to which a supply voltage (Vdd) is applied, the first switching element (T1) corresponding to a reception of electromagnetic radiation is electrically conductively switched by a receiver and an evaluation signal (V _IN ) between the measuring resistor (R _mess ) and the first switching element (T1) is tapped, characterized in that the first switching element (T1) in series with a parallel circuit of the measuring resistor ( R _mess ) and an additional current _source circuit (A _add ) is connected, wherein the additional current _source circuit (A _add ) in the case of an over-radiation of the receiver causes an additional current flow through the first switching element (T1), so that the first switching element (T1) even in the case of an over-radiation the receiver works with a favorable signal-to-noise ratio , Evaluation circuit according to claim 1, characterized in that the first switching element (T1) is a photodiode or a phototransistor. Evaluation circuit according to claim 1 or 2, characterized in that the measuring resistor (R _mess ) is a measuring resistor with a constant resistance value. Evaluation circuit according to one of the preceding claims, characterized in that the additional _{current source} circuit (A _add ) _comprises a series circuit of a first resistor (R1) and a second switching element (T2), which is connected in parallel to the measuring resistor (R _mess ); in that the resistance value of the first resistor (R1) of the auxiliary _{current source circuit} (A _add ) is lower than the resistance value of the measuring resistor (R _mess ); and that the second switching element (T2) is switched to an electrically conductive state in the event of an over-radiation of the receiver. Evaluation circuit according to claim 4, characterized in that the second switching element (T2) is a transistor whose gate terminal via at least one further resistor (R2, R3) is permanently at the potential of the evaluation signal (V _IN ). Evaluation circuit according to Claim 5, characterized in that the additional current source circuit (A _add ) has a current-smoothing capacitor (C1) in order to keep the additional current flow through the first switching element (T1) at a constant value, regardless of fluctuations in the potential of the evaluation signal (V _IN ) , Evaluation circuit according to claim 4, characterized in that the second switching element (T2) is a transistor whose gate terminal via at least one further resistor (R2) and a storage capacitor (C2) at least to the evaluation of the evaluation circuit for switching the second switching element (T2 ) is driven. Evaluation circuit according to claim 7, characterized in that one terminal of the storage capacitor (C2) is connected to the supply voltage (Vdd), while the other terminal of the storage capacitor (C2) in each case before the evaluation of the evaluation via a third switching element (T3) with the potential the evaluation signal (V _IN ) is connected. Evaluation circuit according to claim 8, characterized in that the third switching element (T3) is a transistor which is controlled via a controller of the receiver. Touch-sensitive push button, with an optical sensor having an electromagnetic radiation emitting receiver and a receiver receiving electromagnetic radiation; and an evaluation circuit according to one of claims 1 to 9 for evaluating a measurement signal (V _E ) generated by the receiver of the optical sensor and generating a corresponding evaluation signal (V _IN ). A remote control or communication receiving part, comprising an optical sensor having a receiver receiving electromagnetic radiation; and an evaluation circuit according to one of claims 1 to 9 for evaluating a measurement signal (V _E ) generated by the receiver of the optical sensor and generating a corresponding evaluation signal (V _IN ).

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