Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-11-12
2002-02-19
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100, C327S514000
Reexamination Certificate
active
06348682
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to photodetector circuits. More particularly, this invention relates to the portions of a photodetector circuit that are used to convert a light signal to an electrical signal.
A photodetector is a device used for converting the photon energy of a light source into a photocurrent. In general, photodetectors are used in many practical applications such as communications, security systems, optical analyzing instruments, and other modern electronic devices. Typically, photodetector circuits include an amplifying stage connected to the photodetector for producing an analog voltage signal proportional to the photocurrent.
In many applications, the analog voltage signal from the amplifying stage is applied to a comparator circuit in order to determine the relative strength of the light signal with respect to a preset threshold voltage. If the analog signal is less than the threshold voltage, the comparator circuit may trip and produce a logic high at its output. However, if the analog signal is greater than the threshold voltage, the comparator circuit may generate a logic low at its output.
This type of circuit is often used to create an object detector that can determine the presence or absence of an object within a specific region. For example, a photodetector circuit may be arranged so that a light emitting diode (LED) constantly projects a beam of light on to the photodetector. As long as the light beam is uninterrupted, the photodetector produces a sufficient photocurrent, and the output of the comparator circuit remains at a logic low. However, when an object blocks the light signal, the photodetector turns OFF, and the output of the comparator circuit becomes a logic high.
A block diagram of a typical prior art photodetector system
100
is shown in FIG.
1
. System
100
generally includes a photodiode
110
, an amplifier circuit
120
and a comparator circuit
130
. Infrared light incident upon photodiode
110
creates a small photocurrent that is changed into a voltage and amplified by amplifier circuit
120
. Comparator circuit
130
compares the amplified signal with a preset threshold value to determine whether it is greater or less than the threshold value. If the amplified signal is less than the threshold, comparator
130
trips generating a logic high at its output. This means that the level of infrared light sensed by photodiode
110
has fallen below a minimum value, indicating a lack of infrared light. When the amplified signal is greater than the threshold value, comparator
130
supplies a logic low at its output, indicating that a sufficient amount of infrared light is detected.
Photodetector system
100
may be used in conjunction with other components for determining the presence or absence of objects. For example, assume a light emitting diode (shown in
FIG. 2
as LED
160
) is positioned to constantly project a beam of infrared light upon photodiode
110
. This creates a “sensing field”
170
between LED
160
and photodiode
110
within which objects can be detected. As long as the path of light from LED
160
to photodiode
110
is uninterrupted, a sufficient amount of current will be continuously generated by photodiode
110
so that the output of comparator
130
remains at a logic low. However, when an object enters sensing field
170
, the beam of infrared light is interrupted and photodiode
110
turns “OFF” (i.e., ceases to conduct). This causes comparator
130
to trip and produce a logic high at its output. The presence or absence of an object within sensing field
170
may be determined by examining the output of comparator
130
. If the output is a logic high, an object is present in the sensing field, if the output is a logic low an object is not present.
A schematic diagram of photodetection system
100
is shown in FIG.
2
. As shown in
FIG. 2
, photodiode
110
is reverse-biased with its anode connected to ground through resistor
111
and its cathode connected to a +5V power source. LED
160
is forward biased with its anode connected to +24V power source and its cathode selectively connected to ground through one of resistors
151
-
158
. Each resistor has a different value (
151
>
152
> . . .
158
). The intensity of the infrared light supplied by LED
160
is determined by the value of the resistor it is connected to. For example, if LED
160
is connected to the resistor with the smallest value (i.e., resistor
158
), a relatively large amount of current will flow through it, causing LED
160
to produce the greatest quantity of light. Conversely, If LED
160
is connected to the resistor with the largest value (i.e., resistor
151
), a relatively small amount of current will flow through it, causing LED
160
to provide the least quantity of light.
When photodiode
110
receives an infrared beam from LED
160
, the reverse-bias leakage current across it increases causing a voltage to be generated across resistor
111
(R
L
). This voltage is applied to the non-inverting input of operational amplifier (op-amp)
120
through a high-pass filter
112
formed by capacitor
114
and resistor
115
. The gain of op-amp
120
is controlled by the ratio of resistors
116
and
117
. The amplified output voltage V
o
of op-amp
120
is applied to the inverting terminal of comparator
130
. A threshold voltage V
R
created by series coupled resistors
131
and
132
is applied to the non-inverting terminal of comparator
130
. When the output voltage of op-amp
120
is less than the threshold voltage, the output of comparator
130
is high, when the output of op-amp
120
is less, it is low.
The output signal of comparator
130
is fed to a microcomputer
140
. Microcomputer
140
reads this output signal so that it may determine whether an object is present within sensing field
170
. In addition, the output of op-amp
120
is also fed to microcomputer
140
so that it may monitor the magnitude of the amplifier's output signal. Microcomputer
140
selects a resistor value based on this output signal and transmits a control signal to driver
150
in order to adjust the intensity of the light produced by LED
160
.
One deficiency of photodetection system
100
is that LED
160
, which is connected to the +24V power supply, is constantly on and therefore consuming a significant amount of power. In addition, changing the overall gain of photodetection system
100
poses several practical problems. For example, changing the gain of system
100
involves adjusting the value of the feedback resistor connected to an input pin of op-amp
120
. This is generally an undesirable way of adjusting the gain due to the sensitivity of the op-amp to conditions at the input pin. Furthermore, having an external pin that connects to the input of op-amp
120
adds parasitic capacitance to the op-amp's input, which reduces its phase margin. Although this problem can be solved by increasing power consumption, this solution is undesirable.
Thus, in view of the foregoing, it would be desirable to provide a photodetector circuit that operates at reduced power levels. It would also be desirable to provide a photodetector circuit that has an overall gain that can be adjusted by connecting an external resistor to a single external package pin.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a photodetector circuit that operates at reduced power levels.
It is another object of the present invention to provide a photodetector circuit that has an overall gain that can be adjusted by connecting an external resistor to a single external pin.
In accordance with these and other objects of the present invention a photodetector circuit suitable for acquiring and reporting data indicative of the relative strength of a light signal is provided. The photodetector circuit is configured to alternate between standby and active periods in order to reduce overall power consumption.
The photodetector circuit includes a photodiode and a switch timing circuit that periodi
Allen Stephone
Fish & Neave
Institute of Microelectronics
Shanahan Michael E.
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