Abstract:
A pulsed photoemitter circuit may be arranged to expose a pulsed photodetector circuit. Both the photoemitter and the photodetector circuits may be exposed to a pulsed signal which may be effective to improve the response time of the photoemitter/detector pair. In one embodiment of the present invention, the phototransistor load is coupled to a photoemitter control signal.

Description:
BACKGROUND 
     This invention relates generally to optical couplers including a photoemitter/detector pair. 
     Commonly, a photoemitter is aligned with a photodetector in an optical coupler to detect the imposition of an object between the photoemitter/detector pair. For example, a shutter may open or close between the pair and this may be detected by emission from the photoemitter that is detected (or not) by the photodetector. 
     The photoemitter may be a light emitting diode (LED) which emits light in the visible or infrared spectrum. The photodetector may conventionally be a phototransistor. Commonly, such photoemitter/detector pairs are used in connection with a mouse cursor control device in a computer system. One type of mouse is utilized in connection with wireless systems. In such cases, the wireless mouse may include its own internal battery. Thus, the need for a low power photoemitter/detector pair may be particularly acute in such applications. 
     Techniques have been developed to decrease the power dissipation of the photoemitter/detector pairs. One such technique is to pulse the LED. Pulsing the LED may reduce power consumption but it also may be adversely affected by the parasitic capacitance of the photodetector. 
     Another technique for reducing the power consumption of the phototransistor is to increase the value of its load resistor. However, this approach may slow down the response time of the phototransistor. 
     Thus, there is a continuing need for better ways to reduce the power consumption of photoemitter/detector pairs. 
     SUMMARY 
     In accordance with one aspect of the present invention, a photodetector circuit may include a pulsed photodetector device. A load is coupled to the photodetector device. 
     Other aspects are set forth in the accompanying detailed description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a circuit diagram in accordance with the prior art; 
     FIG. 1B is a circuit diagram of one embodiment of the present invention; 
     FIG. 2 is an oscilloscope trace of the output voltage of a photodetector in accordance with a prior art embodiment; 
     FIG. 3 is an oscilloscope trace of the output voltage of a photodetector in accordance with one embodiment of the present invention; 
     FIG. 4 is an oscilloscope trace in accordance with an embodiment of the prior art; 
     FIG. 5 is an oscilloscope trace in accordance with one embodiment of the present invention; 
     FIG. 6 is an oscilloscope trace in accordance with the prior art; and 
     FIG. 7 is an oscilloscope trace in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1A and 1B, a photoemitter circuit  12  may include a photoemitter  14  such as a light emitting diode (LED) and a current limiting resistor  16 . The circuit  12  is arranged in opposition to a photodetector circuit  18   a  or  18   b . The circuits  18   a  and  18   b  include a photodetector  20 , such as a phototransistor, and a load resistor  24 . The photoemitter  14  may emit light in the visible or infrared spectrum. The anode of the photoemitter  14  and the collector of the photodetector  20  are connected to a supply voltage, indicated as “V”, in one embodiment of the invention. 
     A repetitive, negatively pulsed signal  32 , indicated as LED_ON_ 0  in FIG. 2, is coupled to a node  30 . In FIG. 1A, the photodetector circuit  18   a  is not pulsed and is connected to ground. In FIG. 1B the photodetector circuit  18   b  is continuously pulsed through a connection to the node  30 . However, the circuits  12  and  18   b  may be separated and separately coupled each to its own supply voltage and enabling signals enabled concurrently, in some embodiments of the present invention. 
     The output voltage (Qout)  22  of each photodetector circuit  18  may be measured across the load resister  24 . One terminal of the load resistor  24  is coupled to the emitter of the photodetector  20 , which may be a bipolar phototransistor, and the other terminal is coupled to the pulsed signal  30 . Tying the phototransistor load resistor  24  to the pulsed signal may result in faster response time which can save power by allowing a lower duty cycle circuit  18   b.    
     In one illustrative embodiment of the present invention, the current limiting resistor  16  may be 300 ohms, and the load resistor  24  may be 12 kilohms. The pulsed signal LED_ON_ 0 , waveform  32 , may be a negatively pulsing repetitive signal with an on time of 10 microseconds or less and an off period as long as 2 milliseconds without significant voltage drift in one embodiment of the present invention. For example, the signal  32  may be negatively pulsing repetitive signal that has a minimum of 0 volts and a positive maximum illustrated as 3 volts in one embodiment of the invention. 
     As a result of the application of the pulsed signal LED_ON_ 0  (waveform  32 ) to the current limiting resistor  16 , the output voltage (Qout) of the circuit  10   a , shown in FIG. 1A, has the waveform  34  shown in FIG. 2 with the grounded photodetector circuit  18   a . Thus, in the illustrated embodiment, the output voltage  34  of the photodetector circuit  18   a  rises when the photoemitter  14  is pulsed to produce light. Eventually the output  22  reaches the voltage level  36 , after about 400 microseconds in one example. 
     In contrast, with the circuit  10   a  of FIG. 1A, the photodetector circuit  18   b , coupled to the node  30 , is pulsed and the voltage (Qout) reaches the level  36 , shown in FIG. 3, in one embodiment of the invention. The output voltage (Qout)  38  starts at the approximate level  36  that would have eventually reached if it were allowed to continue to rise with the grounded connection (FIG.  2 ). The output voltage  38  starts out at the level  36  because it picks up where it left off with the previous pulse of the continuous pulse stream. 
     Referring to FIG. 4, the light output from the photoemitter  14  may be partially blocked from the photodetector  20 . In such case, the output voltage  34   a , for the unpulsed photodetector  18   a , does not rise as much as was illustrated in FIG.  2 . Instead, the output voltage  34   a  reaches a maximum level  36   a.    
     In the circuit  10 b with the load  24  tied to the pulsed signal LED_ON_ 0 , the output voltage  38   a  starts out approximately at the level  36   a  of the unpulsed example, as shown in FIG.  5 . 
     Turning next to FIG. 6, a shorter pulse  32   a  replaces the pulse  32 . In contrast to the prior examples, the pulses  32   a  are shortened to 50 microseconds from 400 microseconds. In this case, the output voltage (Qout), with the load resistor  24  connected to ground, is as indicated at  34   b . The characteristics of the signal  34   b  suggest that the rise time is not fast enough, given the shortened pulse width. 
     Again in contrast, with the shortened width pulse  32   a  and the circuit  18   b , connected to the node  30 , an effective output signal  38   b  is produced, as shown in FIG.  7 . 
     Thus, pulsing both the photoemitter and the phototransistor circuits  12  and  18   b  results in a faster response which may save power by allowing a lower duty cycle circuit. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.