Abstract:
Provided is a proximity sensor using a photosensor, which is easy to use and reduced in power consumption. In the proximity sensor, a first photosensor is used to detect a change in amount of ambient light entering the first photosensor, which is caused when a finger is coming close thereto, and a detection signal is output based on a result of the detection. The photosensor includes, for example, one or a plurality of PN junction elements connected in parallel.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2010-071084 filed on Mar. 25, 2010, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a proximity sensor, and more particularly, to a proximity sensor using a photosensor reduced in power consumption compared with a conventional one. 
     2. Description of the Related Art 
     A conventional proximity sensor using a photosensor is capable of detecting a finger or the like coming close thereto by emitting light by itself and detecting reflection light on the finger or the like coming close thereto by means of the photosensor (see, for example, Japanese Patent Application Laid-open No. 2004-56905 (FIG. 1)). 
       FIG. 10  illustrates a circuit block diagram of a non-contact switch using the conventional proximity sensor. The non-contact switch  10  includes a proximity sensor  29  of type using reflection light having a light receiving part  27  and a light projecting part  28 , a switch circuit  31  connected in series to a load such as a lighting tool  30  as an example of an electronic device, a triac (example of switch means)  32  constituting a semiconductor switch element of the switch circuit  31 , a switch control section  33  for turning ON/OFF the triac  32  based on an output of the proximity sensor  29 , and a power supply section  34  for supplying power to those components. 
     The proximity sensor  29  has a well-known configuration, in which the light projecting part  28  using a light-emitting diode emits an infrared pulse to be reflected on a human hand  35  as an example of a target, and when the light receiving part  27  using a phototransistor or a photodiode receives light reflected off the human hand  35 , the proximity sensor  29  sends an output of ON to the switch control section  33 . 
     When a signal is input to a gate of the triac  32  provided in the switch circuit  31 , the triac  32  becomes a conductive element with respect to AC power supply after the input of the signal until the polarity of AC power supply is changed. If the proximity sensor  29  is turned ON, the triac  32  is turned ON based on a signal transmitted from the switch control section  33 . 
     When the proximity sensor  29  detects the human hand  35  or the like, if the human hand  35  moves back and forth in front of the proximity sensor  29 , the proximity sensor  29  is turned ON/OFF for each detection of the human hand  35 . This may result in unnaturally high responsivity of the proximity sensor  29 . As a countermeasure, for example, the switch control section  33  is provided with a timer circuit for about 1 to 2 seconds so as to realize a delay circuit for preventing the triac  32  from being turned OFF unless the timer circuit counts up immediately after the proximity sensor  29  is turned ON. 
     The switch control section  33  is further provided with an operation indicator  23  which lights up when the proximity sensor  29  is turned ON. The operation indicator  23  lights up only when the proximity sensor  29  is turned ON and the triac  32  is turned ON. Alternatively, the operation indicator  23  may employ a two-color light-emitting diode so that green (or yellow) light may be emitted when the proximity sensor  29  is OFF while red light may be emitted when the proximity sensor  29  is ON. Note that, the triac  32  may be connected to a neon tube at both ends thereof for display during OFF of the triac  32 . 
     With such a configuration of the non-contact switch  10  of the conventional technology, under a state in which no power is supplied to the lighting tool  30 , that is, when the triac  32  of the non-contact switch  10  is in a non-conductive state, the power supply voltage is applied across the triac  32 , and accordingly the power supply section  34  rectifies the power supply voltage to a constant voltage to be supplied to the switch circuit  31 , the proximity sensor  29 , and the switch control section  33 . Then, when the proximity sensor  29  detects the human hand  35  and thereby starts to operate, the proximity sensor  29  supplies an output of the detection to the switch control section  33  so that a signal may be sent to the switch circuit  31 , specifically, a signal may be sent to the triac  32  with a phase angle of 15° to 20° with respect to the half wavelength) (180°, to thereby turn ON the switch circuit  31 . 
     Across the triac  32 , a small voltage as a part of the power supply voltage is generated. The power supply section  34  rectifies the generated voltage to obtain minute power, and converts the power to a constant DC voltage to be supplied to the switch circuit  31 , the proximity sensor  29 , and the switch control section  33 . This configuration enables power supply even if the switch circuit  31  continues to be turned ON. 
     The conventional proximity sensor using a photosensor employs a proximity sensor of type using reflection light and needs to emit light by itself, which leads to a problem of very large current consumption, and further the light receiving part for detecting the reflection light has a complicated circuit configuration, which leads to a problem of large current consumption. In addition, there is another problem that the battery life for battery drive significantly reduces, which makes it difficult to utilize battery drive. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problems, and therefore provides a proximity sensor which is reduced in current consumption compared with a conventional proximity sensor. 
     In order to solve the above-mentioned problems inherent in the conventional proximity sensor, a proximity sensor according to the present invention includes: at least one first photosensor for detecting a target; and at least one second photosensor positioned in an area in which the target is not detected, for detecting ambient light. 
     The proximity sensor according to the present invention does not need to emit light by itself and accordingly has a simple circuit configuration, and hence current consumption may be reduced compared with the conventional proximity sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a schematic configuration diagram of a proximity sensor according to the present invention; 
         FIG. 2  is a schematic circuit diagram illustrating a first embodiment of the proximity sensor according to the present invention; 
         FIG. 3  is a schematic circuit diagram illustrating a second embodiment of the proximity sensor according to the present invention; 
         FIG. 4  is a schematic circuit diagram illustrating a third embodiment of the proximity sensor according to the present invention; 
         FIG. 5  is a schematic circuit diagram illustrating a fourth embodiment of the proximity sensor according to the present invention; 
         FIG. 6  is a schematic circuit diagram illustrating a fifth embodiment of the proximity sensor according to the present invention; 
         FIG. 7  is a schematic circuit diagram illustrating a sixth embodiment of the proximity sensor according to the present invention; 
         FIG. 8  is a circuit diagram of a level shift circuit included in the proximity sensor according to the present invention; 
         FIG. 9  is another circuit diagram of the level shift circuit included in the proximity sensor according to the present invention; and 
         FIG. 10  is a schematic circuit block diagram illustrating a circuit configuration of a conventional proximity sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, referring to the accompanying drawings, embodiments of the present invention are described below. 
       FIG. 1  is a schematic configuration diagram of a proximity sensor according to the present invention.  FIG. 1  illustrates a proximity sensor  100  viewed from side, and the proximity sensor  100  includes a first photosensor  101  and a second photosensor  102 . 
     The first photosensor  101  is placed in an area which is shielded from light by a finger  103  or the like when the proximity sensor  100  detects the finger  103  or the like as coming close thereto, to thereby detect that light is blocked by the finger  103  or the like. The second photosensor  102  is placed in an area which is not shielded from light by the finger  103  or the like when the proximity sensor  100  detects the finger  103  or the like as coming close thereto, to thereby detect ambient brightness. 
     When ambient brightness is high and the proximity sensor  100  is shielded from light by the finger  103  or the like, an amount of light entering the first photosensor  101  reduces, whereas an amount of light entering the second photosensor  102  does not change. In this case, the proximity sensor  100  outputs a detection signal. 
     When ambient brightness is high and the proximity sensor  100  is not shielded from light by the finger  103  or the like, neither the amount of light entering the first photosensor  101  nor the amount of light entering the second photosensor  102  changes. In this case, the proximity sensor  100  outputs a non-detection signal. 
     When ambient brightness is low, the amount of light entering the second photosensor  102  reduces. In this case, the proximity sensor  100  outputs a non-detection signal even when the first photosensor  101  thereof is shielded from light by the finger  103  or the like. Because the first photosensor  101  cannot discriminate between the case where the ambient is dark and the case where light is blocked by the finger  103  or the like, the second photosensor  102  detects the ambient brightness so as to discriminate between the case where the ambient is dark and the case where light is blocked by the finger  103  or the like. 
     This way, the proximity sensor  100  according to the present invention is capable of, under high ambient brightness, outputting a detection signal when the proximity sensor  100  is shielded from light by the finger  103  or the like, while outputting a non-detection signal when the finger  103  or the like moves away from the proximity sensor  100 . Under low ambient brightness, on the other hand, the proximity sensor  100  is capable of outputting a non-detection signal. 
     Note that, the proximity sensor  100  according to the present invention described above is configured to output a non-detection signal under low ambient brightness, but, of course, may be configured to output a detection signal under low ambient brightness. Further, it should be understood that the same operation can also be performed when the first photosensor  101  and the second photosensor  102  each include a plurality of photosensors. Still further, it should be understood that the proximity sensor  100  may have a concave portion formed in the surface thereof so that the first photosensor  101  and the second photosensor  102  are disposed at the center of the concave portion instead of the surface of the proximity sensor  100  to prevent degradation of the detection sensitivity due to obliquely entering light. 
     First Embodiment 
       FIG. 2  is a schematic circuit diagram illustrating a first embodiment of the proximity sensor according to the present invention. As illustrated in  FIG. 2 , the first photosensor  101  includes two photodiodes connected in parallel, each of which has an anode connected to a reference power supply terminal GND and a cathode connected to an output terminal  203 . The second photosensor  102  includes a photodiode in which an anode is connected to the output terminal  203  and a cathode is connected to a positive power supply terminal VDD. 
     Next, an operation of the proximity sensor is described. For example, it is assumed that the photodiodes used in the first photosensor  101  and the second photosensor  102  have the same sensitivity. Because the first photosensor  101  uses the two photodiodes connected in parallel, if the amount of light entering the first photosensor  101  is reduced to half or less of that entering the second photosensor  102 , a signal of the output terminal  203  may be inverted from Low (hereinafter abbreviated as L) to High (hereinafter abbreviated as H). In other words, when the finger  103  or the like is coming close to the first photosensor  101  under high ambient brightness, the finger  103  or the like may be detected at a timing when the amount of light entering the first photosensor  101  is reduced to half of that entering the second photosensor  102 . Accordingly, the detection may be made even before the finger  103  or the like completely blocks light, and further a detection distance less varies with the change in ambient brightness. Besides, the amount of light to be detected may be adjusted by changing the numbers of photodiodes used in the first photosensor  101  and the second photosensor  102 , which is easy adjustment. On the other hand, when the ambient brightness becomes low, a dark current flowing through the first photosensor  101  becomes larger than a dark current flowing through the second photosensor  102 , and hence the signal of the output terminal  203  changes to L. 
     It should be understood that the same detection can also be made when two or more second photosensors  102  are connected in parallel. 
     Second Embodiment 
       FIG. 3  is a schematic circuit diagram illustrating a second embodiment of the proximity sensor according to the present invention. As illustrated in  FIG. 3 , the first photosensor  101  includes two photodiodes connected in parallel, each of which has an anode connected to a reference power supply terminal GND and a cathode connected to an input terminal  302  of a level shift circuit  301 . The second photosensor  102  includes a photodiode in which an anode is connected to the input terminal  302  of the level shift circuit  301  and a cathode is connected to the reference power supply terminal GND. The level shift circuit  301  has an inverted signal output terminal  303  connected to an output terminal  203 . 
     Next, an operation of the proximity sensor is described. When ambient brightness is high and the finger  103  or the like is not coming close to the first photosensor  101 , a photocurrent flows from the cathodes to the anodes of the first photosensor  101  while a voltage and a current are generated at the cathode of the second photosensor  102 . In this case, the photocurrent of the first photosensor  101  flows twice as much as the generated current of the second photosensor  102 , and hence the input terminal  302  of the level shift circuit  301  has a voltage of L. Therefore, H is output to the inverted signal output terminal  303  of the level shift circuit  301 , and then H is output to the output terminal  203 . 
     When ambient brightness is high and the finger  103  or the like is coming close to the first photosensor  101 , a photocurrent flows from the cathodes to the anodes of the first photosensor  101  while a voltage and a current are generated at the cathode of the second photosensor  102 . In this case, the amount of light entering the first photosensor  101  is reduced to less than half of that entering the second photosensor  102  because of the finger  103  or the like, and hence the photocurrent of the first photosensor  101  is smaller than the generated current of the second photosensor  102 . Accordingly, the input terminal  302  of the level shift circuit  301  has the same voltage as the generated voltage of the second photosensor  102 . Therefore, L is output to the inverted signal output terminal  303  of the level shift circuit  301 , and then L is output to the output terminal  203 . 
     Under low ambient brightness, no voltage is generated at the anode of the second photosensor  102 , and hence the input terminal  302  of the level shift circuit  301  has a voltage of L. Then, a signal of H is output from the inverted signal output terminal  303 , and a signal of H is also output from the output terminal  203 . 
     As described above, the second embodiment of the proximity sensor according to the present invention has the same function and feature as the first embodiment of the proximity sensor according to the present invention illustrated in  FIG. 2 , except that the polarity of the output signal is reverse. Further, the second photosensor  102  consumes no photocurrent, which flows in the first embodiment as current consumption, and hence lower current consumption may be achieved. 
     It should be understood that the same detection can also be made when two or more second photosensors  102  are connected in parallel. 
       FIG. 8  illustrates a circuit diagram of the level shift circuit  301  used in the above-mentioned second embodiment of the proximity sensor according to the present invention. As illustrated in  FIG. 8 , a P-channel MOS transistor  801  has a source connected to the positive power supply terminal VDD, a gate connected to a node N 1 , and a drain connected to a current inflow terminal of a constant current circuit  811 . An N-channel MOS transistor  806  has a source connected to the reference power supply terminal GND, a gate connected to the input terminal  302 , and a drain connected to the inverted signal output terminal  303 . A P-channel MOS transistor  802  has a source connected to the positive power supply terminal VDD, a gate connected to the inverted signal output terminal  303 , and a drain connected to a current inflow terminal of a constant current circuit  812 . A depletion type N-channel MOS transistor  805  has a source connected to the input terminal  302 , a gate connected to the reference power supply terminal GND, and a drain connected to the node N 1 . A constant current circuit  810  has a current inflow terminal connected to the input terminal  302  and a current outflow terminal connected to the reference power supply terminal GND. The constant current circuit  811  has a current outflow terminal connected to the inverted signal output terminal  303 . The constant current circuit  812  has a current outflow terminal connected to the node N 1 . Although not illustrated, the positive power supply terminal VDD is supplied with a positive voltage from a power source while the reference power supply terminal GND is supplied with a zero-volt voltage from the power source. Note that, the constant current circuit  810  and the constant current circuit  812  are configured such that the constant current circuit  810  has a larger constant current value than the constant current circuit  812 . 
     Next, an operation of the level shift circuit  301  is described. First, if L is input to the input terminal  302 , the N-channel MOS transistor  806  is turned OFF and the depletion type N-channel MOS transistor  805  is turned ON. The voltage at the node N 1  is discharged to around the voltage at the reference power supply terminal GND with an ON-state current of the depletion type N-channel MOS transistor  805 . Then, the P-channel MOS transistor  801  is turned ON to raise the voltage at the inverted signal output terminal  303  of the level shift circuit  301  to around the voltage at the positive power supply terminal VDD. Because the voltage at the inverted signal output terminal  303  of the level shift circuit  301  is raised to around the voltage at the positive power supply terminal VDD, the P-channel MOS transistor  802  is turned OFF. This way, H is output to the inverted signal output terminal  303 . 
     Next, if the generated voltage of the second photosensor  102  is input to the input terminal  302 , the depletion type N-channel MOS transistor  805  is turned OFF and the N-channel MOS transistor  806  is turned ON. The voltage at the inverted signal output terminal  303  is discharged to the voltage around the reference power supply terminal GND with an ON-state current of the N-channel MOS transistor  806 . Then, the P-channel MOS transistor  802  is turned ON to raise the voltage at the node N 1  to around the voltage at the positive power supply terminal VDD. Because the voltage at the node N 1  is raised to around the voltage at the positive power supply terminal VDD, the P-channel MOS transistor  801  is turned OFF. This way, L is output to the inverted signal output terminal  303 . 
     As described above, the level shift circuit  301  illustrated in  FIG. 8  has a function of converting the signal of the second photosensor  102  of generated-voltage level into an inverted signal of CMOS level and then outputting the signal. Further, any of the MOS transistors on a current path from the positive power supply terminal VDD to the reference power supply terminal GND is turned OFF, and hence only a leakage current of the turned-OFF MOS transistor results in current consumption. Note that, the current value of the constant current circuit  810  is designed to be too small to affect a ratio of flowing currents between the first photosensor  101  and the second photosensor  102  illustrated in  FIG. 3 . 
       FIG. 9  illustrates another circuit diagram of the level shift circuit  301  used in the above-mentioned second embodiment of the proximity sensor according to the present invention, which is realized by a different configuration from  FIG. 8 . As illustrated in  FIG. 9 , a depletion type N-channel MOS transistor  902  has a drain connected to the positive power supply terminal VDD, a source connected to the node N 1 , and a gate connected to the inverted signal output terminal  303 . A P-channel MOS transistor  903  has a drain connected to the inverted signal output terminal  303 , a source connected to the node N 1 , and a gate connected to the input terminal  302 . An N-channel MOS transistor  904  has a drain connected to the inverted signal output terminal  303 , a source connected to the reference power supply terminal GND, and a gate connected to the input terminal  302 . A constant current circuit  901  has a current inflow terminal connected to the input terminal  302  and a current outflow terminal connected to the reference power supply terminal GND. Note that, although not illustrated, the positive power supply terminal VDD is supplied with a positive voltage from a power source while the reference power supply terminal GND is supplied with a zero-volt voltage from the power source. 
     Next, an operation of the level shift circuit  301  is described. If L is input to the input terminal  302 , the N-channel MOS transistor  904  is turned OFF. Then, the node N 1  has an absolute value of a threshold voltage of the P-channel MOS transistor  903 , and accordingly an absolute value of a threshold voltage of the depletion type N-channel MOS transistor  902  becomes larger than the voltage at the node N 1  to turn ON the depletion type N-channel MOS transistor  902 . After the depletion type N-channel MOS transistor  902  is turned ON, the voltage at the node N 1  becomes higher than the absolute value of the threshold voltage of the P-channel MOS transistor  903  to turn ON the P-channel MOS transistor  903 . After the P-channel MOS transistor  903  is turned ON, the voltage at the inverted signal output terminal  303  is raised to be equal to the voltage at the node N 1 . Then, the voltage at the node N 1  rises to the voltage at the positive power supply terminal VDD with a current of the depletion type N-channel MOS transistor  902  which still remains in the ON state along with the rise of the voltage at the inverted signal output terminal  303 . Therefore, the inverted signal output terminal  303  outputs H. 
     Next, if H is input to the input terminal  302 , the N-channel MOS transistor  904  is turned ON to discharge the voltage at the inverted signal output terminal  303  to the voltage at the reference power supply terminal GND with a current of the N-channel MOS transistor  904 . Accordingly, the inverted signal output terminal  303  outputs L. Then, the voltage at the node N 1  takes a value obtained by adding the absolute value of the threshold voltage of the P-channel MOS transistor  903  to the voltage at the input terminal  302 . The value exceeds the absolute value of the threshold voltage of the depletion type N-channel MOS transistor  902 , and hence the depletion type N-channel MOS transistor  902  is turned OFF. 
     As described above, the level shift circuit illustrated in  FIG. 9  may have the same function as that of the above-mentioned level shift circuit illustrated in  FIG. 8 . Further, any of the MOS transistors on a current path from the positive power supply terminal VDD to the reference power supply terminal GND is turned OFF, and hence current consumption is equal to the current consumption of the above-mentioned level shift circuit illustrated in  FIG. 8 . Note that, the current value of the constant current circuit  901  is designed to be too small to affect the ratio of flowing currents between the first photosensor  101  and the second photosensor  102  illustrated in  FIG. 3 . 
     Third Embodiment 
       FIG. 4  is a schematic circuit diagram illustrating a third embodiment of the proximity sensor according to the present invention. As illustrated in  FIG. 4 , the first photosensor  101  includes a photodiode in which an anode is connected to a gate and a drain of an N-channel MOS transistor  401  and a cathode is connected to the positive power supply terminal VDD. The second photosensor  102  includes a photodiode in which an anode is connected to a drain of an N-channel MOS transistor  402  and the output terminal  203  and a cathode is connected to the positive power supply terminal VDD. The N-channel MOS transistor  401  has a source connected to the reference power supply terminal GND. The N-channel MOS transistor  402  has a source connected to the reference power supply terminal GND and a gate connected to the gate of the N-channel MOS transistor  401 . 
     Next, an operation of the circuit is described. The N-channel MOS transistor  401  and the N-channel MOS transistor  402  together form a current mirror circuit, in which a current flowing through the N-channel MOS transistor  401  is doubled and mirrored into the N-channel MOS transistor  402 . For example, it is assumed that the photodiodes used in the first photosensor  101  and the second photosensor  102  have the same sensitivity. When ambient brightness is high and the finger  103  or the like is not coming close to the first photosensor  101 , the current of the N-channel MOS transistor  402  determined by mirroring the current flowing through the first photosensor  101  is larger than the current flowing through the second photosensor  102 , and accordingly L is output to the output terminal  203 . When ambient brightness is high and the finger  103  or the like is coming close to the first photosensor  101 , the amount of light entering the first photosensor  101  is reduced to less than half that entering the second photosensor  102  because of the finger  103  or the like. Therefore, the current of the N-channel MOS transistor  402  determined by mirroring the current flowing through the first photosensor  101  becomes smaller than the current flowing through the second photosensor  102 , and accordingly H is output to the output terminal  203 . Note that, under low ambient brightness, no current flows through the second photosensor  102  and accordingly the output terminal  203  becomes L. This way, there may be realized a proximity sensor  100  capable of detecting that the amount of light entering the first photosensor  101  has been reduced to less than half that entering the second photosensor  102  because of the finger  103  or the like. 
     As described above, the third embodiment of the proximity sensor according to the present invention has the same function and feature as the first embodiment of the proximity sensor according to the present invention illustrated in  FIG. 2 . Changing the current ratio between the first photosensor  101  and the second photosensor  102  is possible by changing the mirror ratio of the current mirror circuit, without increasing the number of photosensors. Therefore, downsizing is possible and further the sensitivity of the proximity sensor may be adjusted with ease through the change in mirror ratio of the current mirror circuit. 
     Fourth Embodiment 
       FIG. 5  is a schematic circuit diagram illustrating a fourth embodiment of the proximity sensor according to the present invention. As illustrated in  FIG. 5 , the first photosensor  101  includes a photodiode in which an anode is connected to a gate and a drain of an N-channel MOS transistor  401  and a cathode is connected to the reference power supply terminal GND. The second photosensor  102  includes a photodiode in which an anode is connected to a drain of an N-channel MOS transistor  402  and an input terminal  302  of a level shift circuit  301  and a cathode is connected to the reference power supply terminal GND. The N-channel MOS transistor  401  has a source connected to the reference power supply terminal GND. The N-channel MOS transistor  402  has a source connected to the reference power supply terminal GND and a gate connected to the gate of the N-channel MOS transistor  401 . The level shift circuit  301  has an inverted signal output terminal  303  connected to an output terminal  203 . Note that, the level shift circuit  301  has the same configuration as that of the level shift circuit  301  used in the above-mentioned second embodiment of the proximity sensor according to the present invention, and hence description of the configuration and operation thereof is omitted. 
     Next, an operation of the circuit is described. The N-channel MOS transistor  401  and the N-channel MOS transistor  402  together form a current mirror circuit, in which a current flowing through the N-channel MOS transistor  401  is doubled and mirrored into the N-channel MOS transistor  402 . For example, it is assumed that the photodiodes used in the first photosensor  101  and the second photosensor  102  have the same power generation characteristics. 
     When ambient brightness is high and the finger  103  or the like is not coming close to the first photosensor  101 , a current is generated at the first photosensor  101 . A current of the N-channel MOS transistor  402  determined by mirroring the generated current is larger than a generated current from the second photosensor  102 . Therefore, the input terminal  302  of the level shift circuit  301  becomes L and the output terminal  203  becomes H. 
     When ambient brightness is high and the finger  103  or the like is coming close to the first photosensor  101 , the amount of light entering the first photosensor  101  is reduced to less than half that entering the second photosensor  102  because of the finger  103  or the like. Therefore, the current of the N-channel MOS transistor  402  determined by mirroring the generated current from the first photosensor  101  becomes smaller than the generated current from the second photosensor  102 . Consequently, the input terminal  302  of the level shift circuit  301  has the generated voltage of the second photosensor  102 , and the output terminal  203  becomes L. 
     Under low ambient brightness, no voltage is generated in the second photosensor  102 , and hence the input terminal  302  of the level shift circuit  301  is L and the output terminal  203  becomes H. This way, there may be realized a proximity sensor capable of detecting that the amount of light entering the first photosensor  101  has been reduced to less than half that entering the second photosensor  102  because of the finger  103  or the like. 
     As described above, the fourth embodiment of the proximity sensor according to the present invention has the same function and feature as the second embodiment of the proximity sensor according to the present invention illustrated in  FIG. 3 . Changing the current ratio between the first photosensor  101  and the second photosensor  102  is possible by changing the mirror ratio of the current mirror circuit, without increasing the number of photosensors. Therefore, downsizing is possible and further the sensitivity of the proximity sensor may be adjusted with ease through the change in mirror ratio of the current mirror circuit. 
     Fifth Embodiment 
       FIG. 6  is a schematic circuit diagram illustrating a fifth embodiment of the proximity sensor according to the present invention. As illustrated in  FIG. 6 , the fifth embodiment has a configuration of detecting a change in amount of light entering the first photosensor  101 . Note that, using exactly the same configuration as that illustrated in  FIG. 6 , there may be realized another configuration of detecting the change in amount of light entering the second photosensor  102  illustrated in  FIG. 1 , and hence description thereof is omitted. 
     First, connection in the circuit is described. The first photosensor  101  includes a photodiode in which an anode is connected to the gate and the drain of the N-channel MOS transistor  401 , the input terminal  302  of the level shift circuit  301 , and an input of a delay circuit  603 , and a cathode is connected to the positive power supply terminal VDD. The N-channel MOS transistor  401  has a source connected to the reference power supply terminal GND. The N-channel MOS transistor  402  has a source connected to the reference power supply terminal GND, a gate connected to an output of the delay circuit  603 , and a drain connected to the input terminal  302  of the level shift circuit  301 . The level shift circuit  301  has an inverted signal output terminal  303  connected to the output terminal  203 . A delay circuit  601  has an input connected to one end of a resistor  601 , an output connected to another end of the resistor  601  and one end of a capacitor  602 . Another end of the capacitor  602  is connected to the reference power supply terminal GND. Note that, the level shift circuit  301  has the same configuration as that of the level shift circuit  301  used in the above-mentioned second embodiment of the proximity sensor according to the present invention, and hence description of the configuration and operation thereof is omitted. 
     Next, an operation of the fifth embodiment of the proximity sensor is described. The N-channel MOS transistor  401  and the N-channel MOS transistor  402  together form a current mirror circuit. It is assumed that a current that flows through the first photosensor  101  when the finger  103  or the like is not coming close to the first photosensor  101  is a current  21 , and that a mirror ratio of the current mirror circuit is 1:1. A current I then flows through each of the N-channel MOS transistors  401  and  402 . Then, the input terminal  302  of the level shift circuit  301  has a gate voltage of the N-channel MOS transistor  401  which allows the current I to flow. Therefore, L is output to the output terminal  203 . When the finger  103  or the like is coming close to the first photosensor  101  and the current flowing through the first photosensor  101  reduces to less than I, the gate of the N-channel MOS transistor  402  keeps a constant voltage for a given period of time by means of the capacitor  602 . Accordingly, the N-channel MOS transistor  402  operates to allow the current I to flow. The current flowing through the N-channel MOS transistor  402  is larger than the current flowing through the first photosensor  101 , and hence the input terminal  302  of the level shift circuit  301  becomes L and the output terminal  203  becomes H. After a while, the electric charges are removed from the capacitor  602 , and a current I/2 flows through each of the N-channel MOS transistors  401  and  402 . Then, the input terminal  302  of the level shift circuit  301  is raised again to the gate voltage of the N-channel MOS transistor  401  which allows the current I to flow. Consequently, the output terminal  203  becomes L again. This way, there may be realized a proximity sensor capable of detecting that the amount of light entering the first photosensor  101  has been reduced to half that entering the second photosensor  102  because of the finger  103  or the like, and further outputting H to the output terminal  203  for a given period of time. 
     The single proximity sensor using the above-mentioned photosensor  101  cannot discriminate between the case where the ambient brightness changes and the case where the finger  103  or the like is coming close thereto. Accordingly, another proximity sensor having exactly the same configuration using the second photosensor  102  is added to an area which is not to be covered by the finger  103  or the like. In such a case, when the proximity sensor using the second photosensor  102  responds, it is determined that the ambient brightness has changed. This way, the discrimination is made between the case where the finger  103  or the like blocks light and the case where the ambient is dark. 
     In a case where the finger  103  or the like is slowly coming close to the first photosensor  101 , the gate of the N-channel MOS transistor  402  does not keep a constant voltage for a given period of time, and gradually reduces the current flowing through the NMOS transistor  402 . In such a case, the input to the level shift circuit  301  never becomes L. L is output only when the finger  103  or the like has come close to the first photosensor  101  before the given period of time during which the gate of the N-channel MOS transistor  402  keeps a constant voltage elapses. Note that, the given period of time may be adjusted by the magnitude of the resistor  601  and the capacitor  602 . 
     As described above, according to the fifth embodiment of the proximity sensor of the present invention, there may be realized a proximity sensor capable of detecting that a finger or the like has come close to a given distant at a given speed or faster. Further, in a case where the surface of the first photosensor  101  becomes dirty to reduce the amount of light entering the first photosensor  101  to half of that entering the second photosensor  102 , all the proximity sensors according to the first to fourth embodiments of the present invention make an erroneous detection. However, the proximity sensor according to the fifth embodiment of the present invention does not make an erroneous detection in such a case because of the configuration capable of detecting that the amount of incident light has changed by a desired amount at a desired rate. 
     It should be understood that the resistor  601  illustrated in  FIG. 6  may be short-circuited to interpose a resistor between the node A and the node B so that the same function is realized. Further, the delay circuit may also be realized by another type of using no resistor nor capacitor. 
     Sixth Embodiment 
       FIG. 7  is a schematic circuit diagram illustrating a sixth embodiment of the proximity sensor according to the present invention. The difference from the configuration of the fifth embodiment of the proximity sensor according to the present invention illustrated in  FIG. 6  only resides in that the cathode of the first photosensor  101  is connected to the reference power supply terminal GND. 
     Assuming that the current supplied from the first photosensor  101  serves as a generated current, the proximity sensor according to the sixth embodiment operates to detect the generated current in a similar manner to the fifth embodiment of the proximity sensor according to the present invention. Using the configuration of detecting the generated current of the first photosensor  101  reduces current consumption by the photocurrent. Note that, similarly to the fifth embodiment, the second photosensor  102  may be used to discriminate from the case where the ambient is dark. 
     Hereinabove, the first to sixth embodiments of the proximity sensor according to the present invention have described the case of detecting that the light shield amount of the first photosensor  101  due to the finger  103  or the like is reduced to about half. However, it should be understood that the light shield amount to be detected may be adjusted by changing the numbers of the first photosensors  101  and the second photosensors  102  or the electric generation performance thereof. Further, the first to fourth embodiments of the proximity sensor according to the present invention each have a configuration of comparing the currents of the first photosensor  101  and the second photosensor  102  with a ratio of 2:1. However, it should be understood that the same function may also be realized, except for the function of discriminating from the case where the ambient is dark, when the ratio is reversed to invert the output signal. 
     It should be understood that the first photosensor  101  and the second photosensor  102  may be any sensor, including a photodiode and an LED, as long as the sensor has diode characteristics and photoelectric conversion characteristics.