As shown in the figure above, the timer consists of a photosensitive sensor head, 555 and C2, C4-C7 and so on. Used for automatic metering and timing when the photo is zoomed in. The photosensitive sensor adopts the triode 3DU type. When K1 is in the automatic exposure "1", press AN, then 555 is set, VT4 is turned on, J is sucked, the amplifier lamp is lit, and the exposure starts. The photosensitive tube has different injection currents according to the intensity of the reflected light on the magnifying paper, and the current flowing through VT3 is different, so the charging current and exposure time of C2, C4 (C5-C7) are different. Therefore, the amplifying machine lights up and the exposure time is dependent on the intensity of the light and the contrast of the film to realize automatic metering and timing. China leading manufacturers and suppliers of Ev Charger,AC Charging Station, and we are specialized in AC Charging Pile,AC Ev Charger, etc. Ev Charger,AC Charging Station,AC Charging Pile,AC Ev Charger Shenzhen Hongjiali New Energy Co., Ltd. , https://www.hjlcharger.com