LOW_LEVEL_EXPOSURE_METER

Published:2009/7/13 3:55:00 Author:May | From:SeekIC

Uses Siemens BPX 63 photodiode having sensitivity of 10 nA per lux in circuit which ensures that apdrture setting is affected only by useful lightand not by noise signals. When used at low light levels, circuit recovers quickly from temporary light bursts. Switches S1 and S2 are closed when camera shutter is not open; opamp output is then connected to its inverting input through FET T1. At commencement of exposure, S1 and S2 open to give amplification of over 3000. Integrating capacitor C1 is then charged by photocurrent, making output voltage vary linearly with time. Base-emitter junction of T3 begins to conduct at output voltage of 1 V. Exposure is completed when C1 provides feedback via T3 so no cuwent flows through load resistor RL. Supply is ±3 V.- Photodiode BPX 63-All It Needs Is Starlight, Siemens, Iselin,NJ.



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