Ambient light filter

• US 5,874,731 A
• Filed: 11/18/1997
• Issued: 02/23/1999
• Est. Priority Date: 03/20/1995
• Status: Expired due to Term

• Current Assignee: Itron, Inc.
• Original Assignee: Schlumberger Industries Incorporated (Schlumberger NV)
• Inventors: Swanson, Scott C.
• Primary Examiner(s): Westin, Edward P.
• Assistant Examiner(s): Lee, John R.
• Application Number:
  US08/972,494
• Time in Patent Office: 462 Days
• Field of Search: 250/214 B, 250/214 R, 250/222.1, 250/221, 250/231.17, 250/231.14, 250/231.13, 250/231.1, 250/214 AL, 340/870.02, 340/870.03
• US Class Current: 250/214.00B
• CPC Class Codes: G01R 11/02 Constructional details
• CPC Class Codes: G01R 11/18 Compensating for variations in ambient conditions
• CPC Class Codes: G01R 22/066 Arrangements for avoiding or indicating fraudulent use
• CPC Class Codes: G08C 23/04 using light waves, e.g. infrared

Abstract

An ambient light filter includes light emitting circuitry and optical detection circuitry. The detection circuitry is positioned to detect light from the emitting circuitry. A microprocessor is provided to separately control the light emitting circuitry and the optical detection circuitry. The microprocessor determines whether light from the emitting circuitry has been detected by removing a measured component equivalent to the ambient light present at the optical detection circuitry.

Claims

1. An ambient light filter comprising:

light emitting circuitry;

optical detection circuitry, positioned to detect light from said light emitting circuitry;

a capacitor coupled to said optical detection circuitry; and

a microprocessor for separately controlling said light emitting circuitry and said optical detection circuitry to take an ambient light measurement when said light emitting circuitry is off, and to take at least a first subsequent measurement when said light emitting circuitry is on, and converting said measurements to values which are processed into a single value proportional to light measured in said first subsequent measurement exclusive of said ambient light measurement, wherein said microprocessor supplies said capacitor with a predetermined quantity of charge and measures discharge time of said capacitor to obtain said ambient light measurement and said first subsequent measurement, said capacitor being discharged through said optical detection circuitry at a rate proportional to intensity of light detected when taking said ambient light measurement and said first subsequent measurement.

2. The light filter of claim 1 wherein said light emitting circuitry comprises at least a first light emitting diode.

3. The light filter of claim 2 wherein said at least first light emitting diode is positioned to create a light path through an aperture in a meter disk when said meter disk is in a specific rotational position.

4. The light filter of claim 3 wherein said optical detection circuitry comprises at least a first phototransistor including:

(a) a first terminal coupled to a node, said node coupled to ground through said capacitor, said node further coupled to a timer input and a charging output of said microprocessor;

(b) a second terminal coupled to an output of said microprocessor; and

(c) a third terminal positioned to detect light emitted from said at least first light emitting diode when said aperture of said meter is positioned in said light path.

5. The light filter of claim 2 wherein said at least first light emitting diode is positioned to reflect light off a surface of a meter disk and wherein said optical detection circuitry is positioned to detect the reflection of said light off said surface.

6. The light filter of claim 2 wherein said at least first light emitting diode is positioned to create a light path through at least a first shutter along an edge of a meter disk and wherein said optical detection circuitry is positioned in said light path when said meter disk is in a specific rotational position.

7. A method for filtering ambient light from a photosensor, the method comprising the steps of:

deactivating a known light emitting source;

exposing said photosensor to said ambient light;

operating said photosensor to establish a first light measurement while said known light emitting source is known to be off by discharging a predetermined quantity of charge through said photosensor over a first time interval;

activating said known light emitting source;

exposing said photosensor to said ambient light and to said known light emitting source;

operating said photosensor to establish a second light measurement while said known light emitting source is active by discharging said predetermined quantity of charge through said photosensor over a second time interval; and

subtracting said first light measurement from said second light measurement to achieve a light measurement that excludes said ambient light.

8. The method of claim 7 further comprising the step of incrementing meter rotation information when said difference between said first and second measurements is attributable to said known light emitting source.

9. The method of claim 7 further comprising the step of incrementing a set of meter rotation information when said light measurement that excludes ambient light is equivalent to the light generated by said known light emitting source.

10. The method of claim 7;

wherein said step of operating said photosensor to establish a first light measurement further includes the steps of;

placing a first output of a microprocessor in a high impedance condition, said first output coupled to a first terminal of said photosensor;

charging a node coupled to a second terminal of said photosensor by asserting a second output of said microprocessor for a fixed period of time;

placing said second output in a high impedance condition at the end of said fixed period of time;

asserting said first output low to render said photosensor conductive during said first time interval if a certain amount of light is present; and

measuring said first time interval to establish said first light measurement.

11. The method of claim 10, wherein said step of operating said photosensor to establish a second light measurement further includes the steps of:

placing said first output of said microprocessor in a high impedance condition, said first output coupled to said first terminal of said photosensor;

charging said node coupled to said second terminal of said photosensor by asserting said second output of said microprocessor for a fixed period of time;

placing said second output in a high impedance condition at the end of said fixed period of time;

asserting said first output low to render said photosensor conductive during said second time interval if a certain amount of light is present; and

measuring said second time interval to establish said second light measurement.

12. The method of claim 7, wherein said first time interval is proportional to an intensity of said ambient light, and wherein said second time interval is proportional to a combined intensity of said ambient light and said known light emitting source.

13. An ambient light filter comprising:

light emitting circuitry;

optical detection circuitry comprising;

a resistor;

a phototransistor having a photosensitive first terminal, a second terminal coupled to said resistor, and a third terminal coupled to ground; and

an analog to digital converter having an input and an output, said input being coupled to said second terminal of said phototransistor; and

a microprocessor coupled to said light emitting circuitry and to said output of said analog to digital converter for removing ambient light from total detected light by taking an ambient light measurement when said light emitting circuitry is off and by taking at least a first subsequent measurement when said light emitting circuitry is on, wherein said analog to digital converter samples voltage levels present at said second terminal during said ambient light measurement and said first subsequent measurement and converts said detected voltage levels to digital representations, said digital representations being proportional to the intensity of light received during said ambient light measurement and said first subsequent measurement.

14. An ambient light filter comprising:

light emitting circuitry;

optical detection circuitry comprising;

detection means for detecting incident light,

charge storage means for storing a predetermined quantity of charge, and

discharging means for discharging said predetermined quantity of charge; and

a microprocessor coupled to said light emitting circuitry and to said optical detection circuitry, said microprocessor comprising;

charging means for supplying said predetermined quantity of charge to said optical detection circuitry; and

timing means for measuring discharge time of said predetermined quantity of charge from said optical detection circuitry, wherein said discharge time is related to intensity of said incident light.

15. The ambient light filter of claim 14, wherein said charge storage means comprises a capacitor.

16. The ambient light filter of claim 15, wherein said detection means comprises a first terminal of a phototransistor and said discharging means comprises a path between a second terminal and a third terminal of said phototransistor.

17. The ambient light filter of claim 16, wherein said microprocessor further comprises a variable impedance port coupled to said third terminal of said phototransistor, said variable impedance port being switchable between a high impedance state and a low impedance state.

18. A method for removing ambient light from total light received at a photosensor, thereby determining a light level received from a known emitter, the method comprising the steps of:

measuring said ambient light illuminating said photosensor, comprising the steps of;

deactivating said known emitter;

charging a capacitor to a predetermined voltage;

permitting said photosensor to become conductive when illuminated by said ambient light;

discharging said capacitor through said photosensor;

measuring a first discharge time; and

calculating an ambient light intensity from said first discharge time;

measuring said total light illuminating said photosensor comprising the steps of;

activating said known emitter;

charging said capacitor to a predetermined voltage;

permitting said photosensor to become conductive when illuminated by said total light;

discharging said capacitor through said photosensor;

measuring a second discharge time; and

calculating a total light intensity from said second discharge time; and

determining said light level received from said known emitter by subtracting said ambient light intensity from said total light intensity.