Photo enlargement Automatic metering timer electronic circuit diagram

As shown in the figure above, the timer consists of a photosensitive sensor head, 555, C2, C4-C7, etc. Used for automatic metering and timing when photos are enlarged. The photosensitive sensor adopts triode 3DU type. When K1 is in automatic exposure "1", press AN, 555 is set, VT4 is turned on, J is closed, the amplifier light is on, and the exposure starts. According to the intensity of the reflected light on the amplified paper, the photosensitive tube injects different currents, and the current flowing through VT3 is different, so the charging current and exposure time of C2 and C4 (C5-C7) are different. Therefore, the lighting of the enlarger lamp and the exposure time depend on the intensity of light and the contrast of the film, realizing automatic metering and timing.