555 photo amplification automatic metering timer circuit

Published:2011/5/23 4:03:00 Author:TaoXi | Keyword: photo amplification, automatic metering, timer | From:SeekIC

As the figure 17-9 shows, the timer is composed of the photosensitive sensor head, the 555 and the C2, C4～C7.etc. And this circuit can be used in wide range of applications such as the photo amplification automatic metering timer. The photosensitive sensor uses the phototransistor 3DU type, when K1 is in the position of AE 1 , you press AN, the 555 sets, VT4 conducts, J closes, and the amplifier light turns on, the exposure starts. The photodiode injects different kinds of current to VT3 according to the strength of the enlarging paper's reflected light. So the time of pin-6 charges to the 2/3VDD threshold level is different.



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