| 摘要 |
1533055 Photographic light meters ASAHI KOGAKU KOGYO KK 1 Nov 1976 [4 Nov 1975] 45303/76 Headings G2A and G2X [Also in Division H3] A digital light metering system for a camera has an exposure time or aperture computing circuit 1; an oscillator 6, pulse counter 5 and D/A converter 3; and a comparator 2 for comparing the outputs of circuits 1 and 3 and operating a latch circuit 7 when the counts are equal whereby an LED array 9 indicates the computed time or aperture; the D/A converter 3 comprising four parallel connected transistors 23, 25, 27, 29 with emitter resistors 24, 26, 28, 30 arranged to provide constant currents in the ratio 1:2:4:8, the collectors being connected via a resistor 31 to a constant voltage V6 and the emitters being connected to respective output terminals A, B, C, D of the counter 5, to control conduction of the individual transistors. In this way the voltage drop V5 across the resistor 31 is proportional to the analogue equivalent of the digital count. The computing circuit 1 Fig. 2 (not shown) includes a photo-diode (11) and a resistor (15) set to either S V -A V of to S V -T V . |
