Patent
1976-11-02
1978-10-03
Adams, Russell E.
354 60R, G03D 708
Patent
active
041187121
ABSTRACT:
A digital light meter system for a camera includes a light measuring and computing circuit 1 whose analog output is supplied to a comparator 2 along with the output from a D/A converter 3 fed by a digital counter 5 coupled to an oscillator 6. When the comparator detects signal balance a logic circuit 4 locks the counter outputs in a latch 7, which values are supplied to a B/D converter 8 whose outputs drive an LED display 9.
The light measuring and computing circuit provides an output proportional to the logarithm of the apex value of exposure time (or aperture opening) as a function of the sensed brightness level and preset values of film sensitivity and aperture opening (or exposure time).
The D/A converter includes a constant voltage source 32-34, four parallel transistors 23, 25, 27 and 29 having common collectors, and a resistor 31 connected between the constant voltage source and the collectors. The transistors are base driven from a constant current source 18-22, their emitters have area ratios of 1:2:4:8, and their conductions are controlled by the outputs from the counter 5, whereby their combined collector potentials are proportional to the logarithm of the analog equivalent of the counter value.
REFERENCES:
patent: 3995284 (1976-11-01), Kitaura et al.
"Current-Steering Chip Upgrades Performance of DA Converter" Electronics, Apr. 4, 1974, pp. 125-130.
"A Monolithic 10 b Digital-to-Analog Converter . . . " IEEE Journal of Solid State Circuits, vol. SC8, #6, Dec. 1973, pp. 396-403.
"Dynamic Element Matching for High-Accuracy Monolithic D/A Converters," IEEE Journal of Solid State Circuits, Dec. 1976, pp. 795-800.
Adams Russell E.
Asahi Kogaku Kogyo Kabushiki Kaisha
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