Photographic exposure meter circuit having temperature compensation
AssigneeAsahi Kogaku Kogyo Kabushiki Kaisha
InventorMasahiro Kawasaki
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Abstract
An exposure meter circuit includes a photo diode 2 and associated components 1, 3-6 for producing a first logarithmically compressed voltage V.sub.1 proportional to the apex value B.sub.v of the sensed brightness level. The temperature characteristic of the transistor 1 coupled to the diode 2 is compensated for by developing a second logarithmically compressed voltage V.sub.2 and subtracting V.sub.1 from it in an operational amplifier 9. The voltage V.sub.2 is produced by a transistor 11 whose base is coupled to the cathode of a light emitting diode 13 and whose collector output is returned to the power supply through a diode wired transistor 12. The temperature characteristic of the LED is the same as that of a transistor, whereby compensation is realized.
Claims4
In a temperature compensating photographic exposure meter circuit including means incorporating a photo sensitive element for producing a first logarithmically compressed voltage proportional to the apex value of the brightness of a photographic object, the improvement characterized by:
- a. a transistor (11),
- b. a resistor (10) connected to the emitter of said transistor,
- c. a light emitting diode (13) having its cathode connected to the base of said transistor,
- d. means (12) for converting the collector output of said transistor into a second logarithmically compressed voltage, and
- e. meter indication means responsive to the difference between said first and second logarithmically compressed voltages.
Dependencies are inferred from the claim text.