JPH0687036B2 - Photometric device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention generally applies to illuminance (light intensity).
-Regarding a frequency conversion type electronic photometric device, in particular, it is possible to arbitrarily set its input / output characteristics within a considerable range by using a junction capacitance of a photodetector instead of using a capacitance element in an oscillation circuit. The present invention relates to a highly reliable electronic photometric device having a wide photometric range.

[0002]

2. Description of the Related Art As is well known, photodetectors such as photodiodes and phototransistors are used in electronic photometers. For example, the conventional illuminance (light quantity) -frequency conversion method using photodiodes is used. FIG. 4 shows an example of this photometric device. This photometric device includes a photodiode PD which is a light detecting element and a capacitor C which is a capacitance type load.
And the analog switch 23 and the first C-MOS type Schmitt inverter 24 constitute a light quantity-frequency conversion circuit 30, and the photodiode PD and the capacitor C are connected in series via the analog switch 23 and are incident. The current I _P flowing through the photodiode PD in proportion to the illuminance or the amount of light is accumulated in the capacitor C via the analog switch 23 and converted into the voltage V _C. The voltage V _C accumulated in the capacitor C is converted into a pulse signal by the first Schmitt inverter 24 of C-MOS type. The first Schmitt inverter 24 has two threshold voltages, a high level threshold voltage V _TH and a low level threshold voltage V _TL , and a high level output voltage V _TH when the input voltage is lower than the high level threshold voltage V _{TH. H} is generated, and when the input voltage reaches the high level threshold voltage V _TH , the output voltage switches from the high level V _H to the low level V _L , and the input voltage drops to the low level threshold voltage V _TL . The low level output voltage V _L is maintained until the input voltage drops to the low level threshold voltage V _TL , and the output voltage switches from the low level V _L to the high level V _H. Therefore, when the light is not incident on the photodiode PD and the current I _P does not flow, that is, when the charge is not accumulated in the capacitor C, the output voltage is the high level V _H , and
When the charging voltage V _C of the capacitor C is equal to the high-level threshold voltage V _TH, the Schmitt inverter 24
The output voltage of the switch from the high level to the low level V _L. Further, when the charge accumulated in the capacitor C is reduced to the low level threshold voltage V _TL of the Schmitt inverter 24 by discharging, the output voltage of the Schmitt inverter 24 is switched from the low level _VL to the high level V _H. Therefore,
The first Schmitt inverter 24 outputs a pulse voltage having a cycle corresponding to the charging / discharging of the capacitor C, and thus the illuminance or the amount of light incident on the photodiode PD is converted into a frequency signal.

On the other hand, the analog switch 23 has two fixed contacts x ₀ and x ₁ and one movable contact x, the movable contact x is connected to the capacitor C, and the first fixed contact x ₀ is connected to the photodiode PD. Connected, and the second fixed contact x ₁ is connected to the discharge resistor R. The movable contact x of the analog switch 23 has a first fixed contact x ₀ when a high level output voltage V _H is applied from the first Schmitt inverter 24 through the control line 24a.
When the low-level output voltage V _L is applied, the movable contact x is connected to the second fixed contact x ₁ .

In the above structure, the photodiode PD
In the initial state in which no current flows through the capacitor C and thus no charge is stored in the capacitor C, the output of the first Schmitt inverter 24 is at the high level V _H , so the movable contact x of the analog switch 23 is the first fixed contact x. Connected with ₀ . When the measurement of the amount of light is started, the photodiode P
A current I _P flows through D, and the capacitor C is charged. When the charging voltage V _C of the capacitor C reaches the high level threshold voltage V _TH of the first Schmitt inverter 24,
The output voltage of the Schmitt inverter 24 is high level V
_Switch from _H to low level V _L. As a result, the movable contact x of the analog switch 23 is switched to the second fixed contact x ₁ side, and the charging voltage V _C of the capacitor _C is discharged via the resistor R. Charging voltage V _{C of} capacitor C due to discharge
Falls to the low level threshold voltage V _TL of the first Schmitt inverter 24, the output voltage of the Schmitt inverter 24 changes from the low level V _L to the high level V _H.
Switch to. As a result, the movable contact x of the analog switch 23 is again connected to the first fixed contact x _0, and the charging current flows through the capacitor C. Hereinafter, as a result of repeating the same operation, the output voltage of the first Schmitt inverter 24, that is, the output voltage V from the light quantity-frequency conversion circuit 30.
_As shown in FIG. 5, ₁ has a pulse waveform having a frequency corresponding to the charging / discharging cycle of the capacitor C.

In the present example, the output voltage V ₁ of the first Schmitt inverter 24 is supplied to the same C-MOS type second Schmitt inverter 25 as the first Schmitt inverter 24. This second Schmitt inverter 2
5 also has two threshold voltages, a high level threshold voltage V _TH and a low level threshold voltage V _TL , and operates similarly. That is, the low-level voltage output V _L is generated when the high-level voltage signal V _H is input from the first Schmitt inverter 24, and the high-level voltage output V _L is input when the low-level voltage signal V _L is input. Generate output V _H. Therefore, the second Schmitt inverter 25 generates an output voltage of the same frequency F ₀ which is the inverted pulse waveform of the first Schmitt inverter 24,
It is supplied to the microcomputer 22 which is a calculation and measurement unit.

The microcomputer 22 counts the number of pulses of the frequency signal F ₀ input from the light quantity-frequency conversion circuit 30 through the second Schmidt inverter 25, the photodiode PD, and the input / output characteristics of the conversion circuit. RO storing arithmetic processing program
Storage unit 22b including M (read only memory)
And an arithmetic processing unit 22c that calculates the illuminance or the light amount by detecting the magnitude of the output current of the photodiode PD according to the program of the storage unit 22b from the number of pulses within a certain time counted by the counter unit 22a. Therefore, the illuminance or the amount of light incident on the photodiode PD can be obtained in this way.

[0007]

However, in the photometric device having the above configuration, the input / output characteristics of the light quantity-frequency conversion circuit (oscillation circuit) 30, that is, the photodiode PD.
The relationship between the output current I _P and the oscillation frequency F ₀ is determined by the elements that constitute the oscillation circuit and is fixed. Therefore, the range of measurable light quantity (illuminance) is determined by this input / output characteristic curve, and it is not possible to measure the light quantity smaller or larger than a certain level, and there is a drawback that the measurement range is narrow. There is also a drawback that the measurement accuracy near the limit level deteriorates.

According to experiments conducted by the inventor, it was confirmed that the light quantity-frequency conversion circuit 30 oscillates even if the capacitor C, which is a capacitive element of the light quantity-frequency conversion circuit 30, is removed. It was also confirmed that this is because the junction capacitance of the photodiode PD acts as a capacitance for frequency determination. It was originally thought that the junction capacitance of the photodiode PD was a useless long object, but if this can be used instead of the capacitance element, the capacitance element is not required in the oscillation circuit,
It is a win for both.

Therefore, an object of the present invention is to use the junction capacitance of the photodetector as a capacitance for determining the oscillation frequency, to eliminate the need for the capacitance element and to set the input / output characteristics arbitrarily within a considerable range. To provide a highly reliable photometric device with a widened photometric range.

[0010]

The above object is achieved by the photometric device according to the present invention. In summary, the present invention is
The photodetector whose output current changes according to the amount of received light, the junction capacitance of the photodetector charged by the output current from the photodetector, and the charging voltage charged in the junction capacitance are preset. A reference voltage is compared with the voltage detection means for changing the output signal level each time the charging voltage reaches the reference voltage, and a discharge path for the junction capacitance is formed by the change in the output signal level from the voltage detection means. A photometric device comprising switch means and means for applying a variable bias voltage to the photodetection element.

[0011]

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing an embodiment of a photometric device according to the present invention. The photometric device of this embodiment uses a photodiode PD as a photodetector, and uses a diode D1 instead of an analog switch. The diode D1 is provided between the output and the input of the first Schmitt inverter 24. The illuminance (light quantity) -frequency conversion circuit 30 is configured by connecting the variable voltage source VS to the photodiode PD, which has the opposite polarity to PD and is connected via the resistor R1. That is, in the present embodiment, no capacitance element (capacitor) is used in the light quantity-frequency conversion circuit 30, and instead the junction capacitance C _j of the photodiode PD is used as a capacitance for frequency determination.

The reason why the diode D1 is used instead of the analog switch is to eliminate the disadvantage that the characteristic of the light quantity-frequency conversion becomes unstable, the error in the light quantity measurement becomes large, and the reliability is lacked. . That is,
In FIG. 4, when the movable contact x of the analog switch 23 is connected to the fixed contact x ₁ side, that is, when the capacitor C is discharged, the current output from the photodiode PD is the junction capacitance C _j and the analog switch. 23
Since the terminal capacitance C _i of the fixed contact x ₀ is charged, when the movable contact x is switched to the fixed contact x ₀ side, the charges accumulated in these capacitances C _j and C _i are discharged to the capacitor C. To be done. Therefore, at the start of charging, the voltage of the capacitor C rises from V _TL to a slightly higher voltage, and the charging of the capacitor C is performed from this raised voltage until the high level threshold voltage V _TH is reached. become. Therefore, the time required for charging is shortened. As a result, the characteristics of illuminance (light intensity) -frequency conversion become unstable,
The error in measuring the amount of light becomes large, resulting in a drawback of lacking reliability.

In the configuration of the present embodiment shown in FIG. 1, in the initial state in which no current flows in the photodiode PD and therefore no charge is accumulated in the junction capacitance C _j of the photodiode PD, the first Schmitt inverter 24 has a since the output voltage is at the high level V _H, the diode D1 is reverse biased and forms the same function as the switch off. Therefore, no current flows through the resistor R1 and the photodiode P
The junction capacitance C _{j of} D is in a chargeable state. When the measurement of the amount of light is started, a current I _P flows through the photodiode PD and its junction capacitance C _j is charged. This junction capacitance C _j
When the charging voltage V _j of the Schmitt inverter 24 reaches the high level threshold voltage V _TH of the first Schmitt inverter 24, the output voltage of the Schmitt inverter 24 switches from the high level V _H to the low level V _L. This diode D1 is forward biased, since forming the same function as the switch-on, the output current I _P of the charging voltage V _j and the photodiode PD of the junction capacitance C _j of the photodiode PD resistor R
1 and the diode D1 and the charging voltage of the junction capacitance C _j is discharged. Photodiode P by discharge
When the charging voltage V _j of the junction capacitance C _j of D drops to the low level threshold voltage V _TL of the Schmitt inverter 24, the output voltage of the Schmitt inverter 24 switches from the low level V _L to the high level V _H. As a result, the diode D1 is reverse-biased again and turned off,
A charging current flows through the junction capacitance C _j of the photodiode PD. As a result of repeating the same operation thereafter, the waveform of the output voltage V ₁ of the first Schmitt inverter 24 becomes as shown in FIG. 5 described above.

Further, in this embodiment, a waveform shaping circuit having a simple configuration is connected to the output side of the first Schmitt inverter 24, the output voltage V ₁ is waveform shaped, and the low level output voltage V _{L of the} output voltage V ₁ is output. Duration, that is, the photodiode P
The discharge time of the junction capacitance C _j of D is lengthened so that the output waveform can be measured with sufficient accuracy by an inexpensive counter such as a general-purpose counter or a counter with a built-in microcomputer.

The waveform shaping circuit has an output side of the first Schmitt inverter 24 and a second Schmitt inverter 25.
3rd Schmidt inverter 3 in series with the input side of
1 has a configuration in which a diode D2 is connected and a resistor R2 and a capacitor C2 are connected in parallel between the output side of the diode D2 and the ground, and the output signal waveform-shaped by the waveform shaping circuit is the second Schmitt inverter. 25 inputs.

Next, the operation of the waveform shaping circuit will be briefly described. The output voltage V ₁ of the _first Schmitt inverter 24 is inverted by the third Schmitt inverter 31, and the output voltage waveform thereof is as shown in FIG. This output waveform has a high level portion, that is, a portion corresponding to the discharge time Td of the junction capacitance C _j of the photodiode PD charges the capacitor C2 through the diode D2, and a low level portion, that is, the junction of the photodiode PD. The portion of the capacitance C _j corresponding to the charging time Δt is blocked because the diode D2 is reverse biased. Therefore, when the output waveform of the third Schmitt inverter 31 changes from high level to low level, the voltage charged in the capacitor C2 has a time constant τ = C ₂ · R ₂ (where C ₂ is the capacitor C2
, R ₂ is gradually discharged through the resistor R2 according to the resistance value of the resistor R2). Therefore, the voltage waveform supplied to the input of the second Schmitt inverter 25 is as shown in FIG. 4B. As a result, the output voltage waveform of the second Schmitt inverter 25 becomes as shown in FIG.
It is possible to obtain a required time width Td ′ that is longer than the discharge time Td of the junction capacitance C _j of the photodiode PD. This time width Td 'can be set to a desired arbitrary value by appropriately setting the values C ₂ and R ₂ of the capacitor C2 and the resistor R2 of the waveform shaping circuit. It is possible to easily set the time width Td ′ that can be reliably measured by an inexpensive counter having a certain level of performance.

Further, in the present embodiment, the reverse bias voltage applied from the variable voltage source VS to the photodiode PD can be varied by the control signal S _C from the control port 22d of the microcomputer 22. By changing the capacitance value of the junction capacitance C _j of the diode PD, the input / output characteristics of the light quantity-frequency conversion circuit can be set arbitrarily. For example, when a predetermined reverse bias voltage is applied to the photodiode PD from the variable voltage source VS, if the reference input / output characteristic shown by A in FIG. 3 is exhibited, the reverse bias voltage is increased from this state. When the junction capacitance C _j of the photodiode PD is reduced by using the photodiode PD, the input / output characteristic moves substantially parallel to the direction in which the oscillation frequency increases, as shown by B in FIG. As a result, even if the output current I _{P of the} photodiode PD becomes smaller, the oscillation frequency becomes higher than in the case of the reference characteristic, so that the lower limit of the amount of light that can be accurately measured can be lowered. On the contrary, when the reverse bias voltage is decreased and the junction capacitance C _j of the photodiode PD is increased, the input / output characteristics move substantially parallel to the direction in which the oscillation frequency decreases, as indicated by C in FIG. Therefore, the upper limit of the amount of light that can be accurately measured can be increased. Since this change of the input / output characteristics can be performed continuously, it is possible to set the desired optimum input / output characteristics suitable for the light quantity to be measured. There is also an advantage that the accuracy is further increased and the reliability is further improved.

The electronic photometric device according to the present invention not only measures the illuminance (light amount), but also stains or deterioration of various oils and stains such as liquids and gases through which light other than oil can pass. Deterioration can be measured. For example, it is useful when applied to a device for detecting dirt or deterioration of lubricating oil such as automobile engine oil.

The above embodiment is merely an example of the present invention, and the circuit configuration, the elements to be used, etc. can be arbitrarily changed as required. For example, an inverter other than the C-MOS Schmidt inverter or another circuit element may be used, and it goes without saying that a photodetection element other than the photodiode or an element other than the microcomputer may be used. Further, the frequency output generated from the light quantity-frequency conversion circuit may be a pulse other than the square wave pulse.

[0021]

As described above, the photometric device according to the present invention can arbitrarily set the input / output characteristics of the oscillation circuit within a certain range by utilizing the junction capacitance of the photodetector, which is considered to have an adverse effect. As a result, the range of light amount that can be accurately measured is widened. Further, since the desired optimum input / output characteristics suitable for the amount of light to be measured can be set, the measurement accuracy is further enhanced and the reliability is further improved. further,
Since it is not necessary to use a capacitive element in the oscillator circuit, the number of parts used is reduced, downsizing and cost reduction are possible, and the bias voltage applied to the photodetection element is simply changed, so the circuit configuration is also There are many notable effects such as simplification.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of a photometric device according to the present invention.

FIG. 2 is a waveform diagram showing a voltage output in each part of the waveform shaping circuit used in the first embodiment of the present invention shown in FIG.

FIG. 3 is a diagram showing input / output characteristics of the first embodiment of the present invention shown in FIG.

FIG. 4 is a circuit configuration diagram showing a specific example of a conventional photometric device.

FIG. 5 is a waveform diagram showing the voltage output of the Schmitt inverter used in the photometric device.

[Explanation of symbols]

22 microcomputer 24,25,31 Schmitt inverter 30 illuminance (light intensity) - Frequency converter PD photodiodes C _j photo junction diode capacitance C1, C2 capacitor R1, R2 resistances D1, D2 diode VS variable voltage source

Claims (2)

[Claims] 1. A photodetector element whose output current changes according to the amount of light received, a junction capacitance of the photodetector element charged by the output current from the photodetector element, and a charging voltage charged in the junction capacitance. And a reference voltage set in advance, and a voltage detection unit that changes the output signal level each time the charging voltage reaches the reference voltage, and a change in the output signal level from the voltage detection unit A photometric device comprising switch means for forming a discharge path and means for applying a variable bias voltage to the photo-detecting element. 2. The photometric device according to claim 1, wherein the voltage detection means is a pulse oscillation circuit that generates a pulse signal each time the charging voltage reaches the reference voltage.