Constant-current light-sensing system

• US 5,055,669 A
• Filed: 07/16/1990
• Issued: 10/08/1991
• Est. Priority Date: 06/29/1989
• Status: Expired due to Fees

• Current Assignee: Multipoint Control Systems Incorporated
• Original Assignee: Multipoint Control Systems Incorporated
• Inventors: Blake, Frederick H., Long, C. David
• Primary Examiner(s): Nelms, David C.
• Application Number:
  US07/554,176
• Time in Patent Office: 449 Days
• Field of Search: 250/214 AL, 250/214 R, 250/214 B, 315/149, 315/156, 307/311
• US Class Current: 250/214.0AL
• CPC Class Codes: H01L 31/0203 Containers; Encapsulations , e.g. encapsulation of photodiodesfor photovoltaic devices H01L31/048; for organic photosensitive devices H10K30/80

Abstract

A constant-current light sensing system employs a constant-current diode in series with the power lead to a photoconductive cell. This diode provides a substantially fixed current through the cell and enables the system to be designed such that it can make maximum use of the linear portions of the cell'"'"'s output response curve, and maximum use of the range of a potentiometer'"'"'s settings. The cell used in conjunction with the system is received in a housing that is easily mountable to a wall or ceiling surface by an adhesive material.

Claims

1. An improved photoconductive light sensor circuit, comprising:

a photoconductive cell interconnecting a circuit junction and a ground, said photoconductive cell being characterized in that said cell normally has a generally nonlinear electrical resistance that decreases in accordance with exposure of said cell to increasing light levels, and vice versa;

a constant-current, fixed-voltage power source connected to said circuit junction, and operative to continuously transmit a substantially constant current through said photoconductive cell at all times while said light sensor circuit is both operative and said cell has a resistance substantially at or below a certain selected resistance, said constant current source being further operative to transmit a variable current that is less than the magnitude of said constant current when said cell'"'"'s resistance is higher than said selected resistance; and

a voltage-sensing circuit operatively connected to said circuit junction, for sensing the change in voltage at said junction in response to variations of the resistance of said photoconductive cell in response to changes in said cell'"'"'s light exposure.

2. The invention of claim 1, wherein said circuit junction is positioned in a remote location relative to the location of said photoconductive cell.

3. The invention of claim 1, wherein said voltage-sensing circuit includes:

a sensor delay circuit having a sensor delay capacitor; and

a forcing capacitor connected in series to said sensor delay capacitor; and

a by-pass discharge diode bridging around said forcing capacitor.

4. The invention of claim 3, wherein the capacitance of said forcing capacitor is substantially equal to the capacitance of said sensor delay capacitor.

10. The invention of claim 1, wherein said constant-current, fixed-voltage power source includes a constant-current diode interconnecting a fixed-voltage power source and said circuit junction.

5. For use in connection with a light sensor having a photoconductive cell, said cell normally having a variable nonlinear electrical resistance that changes in response to changes in light levels impinging on said cell, such resistance decreasing as light levels increase, and vice versa, a method of linearizing the output of said light sensor by making said sensor nonresponsive to light below a certain level, said method comprising:

selecting a lowest level of desired light sensor sensitivity;

selecting a photoconductive cell for said light sensor, such cell having a certain normal range of output resistances, and wherein such cell also has a certain desired resistance corresponding to said lowest level of desired light sensor sensitivity, said desired resistance being at the upper end of a substantially linear portion of said cell'"'"'s nonlinear output response; and

transmitting a substantially constant current through said selected cell from a fixed-voltage power source at all times while said light sensor is operative, and while said cell'"'"'s output resistance is substantially at or below said desired resistance, wherein the magnitude of said constant current is selected to have a value such that when said selected cell has said desired resistance, said desired resistance and constant current define a substantially maximum desired voltage across said cell, corresponding to said selected lowest level of desired light sensor sensitivity; and

transmitting a variable current through said cell from said fixed-voltage power source when said cell'"'"'s resistance is above said desired resistance, including providing a substantially fixed-voltage drop between said fixed-voltage power source and said cell while said variable current is transmitted through said cell, the magnitude of said fixed-voltage drop being selected so that the difference between the voltage of said fixed-voltage drop and the voltage of said fixed-voltage power source substantially equals said maximum desired voltage across said cell, to limit the voltage across said cell to said maximum desired voltage when said cell'"'"'s resistance is above said desired resistance, and to thereby make said voltage across said cell substantially nonresponsive to changing light levels that are below said lowest level of desired light sensitivity.

6. The method of claim 5, wherein transmitting a substantially constant current through said photoconductive cell includes

using a constant-current diode interconnecting in series said fixed-voltage power source and said photoconductive cell.

7. The method of claim 5, wherein

selecting said lowest level of desired light sensor sensitivity includes selecting 2 foot candles as being the lowest level of sensitivity, and

selecting said photoconductive cell includes selecting a cell having a normal range of output resistances where cell resistance is approximately 11K ohms at 2 foot candles of light input, and approximately 1.5K ohms at 50 foot candles of input, and said certain desired resistance of said cell at said lowest level of sensitivity is approximately 11K ohms, and

transmitting a substantially constant current through said selected cell includes using a constant-current diode interconnecting in series a 12 volt DC low voltage power source and said photoconductive cell, said diode being of a type that causes a current of approximately 1.0 milliamps to be transmitted though said cell until the voltage drop across said diode decreases to approximately 1 volt DC, and of a type that maintains a fixed voltage drop of approximately 1 volt DC thereacross when said cell resistance exceeds 11K ohms, and transmits a variable current through said cell so long as said cell resistance exceeds 11K ohms.

8. The method of claim 5, wherein

selecting said lowest level of desired light sensor sensitivity includes selecting twenty-five foot candles as being the lowest level of sensitivity, and

selecting said photoconductive cell includes selecting a cell having a normal range of output resistances where cell resistance is approximately 30K ohms at 2 foot candles of light input, and approximately 11K ohms at 25 foot candles of input, and approximately 1K ohms at 150 foot candles of input, and said certain desired resistance of said cell at said lowest level of sensitivity is approximately 11K ohms, and

transmitting a substantially constant current through said selected cell includes using a constant-current diode interconnecting in series a 12 volt DC low voltage power source and said photoconductive cell, said diode being of a type that causes a current of approximately 1.0 milliamps to be transmitted through said cell until the voltage drop across said diode decreases to approximately 1 volt DC, and of a type that maintains a fixed voltage drop of approximately 1 volt DC thereacross when said cell resistance exceeds 11K ohms, and transmits a variable current through said cell so long as said cell resistance exceeds 11K ohms.

9. The method of claim 5, wherein

selecting said lowest level of desired light sensor sensitivity includes selecting fifty foot candles as being the lowest level of sensitivity, and

selecting said photoconductive cell includes selecting a cell having a normal range of output resistances where cell resistance is approximately 100K ohms at 2 foot candles of light input, and approximately 11K ohms at 50 foot candles of input, and approximately 0.5K ohms at 300 foot candles of input, and said certain desired resistance of said cell at said lowest level of sensitivity is approximately 11K ohms, and

transmitting a substantially constant current through said selected cell includes using a constant-current diode interconnecting in series a 12 volt DC low voltage power source and said photoconductive cell, said diode being of a type that causes a current of approximately 1.0 milliamps to be transmitted through said cell until the voltage drop across said diode decreases to approximately 1 volt DC, and of a type that maintains a fixed voltage drop of approximately 1 volt DC thereacross when said cell resistance exceeds 11K ohms, and transmits a variable current through said cell so long as said cell resistance exceeds 11K ohms.

11. An improved photoconductive light sensor, comprising:

a photoconductive cell characterized in that said cell normally has a variable, generally nonlinear electrical resistance that decreases in accordance with increasing exposure of said cell to changing light levels, and having a certain desired resistance when a certain threshold level of light impinges on said cell; and

a constant-current power source connected to said photoconductive cell, said constant-current power source including an electric circuit element serially interconnecting said photoconductive cell and a power supply of a fixed voltage, said electric circuit element being characterized in that it is operable to transmit a substantially constant current of a certain magnitude from said fixed voltage power supply through said photoconductive cell at all times when said sensor is in operation, and when said cell has a resistance that is substantially at or below said certain desired resistance, said constant current defining the magnitude of the voltage across said cell, such voltage changing as said cell is exposed to changing light levels, said electric circuit element being further operable to maintain a substantially constant voltage drop thereacross when said cell'"'"'s resistance is greater than said desired resistance, and to transmit a variable current from said power supply through said cell when said cell'"'"'s resistance is greater than said desired resistance, and wherein the sum of the voltage across said photoconductive cell and the voltage drop across said electric circuit element substantially equals the voltage of said power supply, thereby making the voltage across said cell substantially constant and nonresponsive when cell resistance is greater than said desired resistance, to correspondingly make the voltage across said cell substantially nonresponsive to light levels below said threshold level; and

a voltage-sensing circuit operative to sense the change in voltage across said photoconductive cell in response to changes in said cell'"'"'s exposure to light levels.